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The angular distributions for 35Cl(2P3/2,1/2) scattered by Xe(1S0)
at collision energies 2.37-26.1 kcal/mol were detected in crossed-beam experiments. The interaction potentials for the X(1/2),
I(3/2), and II(1/2) electronic states were obtained via an approx. elastic-scattering anal. The well depth ( , in kcal/mol)
and equil. interat. distance (rm, in Å), resp., of the potentials are: 0.80, 3.23 for X(1/2); 0.37, 4.1 for I(3/2).
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A high-pressure, radio-frequency-discharge nozzle-beam source was developed for the prodn. of very intense (> 1018 atoms
sr-1 s-1) supersonic beams of O atoms. An efficient impedance-matching scheme was devised for coupling
the radio-frequency power to O-rare gas mixts. as functions of the gas pressure, temp., and compn. Techniques for localizing
the discharge directly behind the orifice of a specially designed quartz nozzle were developed. The above combine to yield a
beam source which reliably produces a high degree of mol. dissocn. in oxygen-rare gas mixts. at pressures up to 350 torr. At.
O mean translational energies of 0.14-0.50 eV were achieved by using the seeded-beams technique with Mach nos. of < 10. When
He was used as the carrier gas, both O(3PJ) and O(1D2) atoms were present in the
beam; only ground-state atoms were present in Ar seeded mixts. The design, construction, and operation of the beam source are
described and a characterization is given of the at. oxygen beams.
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In the crossed beams examn. of the reactions of O(3P) + C6H6, C6D6 (using a seeded, supersonic, at. O nozzle beam source), the angular and velocity distributions of reaction products are used to
identify the major reaction paths. The initially formed triplet biradical, C6H6O (C6D6O),
either decays by H (D) elimination or becomes stabilized, most likely by a nonradiative transition to the S0 manifold
of ground state PhOH. CO elimination was not a major channel. The branching ratio between H(D) atom elimination and stabilization
was sensitive to both collision energy and isotopic substitution.
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A crossed mol. beam study was made of the reaction O(1D2) + CH4 using a supersonic beam
app. The H atom elimination reaction O(1D2) + CH4 -> CH3OH* -> CH3O + H
greatly exceeds the H2 elimination reaction O(1D2) + CH4 -> H2 + HCHO
which is not obsd. to occur appreciably. This is in disagreement with the results of gas-phase study by C. L. Lin and W. B.
DeMore, (1973).
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In a study of the diffraction of He beams at the GaAs(110) surface, diffraction angular scans and specular intensity scans
were obtained for a wide range of incident angles, azimuths, and at 2 energies (E = 0.063 eV and E = 0.021 eV). The data are
analyzed qual. from a classical scattering viewpoint and the hard wall eikonal scattered wave approxn. A hardwall corrugation
function of the form F(x,y) = 0.5dx cos(2px/Lx) + 0.5dy cos(2py/Ly) yields
a qual. fit to the data with dx .apprx. 1.1 Å and dy .apprx. 0.3 Å, where the deep corrugation corresponds to going across the
surface troughs. This value of dx is approx. half the corrugation of the nuclear positions. Specular intensity scans are analyzed
in terms of interference in the normal momentum transfer, Dk.perp., which yields structural information
about vertical displacements. Requirements to describe appropriately the scattering from strongly corrugated surfaces are discussed.
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The angular and velocity distribution of OH product from the O(1D) + H2 reaction at 2.7 kcal/mol
collision energy was obtained in a crossed mol. beam study. The product is found to be forward-backward sym.; most of the
reaction occurs through insertion of the O to form ground electronic state H2O.
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Collision-induced translation-to-rotation energy transfer was studied
of HD (J = 0) scattering from Pt (111) by using a previously described
approximation (C. et al., 1977). Sharp modulations were observed of J = 0
to J = n (n = 1,2) rotational transition probabilities as functions of
incident angle. These modulations are believed to be due to competitive
scattering into bound surface state resonances of the HD/Pt adsorption
interaction potential, where the final HD rotational state is that of a
nearly free rotor.
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Rotationally mediated selective adsorption of HD on Pt(111) is
examined theoretically by using R-matrix scattering techniques. With a
laterally averaged surface-mole. Morse potential interaction and for an
anisotropic potential term transformed from H2, excellent
agreement is obtained between the resonances and the 1st-order perturbed
bound vibration-free rigid rotor energies.
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Rotationally mediated selective adsorption scattering resonances are
used to make an experimental and theoretical study of the laterally averaged
interaction potential between HD and a weakly corrugated system, Ag(111). The
experimentally observed resonances determined the vibrational levels of the
HD/Ag(111) physisorption potential as a function of bound rotational state.
These vibrational levels show J-dependence shifts due to the orientational
anisotropy of the potential. Exact quantum scattering calculations using a
full laterally averaged potential of the form V0(z,q)
= v0(z)[1 + bP2(cos q)] were
carried out to obtain rotationally inelastic transition probabilities.
Experimental and theoretical resonance energies are compared for 2 forms of
v0(z), a Morse and a variable exponent potential, as a function of b, and are very close to the 1st-order perturbed
energies of a free rotor in bound states of v0(z). Both potential forms give
equally good fits to the data, yielding an optimum value of the asymmetry
parameter, b ~ -0.05. The determination of b is relatively insensitive to small changes in the
v0(z) well depth.
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The reaction O(3P) + CF3I was studied with an rf discharge, supersonic O-atom beam source
in a crossed-beam arrangement. At collision energy 2.2 kcal/mol, the reaction yields exclusively IO. and
CF3.. The angular and velocity distributions of the product show that the reaction remains
confined to the triplet surface, forming a CF3-I-O complex with a half-life of >1 rotational period. Only
a small fraction of the available energy is partitioned into product translation, in good agreement with a statistical model.
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Elastic and rotationally resolved inelastic scattering was studied of
H2, D2 and HD supersonic beams from Ag(111) surface.
Very weak minima were detected in specularly reflected H2 and
D2 beams as a function of incident angle and azimuthal crystal
orientation for several beam energies. These are attributed to adsorption
scattering resonances. The D2-Ag vibrational levels were used to
determine the shape of the potential well for this system. A variable
exponential potential gives an excellent fit to the present experimental data.
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Rotationally-inelastic-diffractive-scattering probabilities were
measured for an HD beam colliding with a smooth Pt(111) surface. These
large T ® R inelastic probabilities were measured
as a function of incidence angle for a 110-meV beam energy. The results
agree with a simple physical model of an eccentrically weighted sphere
colliding with a hard wall, with an attractive well depth of 55 ± 10 meV.
The numerical GR method of N. Garcia (1977) was superior to an eikonal
method in solving this rotational-quantum-boundary-value problem.
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Diffractive and rotationally mediated selective adsorption scattering
resonances are reported for n-H2, p-H2, n-D2,
and o-D2 on Ag(111) (n = normal, p = para, o = ortho). Small
resonance shifts and line-width differences are observed between
n-H2 and p-H2, indicating a weak orientation dependence
of the laterally averaged H2/Ag(111) potential. The
p-H2 and o-D2 levels were used to determine the
isotropic component of this potential, yielding a well depth of ~32 meV.
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Angle- and velocity-resolved inelastic He scattering was used to study
how the surface dynamics of thin rare-gas films evolves on a layer-by-layer
basis. Surface-phonon dispersion relations for ordered 1-, 2-, 3-, and
25-layer Kr films physisorbed on Ag(111) are presented along LM across the entire Brillouin zone. The monolayer
data are dispersionless, indicative of an Einstein oscillator mode. In
comparison, the 25-layer film has a well-developed Rayleigh wave, typical of
a thick crystal surface. Excitation linewidths for monolayer Kr, which vary
across the zone, are also briefly discussed.
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A comprehensive study of the spatially isotropic component of the
laterally averaged molecular H/Ag(111) physisorption potential is presented.
Diffraction selective adsorption scattering resonances for rotationally
state-selected H2 and D2 on Ag(111) were mapped out as
a function of incident polar angle for several crystal azimuths and beam
energies. These resonances were used to determine the bound eigenvalues,
and subsequently the shape, of the potential well. Best fit Lennard-Jones,
Morse, variable exponent, and exponential-3 potentials having well depths of
~32 meV are derived from the data. These measurements are supported by
rotationally inelastic scattering measurements for HD and exact close-coupled
quantum scattering calcns. Debye-Waller attenuation measurements are also
presented for H2, D2, and HD. The ability to detect
these diffractively coupled resonances on a closest-packed metallic surface,
i.e., a surface of extremely low corrugation, suggests that such measurements
can be carried out on a much wider class of surfaces than previously envisioned.
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A detailed investigation of the spatially anisotropic component of the
laterally averaged H2/Ag(111) physisorption potential is presented.
Experimentally derived rotationally inelastic transition probabilities for
H2, D2, and HD, taken as a function of collision energy,
were compared with those resulting from close-coupled quantum scattering
calculations. These calculations utilize exponential-3 and variable exponent
parametrizations of the laterally averaged isotropic potential which reproduce
the experimental bound state resonance spectra for p-H2 and
o-D2 on Ag(111). Complementary information is obtained by analyzing
the magnetic sublevel splittings for physisorbed J = 1 n-H2, using
diffractive selective adsorption resonance energies calculated with 1st-order
perturbation theory. Theoretical predictions for HD/Ag(111) rotationally
mediated selective adsorption resonances were compared with previously reported
experimental results, which show well resolved J-dependent energy shifts
resulting in part from the orientational anisotropy of the potential. Both the
attractive and repulsive parts of the anisotropic potential exhibit only a weak
orientation dependence, in agreement with recent theoretical predictions for
this system.
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The lattice dynamics of Ar, Kr, and Xe overlayers on the Ag(111)
surface is discussed considering monolayer, bilayer, trilayer, and 25
layer films of each of these adsorbates. Data are also presented on the
dispersion relations of selected branches of the phonon spectra of these
overlayers. The data were obtained by the method of angle-resolved
inelastic He scattering. Several models of the lattice dynamics are
compared with the data. The gas-phase potentials of Barker give a
suitable description of the lateral interactions between the adsorbates,
within the accuracy of the available data, provided that the phonon
spectra are calculated for a lattice with the experimentally determined
lattice constants.
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A review with 26 references, mainly of the authors' experiments
involving He scattering by rare-gas overlayers physisorbed on Ag(111).
These experiments include elastic diffraction, selective adsorption,
and inelastic single-phonon scattering by angle-resolved time-of-flight.
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The phys. properties were detd. of ordered overlayers of Ar, Kr, and
Xe physisorbed on Ag(111). The desorption kinetics of the Xe
monolayer/Ag(111) system were investigated also. Desorption is zeroth
order until ~90% of the monolayer has desorbed, then becomes first
order. The inelastic scattering was measured of an 18 meV He beam from
unconstrained monolayers of (111) oriented Ar, Kr, and Xe. The
transitions are mapped across the entire surface Brillouin zone from G to M. The data are dispersionless, indicating
that for
the measured mode the adatoms are behaving as independent Einstein
oscillators. Parametrized physisorption potentials for RG-Ag(111) [RG =
Ar, Kr, Xe] are constructed by using these results. Inelastic scattering
probabilities and linewidths are also presented. The inelastic scattering
probabilities vary by at most a factor of 3 across the entire surface
Brillouin zone, and are reported as a function of incident angle, final
wave vector, and surface temp. Variations in the inelastic scattering
probabilities are indicative of dynamic adatom-substrate coupling. Exptl.
techniques which turn these dynamic couplings on or off for the same
phonon energy are discussed. Limited results for clean Ag(111) are also
presented. It is hoped that these measurements, on such ideal systems as
ordered rare gas monolayers, will provide further impetus for developing
improved theor. treatments of inelastic single phonon scattering.
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The surface dynamics of multilayer Ar, Kr, and Xe physisorbed on
Ag(111) were investigated along the G-M
direction. This was
done on a layer-by-layer basis for 2, 3, and ³20
layers for each of the
rare gases. Unlike the monolayers, the vibrational modes obsd. for the
multilayers show dispersion across the surface Brillouin zone, the amt. of
dispersion increasing with the no. of adsorbed layers. These results
reveal in detail how the surface dynamic properties of a thin film evolve
towards those of a thick crystal as a function of increasing dimension.
Lattice dynamics calcns., which utilize realistic gas phase pair
potentials, reproduce the exptl. obsd. phonon dispersion relations quite
well. The inelastic scattering probabilities and linewidths of the
transitions were also examd. One of the more notable results is that the
inelastic scattering probabilities vary by at most a factor of 2-3 across
the entire surface Brillouin zone. Isothermal desorption measurements for
the Xe overlayers are also discussed. Like the monolayer, the bilayer and
trilayer exhibit nearly zeroth order desorption until ~90% of the
top layer has desorbed, where the desorption kinetics become first order.
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The angle resolved intensities were measured of He (Ei = 18
and 66 meV) elastically scattering from the surfaces of rare gas
overlayers physisorbed on Ag(111). These studies were done on a
layer-by-layer basis for 1, 2, 3, and ~25 ordered overlayers of Ar, Kr,
and Xe. Two types of experiments are described. The first is
diffraction, where the scattered He intensity was measured as a function
of the detector angle, with the incident polar and azimuthal angles held
const. In the 2nd type of experiment, selective adsorption, the specular
intensity was measured as a function of incident angle. The purpose of
these experiments was to examine the He-surface potential, to assess the
relative contributions that various He-rare gas pair potentials,
nonadditive multibody terms, and He-substrate interactions make to the
systems studied. The experiments are compared with the results of
accurate close-coupling calculations, in order to quant. perform these
assessments. The comparisons between the selective adsorption data and
scattering calculations demonstrate the extreme sensitivity that such
measurements have to the He-surface potential. In particular,
observable changes in the calcd. selective adsorption spectra appear when
different He-rare gas potentials are tested, or when various nonadditive
terms are included in the potential. The results suggest that further
refinements in the He-heavy-rare gas pair potentials may be in order.
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UV laser-induced desorption from the surface of a thin NO film
proceeds via 2 mechanisms which are present simultaneously. One mechanism
is attributed to laser induced thermal desorption, while the other is due
to a nonthermal, single photon process. A film of 1-2000 ML (layers) of
NO condensed on a Ag(111) substrate under ultrahigh vacuum conditions at
25-50 K was irradiated by 5 ns pulses of 220-270 nm laser light (4.6-5.5
eV) with 0.5-5 mJ/cm2 (0.1-1 MW/cm2) power
d. at the surface.
Translational energies of desorbed mols. were measured from time-of-flight
(TOF) spectra taken with a mass spectrometer, while the internal energy
distribution of mols. desorbed in the nonthermal channel was detd. by
using a (1 + 1) resonance enhanced multiphoton ionization (REMPI) probe.
The NO monomer in the 2P3/2,1/2 electronic ground states was the only
significant product. There were 2 distinct characteristic TOF components,
which are assocd. with different desorption mechanisms. Each component
had a different velocity and angular distribution, and their relative
yields varied with laser pulse energy and NO layer thickness. The
angular, velocity, rotational, and vibrational distributions suggest
mechanisms for the nonthermal desorption. Factors detg. the relative
extent of thermal and photochem. effects, which control the morphol. of
ablated surfaces are discussed.
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The catalytic oxidn. of CO on a Rh(111) surface was investigated by
using modulated mol. beam techniques. Reaction proceeds via a
Langmuir-Hinshelwood mechanisms. Under exptl. conditions which provide a
high coverage of O adatoms and near zero coverage of adsorbed CO, an
activation energy of 24.5 ± 0.4 kcal/mol and a preexponential factor of (2
± 1) x 10-3 cm2 s-1 were
obtained. The angular distribution of the
product CO2 is sharply peaked toward the surface normal, and
cannot be described by a simple cosnq expression.
These results are discussed in relation to previous work on Pt and Pd
surfaces.
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Mol. beam and time-of-flight methods were used to examine the angular
distributions and velocity distributions of the CO2 product
mols. formed
in the catalytic oxidn. of CO on a Rh(111) single crystal with surface
temp. 700-1000 K. The angular distribution was sharply peaked about the
surface normal, and cannot be described by a simple cosnq expression. No
temp. dependence was obsd. in the angular distribution over the range of
temps. studied. Obsd. velocity distributions were clearly non-Maxwellian
and had av. translational energies in excess of those expected at the
surface temps. The av. velocity depended strongly on the desorption
angle. Mols. desorbing along the surface normal had an av. translational
energy of ~8 kcal/mol. The av. energy decreased with increasing
angle, reaching a value of ~4 kcal/mol at an angle of 60°. All of
the obsd. velocity distributions were narrower than Maxwellian
distributions with the same av. energies. Product velocity distributions
did not vary with surface temp. The obsd. excess energies probably arise
from the crossing of the activation barrier to reaction, with a fraction
of the reaction energy being carried away from the surface by the product
mols.
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The NO mols. subliming into vacuum from a condensed NO film at 50 K
exhibit cosine angular flux and Boltzmann rotational distribution at the
surface temp. This implies that the sticking probability for incident
mols. is independent of angle or rotational energy, at least for levels
populated at 50 K. Some models for rotational distributions of desorbing
mols. and the extent to which desorption can be used as a probe of the
surface dynamics of condensed phases are considered. Though the results
are simple, they touch on issues fundamental to obtaining microscopic
dynamical information for non-equil. interfaces.
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The exptl. detn. and theor. anal. are given for the surface phonon
dispersion relations for clean Ni(111) along the GM symmetry direction. The
surface phonon spectra were obtained with a high-resoln. EELS spectrometer operating
in the off-specular impact scattering regime. Kinematic conditions were varied to
selectively examine the Rayleigh and gap modes and contributions from bulk phonons.
Comparison of the exptl. surface phonon dispersion relations and inelastic scattering
cross sections with lattice dynamics and quantum multiple scattering calcns.
demonstrate that tensile surface stress is present at the level of 1.6 ± 0.2 N/m, and
that the intraplanar surface force const. is 11 ± 3% softer than in bulk Ni.
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The surface phonon spectrum of the ordered Cu3Au alloy
(001) was measured. In addn.
to the Rayleigh wave, a higher energy mode interpreted as the folded Rayleigh mode or
optical surface mode was obsd. From a harmonic pair potentials fit to the inelastic
neutron scattering data of Katano et al (1988), the force const. between the 1st and
2nd layer Cu atoms is stiffened by 20% with respect to the bulk value to match the
folded Rayleigh mode.
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A new mol. dynamics (MD) program was used to calc. the phonon spectrum
of the
stepped Lennard-Jones (LJ) surfaces (533) and (511). Surface phonons were obsd.
on both systems. On the LJ(533) surface at Q = XA, modes localized at the step are
obsd. which are identical in character to the step modes investigated by P. Knipp
(1989). These edge modes (E1 and E2) involve motions of atoms perpendicular to the
step edge and are obsd. by resolving each of the 4 terrace atom contributions to the
spectrum. The observation of step modes in MD simulations is also important, since
only single force const. models in the harmonic approxn. have previously been used to
calc. phonons on stepped surfaces. The temp. dependence of the z-polarized surface
and step modes were also studied as a function of temp. on LJ(533). As the temp.
is raised, the step and surface modes decrease and broaden in frequency. For a
surface temp. equal to 15% of the LJ bulk m.p. Tm, the step
modes are 4 times broader than in the low temp. simulations (~3%
Tm), while the surface modes are only 1.7 times broader. At
these elevated temps., the step modes also couple
strongly to the surface modes, creating a broad energy band on all four terrace
atoms. This mode coupling may be important when considering vibrational energy flow
on stepped surfaces, as well as the rate enhancements that catalytic reactions have
on such surfaces as compared to their low Miller index counterparts.
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Inelastic electron scattering was used to obtain the surface phonon
dispersion relations for clean Ni(111) along the GM symmetry
direction. Kinematic conditions were varied to examine selectively the
Rayleigh mode and "gap" mode,
as well as contributions from bulk phonons. Comparison of the exptl. phonon dispersion
relations and inelastic scattering cross sections with lattice dynamical and quantum
multiple scattering calcns. demonstrate that the intraplanar surface force const. is
11 ± 3% softer than in bulk nickel, and that tensile surface stress is present at the
level 1.6 ± 0.2 N/m.
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Surface kinetics which are nonlinear and involve multiple reactants
are not amenable
to std. modulated mol. beam reactive scattering techniques. To explore these complex
reactions, a new 3 mol. beam arrangement was developed. Two continuous, independently
adjustable beams establish steady-state surface concns., while a 3rd weaker beam is
modulated to induce small concn. perturbations around the selected steady state.
This permits exptl. linearization of nonlinear kinetics over a wide range of
coverages, helping to evaluate coverage-dependent rate consts. and to isolate
individual elementary steps from complex reaction mechanisms. The technique was
applied to the oxidn. of H to H2O on the Rh(111) surface.
Linearization of the H2O
reaction was demonstrated with isotopic substitution forming HDO. The anal. is
illustrated by the simpler example of H-D recombination. HD shows an activation
energy of 20 kcal/mol and preexponential of ~10-2 cm2 s-1 for the linearized
reaction in the low coverage limit.
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The kinetics of the oxidn. of H to H2O on the Rh(111)
surface was studied by using
modulated mol. beam reactive scattering. For reactant pressures <10- Torr and
temps. from 450-1250 K, serial steps were obsd. with apparent activation energies
of 2.5 ± 1 and 10 ± 1 kcal/mol. Pseudo-first-order preexponential factors are 105
and 107 s-1, resp., varying slightly with O coverage.
Reaction is inhibited by excess O. Max. H2O prodn. occurs ~
650 K. At lower temps. the reaction
becomes nonlinear. A new 2-mol.-beam arrangement was used. Two continuous,
independently adjustable beams establish steady-state surface concns., while
a weaker modulated third beam induces small concn. perturbation around the selected
steady state. With this technique surface O coverages were varied, isotopic
substitution used in the 3 beams to produce H2O, D2O
and HDO, and the HDO reaction linearized.
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High-resoln. electron energy loss spectroscopy (HREELS) was used to
study the low-energy vibrational modes of bridge-bonded CO on Ni(111)
after a well-ordered
c(4x2) overlayer structure was formed. In this paper, the spectroscopic observation
of two low-energy modes of bridge-bonded CO on Ni(111) were reported which have not
been previously reported, as well as the momentum-resolved scattering for one of these
modes. At Ts = 120 K, under impact scattering conditions, a
frustrated translation
of CO parallel to the surface was obsd. at an energy loss of 11.8 meV. The
dispersion curve measured along the 11-2 direction of Ni(111) for this
mode was dispersionless, indicating that there is no direct lateral interaction
between the adjacent CO mols. in this structure. At Ts = 170
K, using a transient
neg.-ion S shape-resonance to enhance our sensitivity, a frustrated rotation for
bridge-bonded CO was also obsd. at an energy loss of 37.5 meV. The energy of this
mode is near the value previously estd. from temp. dependent IR line shape
measurements of the CO stretch peak position and lineshape, and is believed to be
responsible for vibrational phase relaxation in the c(4x2)-CO-Ni(111) system.
The energies of the obsd. modes are also consistent with those derived from a
simple cluster calcn.
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A new 3-mol. beam arrangement is introduced that expands the range and
power of modulated beam reactive scattering for studying complex kinetics
at surfaces. This paper presents 2 types of kinetic measurements that
utilize this 3-beam
arrangement. The 1st measurements use 2 continuous, independently adjustable
mol. beams to establish steady-state reaction conditions, while a weaker modulated
3rd beam induces small concn. perturbations around the selected steady state.
This technique permits exptl. linearization of nonlinear kinetics over a wide range
of conditions, allowing one to explore the global behavior of reactions, det.
coverage-dependent rate consts., and isolate individual elementary steps from
complex reaction mechanisms. These capabilities are illustrated with preliminary
results for the oxidn. of H to H2O and for the recombination
kinetics of H on the
Rh(111) surface. The 2nd group of measurements uses time-resolved specular He
scattering as a sensitive in situ probe of both adsorbate coverage and
coverage-dependent surface kinetics. The oxidn. of CO on Rh(111) under
pseudo-first-order conditions is examd. with this new kinetic probe.
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Linearized measurements of the coverage dependent desorption rates of
CO from Rh(111) were made with a novel 3 mol. beam app. To measure these
isothermal and essentially isosteric rates, a new kinetic response amplifier (time-resolved
specular helium scattering) is introduced, which makes use of the large attenuation
cross section that CO has for specular He scattering. The measurements were made
by using 1 intense and continuous CO beam to establish a specific adsorbate coverage
while another low intensity and chopped CO beam was used to modulate
weakly the adsorbate d. around the selected steady state. The transient He reflectivity
waveforms measured during the modulated CO scattering contain the desired kinetic
information, and are typically 1 to 2 orders of magnitude more sensitive
to the desorption kinetics than are the signals arising from direct detection of
desorbing CO. Desorption rates are reported for 0 £ qCO £ 0.22 and
440 K £ Ts £ 555 K. The He diffraction measurements revealed that the CO overlayer was disordered
for all conditions for which kinetics were measured, and that the sticking coeff.
varied with coverage as S0 (1-3q). At
least a second order expansion of the chem.
potential in terms of CO coverage was needed to explain these rates. The measured
He diffraction data, sticking coeff., specular He scattering attenuation vs. CO
coverage, and increase in CO desorption rate with increasing coverage imply
nearest-neighbor repulsive interactions. The exptl. detd. desorption rates can
fit equally well by placing the coverage dependence in either the pre-exponential
factor or in the activation energy. The use of time-resolved specular He scattering
for studying coverage dependent reactions is also discussed.
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In this article we discuss how pseudorandom sequences are generated for use in cross-correlation
modulation experiments and present means for generating all pseudorandom sequences (modulo-two) that have
a maximum length of N=2n–1, with n=2–12. We explain the criteria that the pseudorandom sequences
must satisfy, and find the set of recursion coefficients which are used to generate the pseudorandom sequences.
These sets of recursion coefficients were calculated for n=2–16, with n=2–12 being explicitly presented in this
article. We also explain how each set of recursion coefficients can be used to generate maximum length
pseudorandom sequences of length sufficient for use in cross-correlation chopping.
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Inelastic helium atom scattering was used to measure the surface
phonon dispersion
curves for the (001) face of the ordered phase of Cu3Au along
the [100] (i.e., G-M') direction. The authors
report the spectroscopic
observation of two surface phonon modes on this fcc. alloy, and present a detailed
description of the scattering instrument that was used for making these measurements.
The lower-energy surface phonon mode, the Rayleigh wave, has an energy of 7.1 ± 0.5
meV at M'. The higher-lying feature is an optical mode with an energy of
12.5 ± 1.0 meV, which shows little dispersion across the surface Brillouin zone.
This phonon mode might be interpreted as a folded Rayleigh mode. The exptl.
measured dispersion curves do not agree with those generated by a lattice dynamical
slab calcn. which uses a pair potential force-field that successfully models the
bulk vibrations of the ordered alloy. The best fit to the authors exptl. data
indicates that the force const. between the first and second layer Cu atoms needs
to be stiffened by ~20% with respect to the corresponding bulk value.
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Temp. dependent studies were made of the surface phonon dispersion
relations for fcc. (100), (110), and (111) faces by using mol. dynamics
(MD) simulations and Lennard-Jones potentials to establish the influence of anharmonic
potential terms on the dynamical properties of the surface. This was accomplished
by examg. the temp. dependence of the Q-resolved phonon spectral d. function.
All phonon frequencies decrease linearly as the temp. increases, while at low temps.
the phonon linewidths increase linearly with increasing temp. At higher temps.,
some of the phonon linewidths exhibit a change from a linear to a quadratic
dependence on temp. The temp. at which this T to T2 change occurs is surface
dependent and occurs at the lowest temp. on the (110) surface. The T2 dependence
arises from the increasing importance of higher-order phonon-phonon scattering
terms. The phonons which exhibit a T2 dependence tend to be modes which propagate
perpendicularly or nearly perpendicularly to the direction of max.
root-mean-squared displacement (RMSD). This is esp. true for the linewidth
of the S1 mode at X on the (110) surface where, at T » 15-23% of the
melting temp., the RMSD perpendicular to the at. rows become larger than the RMSD
normal to the surface. These results indicate that the dynamics on the (110)
surface may be influenced significantly by anharmonic potential terms at
T ³ 15%
of the melting temp.
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The surface phonon dispersion curves were calcd. for fcc. (100),
(110), and (111) surfaces by using mol. dynamics (MD) simulations and
Lennard-Jones pair
potentials. In the low-temp. limit, these MD simulations are compared to the
results from slab-technique lattice dynamics calcns. of the type pioneered by
R. Allen, G. Alldredge, and F. de Wette(1971). Comparison of the dispersion
results between these 2 methods serves as a prelude to MD studies of the dispersion
curves at elevated temps. At temps. where the dynamical behavior is well
described within the harmonic approxn., the techniques should provide equal
descriptions of surface phonon spectral densities and phonon frequencies.
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The desorption kinetics (assuming quasi-equil. throughout the
desorption process)
were calcd. for a triangular lattice gas with up to 3rd-nearest neighbor
interactions (e.g., between CO mols. adsorbed on Rh(111)). These calcns.
are compared with linearized and essentially isosteric kinetic data that were
obtained by using a 3 mol. beam scattering arrangement. The exptl. desorption
rates and ordered adsorbate structure data, including phase transition temps.,
are reproduced accurately by transfer matrix calcns. Such calcns. give quant.
values for inter-adsorbate interactions extending out to 3rd-nearest neighbor
distances. Std. Monte Carlo simulations qual. show the correct trends in the
coverage dependent rate data, but are quant. inadequate for this system since
the wrong coverage dependence of the sticking coeff. is assumed implicitly.
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New multiple molecular beam scattering techniques are reviewed with 45
refs. Their use in measuring the coverage dependent kinetics of CO
desorption from and CO oxidation on Rh(111) is described.
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The surface phonon spectral d. functions were calcd. for the (100),
(110), and
(111) surfaces of Ni and Cu by using Finnis-Sinclair (FS) potentials in MD
simulations. The simulated phonon spectral densities are compared to the exptl.
inelastic He atom scattering and HREELS data which are available for the 3
basal faces of Ni and Cu. The overall shape of the calcd. surface and 2nd
layer phonon spectral densities qual. reproduce those obtained from force const.
fits (i.e., lattice dynamical modeling) of the exptl. phonon dispersion data.
Good agreement is also found between the calcd. and exptl. geometric sepns. between
the surface and 2nd layer for a given interface. However, on all surfaces, the
phonon frequencies calcd. with Finnis-Sinclair potentials are lower than the exptl.
measured values. The best agreement between the calcd. results and the exptl.
measured phonon frequencies was obsd. for the (100) and (110) surfaces, while the
poorest agreement was obsd. for the (111) surfaces. Apparently, Finnis-Sinclair
model potentials derived from bulk properties systematically underestimate the many
body binding potential at the surface. This underestn. of the many body binding
term is also manifested in the magnitude of the calcd. surface stress. The
Finnis-Sinclair model potentials are quite adequate for a good qual. and semi-quant.
description of the bonding changes at the surfaces of Ni and Cu.
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This paper examines how the formation of a transient neg. ion during
the scattering
of an electron from CO chemisorbed on Ni(111) can lead to significant enhancement
in the probability for vibrationally inelastic scattering. The authors
specifically report on the incident energy dependence for transient neg. ion
formation for a c(4x2) overlayer. The signature for this resonance is the energy
dependence of the probability for vibrationally inelastic scattering from the CO
intramol. stretch and a CO frustrated rotation, both broadly peaking in the
vicinity of 18 eV. Addnl. support for this scattering mechanism comes from the
monotonic rise in towards the surface normal under otherwise fixed kinematic
conditions. They also observe the presence of weak first overtone scattering for
the CO intramol. stretch under resonant of a S shape resonance which is slightly
lower in energy, and has a shortened lifetime, than in the gas phase. Dispersion
measurements along the [11-2] direction are presented for the c(4x2)
structure, as well as for a satd.
(7/2x7/2) R19.1° CO/Ni(111)
overlayer which
give information about intermol. couplings in these compressed structures.
Discussions are presented, including wave packet arguments, which emphasize that
the presence (or absence) of vibrational excitation in a given vibrational
coordinate following neg. ion formation can be used to infer important details
about femtosecond nuclear coordinate evolution for the system in the excited state.
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Mol. dynamics simulations were performed for Ni (110) and Cu (110) by
using
a Finnis-Sinclair model potential. During the simulations the temp. dependencies
of the mean-square displacements (MSD), the layer-by-layer stress tensors, and
the surface phonon spectral densities were measured. A more pronounced increase
in the MSD perpendicular to the at. rows was obsd. as the temp. was increased as
compared to either the other in-plane direction or along the surface normal.
Also, at each temp. studied, the MSD along the direction normal to the surface
were always larger in the 2nd layer than in the 1st. The authors' calcns. reveal
that the surface phonon frequencies all decrease linearly with increasing temp.
Moreover, the surface phonon linewidths increase linearly with temp. at low
temp., and then exhibit an increased sensitivity to temp. variation, changing
from a T to T2 dependence, ~ 150° before the onset of defect
creation at the surface. These simulation results imply that the Ni
(110) and Cu (110) surfaces do not roughen extensively before the onset of adatom-defect formation,
and, in confirmation of exptl. results, that the rapid decrease of specular
intensity for He or electron scattering at elevated temps. is due to the influence
of anharmonicity in the surface potential.
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The authors have mapped the salient surface phonon features of the
p(2x2)O/Ni (111) system along the G-M' direction of the
surface Brillouin zone by using inelastic electron scattering. Because of the diffuse scattering
properties of the system, the authors have developed a max. entropy deconvolution
routine to ext. enhanced spectroscopic information from the data. With this
routine, a set of exptl. dispersion curves were obtained successfully for the
O/Ni system. The authors addnl. have developed a lattice dynamic model of the
system and used spectral d. curves from this to produce theor. dispersion curves
for comparison to the exptl. generated curves. From these comparisons, the
authors conclude that the bonding interactions in the topmost Ni layers are
described well by a scaling relation which relates intermetallic force consts.
and bond length, and that the various bond lengths present in the surface region
can be referenced to a single force const. description of the bulk Ni-Ni
interaction. The surface force field derived in this way for p(2x2)O/Ni (111)
differs significantly from that of the clean Ni(111) interface.
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The authors describe a series of beam-surface scattering expts. which
examine the internal and translational energy dependence of the mol.
condensation probabilities for CCl4 or SF6 colliding
with their resp.
condensed phases. Thermal excitation of polyat. mol. rotational and
vibrational degrees of freedom is shown conclusively to inhibit the
probability of sticking on impact with a cryogenically cooled surface.
This effect is most pronounced in the limit of low incident kinetic
energy, and essentially vanishes at higher velocities. As part of these
expts., the authors have also obtained the angular and velocity
distributions for reflected SF6 and Kr which were used to
examine the
energy and momentum exchange of these gases with their resp. condensed
phases. These results suggest that heterogeneous laser isotope sepn.
schemes based on precollision mol. excitation may warrant further study.
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Time-correlated single-photon counting is used to measure the
lifetimes of the 6p 2P1/2 and 6p
2P3/2 levels in at. Cs with
accuracies » 0.2-0.3%.
A high-repetition-rate, femtosecond, self-mode-locked Ti:sapphire laser is
used to excite Cs produced in a well-collimated at. beam. The time interval
between the excitation pulse and the arrival of a fluorescence photon is
measured repetitively until the desired statistics are obtained. The
lifetime results are 34.75(7) and 30.41(10) ns for the 6p 3P1/2 and 6p 2P3/2 levels, resp. These lifetimes fall between those extd. from ab initio
many-body perturbation-theory calcns. by Blundell, Johnson, and Sapirstein
(1991) and V. A. Dzuba et al. (1989) and are in all cases within 0.9% of
the calcd. values. The measurement errors are dominated by systematic
effects, and methods to alleviate these and to approach an accuracy of 0.1%
are discussed. The technique is a viable alternative to the fast-beam laser
approaching for measuring lifetimes with extreme accuracy.
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The adsorption of O on the Rh (111) surface (using O2, NO,
and NO2 as O sources) was studied by using thermal desorption of O2, He
diffraction, and time-resolved specular He scattering. At all surface temps.
(TS), the surface coverage of O sats. with qO(sat) = 0.5
ML (monolayers). At TS > 375 K, addnl.
subsurface O is absorbed. The subsurface O will segregate to and desorb from
the Rh (111) surface at TS > 650 K. The rate of subsurface
deposition varies with the source of O, with NO2 and NO > O2. For
absorption of O, Ea = 4.3 ± 0.7 kcal/mol. The difference in enthalpy between the surface and
subsurface O is 4.3 ± 0.3 kcal/mol. For qO < 0.15 ML, O2 desorption
occurs with 2nd order kinetics, with Ea = 56 ± 2 kcal/mol. The shape
of O2 desorption
peaks at total (surface + subsurface) O ~ 0.5 ML is independent of the
source of O, surface or subsurface. Rate anal. indicates that the interat.
interactions between coadsorbed O species are approx. the same magnitude as
the interactions between adsorbed and absorbed O species.
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EELS was used to map the dispersion of the dipole active internal NO
stretch and of the NO frustrated translation (which has not been previously obsd.)
in the c(4x2)NO/Ni (111) system. The dispersion of the dipole active mode
was fitted to a model that assumes electrostatic dipole-dipole coupling
(including image dipoles) between the adsorbates. However, the frustrated
translation showed no dispersion to within the resoln. of the expt. across
the entire surface Brillouin zone of the Ni (111) substrate. These
measurements reveal new information on interadsorbate interactions in an
important model system.
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Electron beams from 5 eV to 2 keV stimulate facile nickel oxide growth
on Ni(111) at 120 K, and that oxidn. occurs extremely slowly at low temps.
when electron irradn. is absent. A model is proposed which quant. accounts
for the data and yields relevant cross sections. These findings are of
fundamental importance to metal oxidn. and corrosion, and may find application
in electron beam and STM lithog.
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Inelastic helium atom scattering has been used to measure the phonons
on a stepped metallic cryst. surface, Ni(977). When the scattering plane is
oriented parallel to the step edges and perpendicular to the terraces, two
branches of step-induced phonons are obsd. These branches are identified
as transversely polarized, step-localized modes that propagate along the
step edge. Anal. reveals significant anisotropy in the force field near
the step edge, with all forces near the step edge being substantially
smaller than in the bulk. Such measurements provide valuable information
on metallic bonding and interface stability near extended surface defects.
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Inelastic helium atom scattering has been used to measure the surface
and step localized phonons on a stepped metallic surface, Ni(977). These
time-of-flight measurements were carried out both perpendicular and
parallel to the step direction. Surface phonon dispersion data collected
across the steps show backfolding of the surface Rayleigh mode, and,
most importantly, dramatic softening as compared to the forces present
at the smooth Ni(111) surface. This softening suggests significant
relaxation perpendicular to the step edge. Single-phonon scattering data
collected along the step direction reveals the presence of two new
step-edge localized modes, as well as the Rayleigh mode for this direction
of the crystal. The Rayleigh mode here does not exhibit the notable
softening that was found for the other direction. Novel in-
and out-of-phase scattering measurements, with respect to the terraces,
lead us to assign the new step induced modes as the two transversely
polarized vibrations which propagate along the direction for the step edge.
An analytic one-dimensional lattice model is proposed which well represents
the dispersion data for these two step modes; its use allows us to det. the
effective local force field in the two transverse directions with respect
to the step edge. The findings reported herein shed new light
on such topics as interface stability, crystal growth, and charge
redistribution in the vicinity of well-characterized extended surface defects.
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O adsorption and oxide growth on Ni(111) was studied at 120 K by high resolution
EELS (HREELS). The authors find that an electron beam can stimulate Ni oxide growth
at all incident electron energies examined, spanning the range from 5 eV to 2 keV.
When electron irradiation is absent, oxidation occurs extremely slowly on this surface
at low temperatures, resulting in mainly chemisorbed O. The authors demonstrate that
HREELS is capable of simultaneously monitoring oxide growth and characterizing the chemical
nature of the O/Ni interface, providing a useful complement to the earlier Auger
spectroscopy based study of electron stimulated oxidation of this interface. The authors
propose a model for the observed effect in which electrons create oxide nucleation centers
on the Ni(111) surface in the presence of chemisorbed O. This model accounts quantitatively
for the data, including extension of the relevant cross sections.
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The effect of surface temp. on the rate of oxidn. of the Ni(111)
surface with and without electron irradn. has been detd. for temps.
between 120 and 340 K. The oxidn. rate in the presence of an electron
beam demonstrates an inverse dependence on the substrate temp., while without
an electron beam we observe a decrease in oxidn. rate with decreasing
substrate temp., decreasing almost to zero at 120 K. Similar rates are
obsd. near room temp. for the two cases. We have found that oxidn. of
this surface can be well described by either of two rate expressions: one
that relates the oxide growth rate to the rate of lateral growth of two
dimensional oxide islands, and another that is first order in oxide and
oxygen coverages at the surface. The phys. implications of each model
are discussed in terms of the nucleation sites created by the electron
beam, and the rate consts. for oxidn. at these nucleation sites. The
authors present evidence that the nucleation sites created by the
electron beam are metastable, with an unusually long half-life of about
600±150 s. The authors have also investigated the dependence of the
cross section for nucleation center creation as a function of incident
electron energy at const. electron flux and const. oxygen exposure. The
energy dependencies of the cross section for nucleation center creation
and the yield of secondary electrons produced by irradn. from the
incident electron beam are compared, leading to consideration of the role
that such secondary electrons may have in the creation of nucleation
centers. The results presented herein delineate the correct low temp.
oxidn. kinetics for Ni(111) in the absence of perturbing electrons. They
also provide a cautionary note for expts. which use electron-based
probes, or optical probes which generate intense swarms of electrons, for
studying the oxidn. kinetics of metals, and perhaps other classes of
interfacial reactions.
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The velocity and angular distributions of CO2 produced by
CO oxidn. on Rh(111) have been measured as a function of surface temp. and oxygen
coverage. Both the velocity and angular distributions are bimodal. The
velocities of one component are well fit by a Maxwell-Boltzmann
distribution at the surface temp., and the angular distribution of its
intensity is cosine. The second component is non-Boltzmann, and the
angular distribution is sharply peaked toward normal. The av. energy of
this feature is a very strong function of the surface temp., increasing
with a slope of 8.7kb, where kb is the Boltzmann
const., between 475 K
and 700 K. Surprisal anal. proves useful in condensing and interpreting
these data.
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The translational energy distribution for the H2O product
from the reaction of H2 and O2 on Rh(111) was
measured as a function of surface temp. at two different oxygen coverages. The results are well
represented by Maxwell-Boltzmann velocity distributions significantly
cooler than the surface temp. For [O] = 0.2 monolayers (ML), the product
H2O is slightly faster than for [O] = 0.1 ML. The energy
distributions are very close to those obsd. for the trapped and desorbed mols. when
scattering low energy H2O mol. beams from the Rh(111). We also
measured the angular dependence of the energy and intensity of the product H2O at
Ts = 650 K. The velocity distribution of the H2O
product is independent of final angle, and the relative intensities are cosine distributed.
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Oxygen can exist either on the surface or in the sub-surface region of
Rh(111). There are clearly different thermal desorption signatures for
these two states. In this Letter, we report on expts. examg. the
dynamical paths for oxygen recombination and desorption arising from the
two states by measuring the velocity distributions of the desorbing
O2.
The results are well represented by identical Maxwell-Boltzmann velocity
distributions, significantly hotter than the surface temp. We conclude
that surface and sub-surface oxygen share a common dynamical state prior
to desorption.
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He diffraction and specular He scattering measurements of O on Rh(111) were
made for TS = 125-625 K and qO = 0.0-0.50 ML.
For TS > 450 K and qO < 0.21
ML, specular He scattering from the O/Rh(111) overlayer is well described by a model for
disordered adsorbates, with a cross section of 62.6 Å2 for 63 meV He.
At higher coverages, these measurements reveal a complex relationship between coverage,
temperature, and the ordered overlayer structures for O/Rh(111). In addition, adsorption
isotherms presented here show second order Langmuir adsorption kinetics for O2 on Rh(111) and a sticking coefficient of about 5% for O2 on 525 K Rh(111).
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The velocity and angular distributions of N produced from the redn. of
NO by H on Rh(III) were measured in the low N coverage limit as a function
of surface temp. The angular and velocity distributions are well fit by
bimodal forms. The high energy channel has av. translational energies
~6 times that expected for mols. accommodated at the surface temp.,
an unusually sharp angular distribution, and angle dependent velocity
distributions. The low energy channel is also hyperthermal, with av.
translational energies about twice thermal, a cosine angular
distribution, and velocity distributions which are independent of angle.
Application of surprisal anal. to the data shows that the high energy
channel may be characterized by constraints on the normal velocity and
the total energy; the low energy channel may be characterized by a single
constraint on the velocity.
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He atom diffraction was used to study the reconstruction kinetics of a
stepped metallic surface (Ni (977)) which sequentially undergoes
step-doubling and -singling upon dosing with low coverages of O. At
390-470 K, < 2% O monolayer was sufficient to transform the initially
prepd. single-stepped surface to a new steady state having a
double-stepped structure. The thermal range over which the doubled phase
exists extends to higher temps. when more O is present. At low O
exposures, this doubled interface reverts to the single-stepped surface
at > 470 K. Singling can also be driven by more extensive levels of O
adsorption. The kinetics of the step-doubling transformation which
occurs at < 470 K is 2nd order with respect to single-step d. while, for
temps. > 470 K, step-singling follows 1st-order kinetics with respect to
the double-step d. The O atoms adsorbed at step edges play a crucial
role in these transformations. Arrhenius anal. was used to ext.
energetics for the step-doubling and -singling reconstructions. These
results delineate the sequence of mechanistic stages which occur during
the initial stages of oxidn. of a stepped metallic interface and which
precede the onset of bulk oxidn. (results which are important for
developing an improved understanding of metallic oxidn. and corrosion).
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Coverage-dependent sticking probabilities and 2nd-order rate consts.
for recombinative desorption of H from Rh (111) were measured by using
mol. beam relaxation spectroscopy (MBRS) and time-resolved specular He
scattering. The sticking probability follows a 2nd order Langmuir
coverage dependence, with s0 equal to 0.01 ± 0.005. Under
isothermal and nearly isosteric conditions over the coverage range 0.2-0.7 ML, the 2nd
order rate const. for desorption essentially is independent of H coverage
(in contrast to kinetic parameters detd. from thermal desorption
spectra).
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The fabrication of silicon nitride membrane substrates and their use
in studies of polymer thin films are described. As an integral part of a
wafer, these membranes are both self-supporting and transparent for
transmission electron microscopy (TEM). Therefore, the same polymer film
can be spin-cast on the substrate and, without being removed, studied by a
variety of techniques, including TEM, and atomic force microscopy (AFM).
to demonstrate the utility of these substrates in characterizing both
global and local film morphology, experimental results are presented on
polystyrene-polymethylmethacrylate diblock copolymers in the ultrathin
film limit, using optical microscopy together with combinations of AFM and
TEM at the same location. The addition of microfabricated structures to
these substrates, such as planar electrodes is also discussed.
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We track individual defects in the microdomain pattern of
cylinder-forming polystyrene-block-polymethylmethacrylate films with atomic force
microscopy to elucidate the evolution of diblock domain topology during
annealing. This evolution takes place through relinking, joining,
clustering, and annihilation of defects. Such processes form the basis for
predicting structural change in polymer films.
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The multiphonon energy exchange between a neutral He atom and a
stepped Ni(977) surface has been measured in order to examine how the
presence of a regular array of atomic-scale steps on a surface modifies energy exchange in the
classical multiphonon scattering regime. At elevated substrate
temperatures, we compare the multiphonon scattering with the predictions of a classical theory that has
previously been used by others for assessing energy exchange involving a
smooth surface. There is a significant discrepancy between the theoretical predictions and
our experimental data, which we attribute to differences between a smooth
and stepped surface. Specifically, changes in the vibrational modes and associated
surface density of states due to the presence of extended surface defects
have a fundamental impact on the details of the energy exchange mechanism.
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The oxidation kinetics of the Ni(111) surface have been quantitatively
examined utilizing kinetic energy selected supersonic beams of molecular
oxygen. Using in situ high-resolution electron-energy-loss
spectroscopy, we have observed notable differences in the oxidation
mechanism for this interface as a function of incident beam kinetic
energy. Exposure of a 300 K surface to a relatively low energy 60 meV
O2 beam leads to oxidation kinetics which follow an island
growth model, qualitatively similar to what is seen with simple ambient
has dosing. In contrast to this, exposure to a relatively high energy 600
meV O2 beam yielded funamentally different oxidation "High density adsorbed oxygen on Rh(111) and
enhanced routes to metallic oxidation using atomic oxygen"
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Exposure of Rh(111) to atomic oxygen leads to the facile formation of
a full-coverage and ordered (1x1)-O monolayer which is stable at room
temperature. This result differs markedly from the half-coverage (2x1)-O
overlayer which forms at saturation when using molecular oxygen. This
demonstrates that kinetic rather than thermodynamic constraints inhibit
the formation of dense oxygen overlayers when O2 is the
oxidant. We also report that O absorption into the bulk proceeds much
more readily when using O rather than O2, a finding with direct
implications for enhanced methods of low-temperature metallic oxidation.
These results demonstrate that there are important fundamental differences
in the way in which low-energy beams of atomic and molecular oxygen
interact with metals.
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The oxidation of the Ni(111) surface by a supersonic atomic oxygen
beam has been quantitatively studied using in situ high-resolution
electron-energy-loss spectroscopy. For a room temperature substrate, a
drastically different oxidation rate for atomic oxygen induced oxidation
than for molecular oxygen based oxidation is observed. This rate was
found to be two orders of magnitude higher than that of molecular oxygen.
The reaction on a 110 K substrate indicated oxygen uptake only to the
chemisorption saturate with no further oxidation. This later finding
agrees with previous results for low surface temperature oxidation using
molecular oxygen, implying that the inhibiting step in low temperature Ni
oxidation is not molecular oxygen dissociation, but a more fundamental
property of the metallic substrate. The chemisorption region of the
atomic oxygen reaction was found to saturate at the same (2x2) overlayer
as results from exposure to molecular oxygen, i.e., a dense (1x1)
overlayer as has been seen on other metals does not form on Ni(111).
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We have studied the effect of an extended array of defects on the
two-dimensional phase behavior of adsorbed hydrogen on a Ni surface using
helium atom scattering. Specifically, the interaction of hydrogen with
the stepped Ni(977) surface was examined and compared with similar
interactions with the flat Ni(111) surface. The phase behavior of
hydrogen on Ni(977) is qualitatively the same as that of hydrogen on
Ni(111); however, the temperature at which the order-disorder transition
occurs is elevated. On the stepped surface, the ordered (2x2)-2H phase
exists at a temperature 40 K higher than on the flat surface. This
reversible phase transition is second order and is best fit with
Tc = 310 K and b = 0.12, indicative of
two-dimensional Ising behavior. Stabilization of the ordered phase is
attributed to pinning from the step edges. The cross section for diffuse
elastic He scattering by adsorbed hydrogen and the temperature-dependent
domain size of ordered hydrogen along the step edges are also discussed.
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In this letter we show that it is possible to guide the formation of a
novel non-close-packed xenon structure on a stepped nickel surface using
an intentionally atomically-patterned substrate. By first defining the
symmetry and desired dimensions of the underlying superlattice, we
successfully template the targeted rare gas overlayer. Such templating
effects, in which the corrugation and structure of the interface can be
tuned by prior adsorption of an adsorbate, should be a general route to
the formation of new self-organizing interfacial nanoscale structures.
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The velocity and angular distributions of NO produced from the
decomposition of NO2 on Rh(111) under both reducing and
oxidizing conditions have been measured at surface temperatures between
500 K and 1000 K. When a concurrent H2 beam is used, which
keeps the surface free of oxygen, the NO product has much more
translational energy than expected for equilibration at the surface
temperature, but is dependent on Ts. There is
total energy scaling; the translational energy is independent of final
angle. A small amount of N2 is also produced. When the
H2 beam is turned off, oxygen builds up on the surface. Under
this oxidizing condition, the NO product has a nearly Maxwell-Boltzmann
velocity distribution at the surface temperature.
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Water adsorbed on clean Rh(111) forms an ordered structure with a
(Ö3xÖ3)R30º diffraction
pattern. This is facilitated by the close match of
surface lattice constants for Rh(111) and the (0001) face of hexagonal
ice, Ih. The pre-adsorption of small quantities of disordered
oxygen improves the long-range ordering of the water overlayer. When a
well-ordered half monolayer of oxygen is grown on the Rh(111) prior to
H2O exposure, there is no evidence of any long-range ordering
of the water. However, when H2O is adsorbed on a
(1x1)-O/Rh(111) surface, where there is a well-ordered monolayer of
adsorbed oxygen, the adsorbed H2O forms an entirely new
high-density structure exhibiting a (1x1) diffraction pattern. The
adsorbed H2O structure is epitaxial with respect to the
underlying oxygen and rhodium. This structure persists for many layers of
adsorbed water. On the clean Rh(111) surface, water molecules are adsorbed
through the oxygen lone pair orbital. When the surface is fully covered
with oxygen, the first layer of water can hydrogen bond to the surface, i.
e., they likely adsorb with one or both of the hydrogen atoms pointing
towards the surface. This creates a template for a novel new structure
which forms at low pressure, producing a high-density crystalline form of
ice. The discovery of this structure suggests that other molecules,
especially those that hydrogen bond, may form entirely new structures on
metals covered with a high-density oxygen overlayer, with associated
consequences for interfacial chemistry.
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In this paper, we elaborate on our previous communication of high
coverages of oxygen on Rh(111) (J. Chem. Phys. 110, 2757 (1999)). When
dosing with O2, half of a monolayer of O is adsorbed. Higher
coverages can be achieved when exposing the surface to O atoms. As the
quantity of adsorbed O increases from a half to a full monolayer, the
overlayer structure undergoes several distinct phase changes. At a full
monolayer, the (1x1)-O structure is stable at surface temperatures less
than ~400 K. Continued dosing with O atoms results in the rapid migration
of O into the bulk. We also report on the chemical reactivity of this
densely oxygen-covered surface with CO, H2, and propene.
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The design and application of a radiant heater assembly for elevated
temperature scanning tunneling microscopy (STM) in ultra-high vacuum (UHV)
is presented. The proximity heater is a non-invasive modification to an
existing commercial room-temperature microscope and is capable of
radiatively heating samples up to 650 K in situ. Imaging at higher
temperatures should be readily accessible with other microscope
construction designs. It is demonstrated that this heater is well suited
for enabling an STM to capture surface morphological transformations such
as the motion of atomic steps on metal surfaces at elevated temperature.
Various design issues and solutions related to variable temperature
UHV-STM are also discussed. We believe the approach described to be
general in nature, offering a direct route to adapting UHV-STM designs for
elevated temperature imaging.
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The downstream composition of a skimmed supersonic binary molecular
beam originally consisting of a 20% neon/80% xenon mixture before
expansion has been studied as a function of nozzle stagnation pressure.
We have found that the neon to xenon ratio dropped dramatically as the
stagnation pressure was increased at low nozzle temperature (303 K), a
drop which cannot be well described by existing theory. Time-of-flight
(TOF) measurements indicate that Xe clustering occurs as the stagnation
pressure is increased. This clustering coincides with the additional Ne
depletion we observe. At a higher nozzle temperature where Xe clustering
does not occur (573 K), this measured mass separation phenomenon is
absent. Similar experiments have been done for another binary mixture, 20%
O2/80% Xe. Similar anomalous mass separation is observed with
this mixture, confirming the attribution of this phenomenon to clustering
of the more massive component of the mixture. These findings have
implications for novel methods of gas-dynamics-based mass separation
potentially including isotope enrichment.
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The heterogeneous combustion of benzene on Rh(111) has been examined
using molecular beams in an ultra-high vacuum environment. For the
reaction conditions studied, CO is the dominant carbon-containing product.
CO2 is a minor component, accounting for a maximum of 10% of
the carbon species at 650 K and dropping to 2% when the temperature is
raised to 1000 K. The relative yields of CO and CO2, as well
as the reaction rate for CO production, are strongly influenced by surface
oxygen concentration, controlled though the relative ratio of oxygen and
benzene fluxes, with the fastest rate of CO production and the greatest
proportion of CO2 occurring under the most oxidizing
conditions. Because the catalytic decomposition of benzene is rapid on the
rhodium surface, the kinetics of CO and CO2 evolution are
dominated by the reaction of atomic carbon and oxygen species on the
surface. We calculate an activation energy for the reaction C(a) + O(a) ® CO(a) of 130 kJ/mol. CO2 is produced
by the further reaction of CO with adsorbed O, with the extent of reaction
being substantially influenced by reaction at defect sites. CO is evolved
with a thermal kinetic energy distribution, while CO2 desorbs
hyperthermally.
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We present a novel and simple method for generating micron scale annular
structures formed from polystyrene-b-polymethylmethacrylate (PS-b-PMMA)
diblock copolymer on a silicon oxide substrate. This method is based on
pre-wetting of the underlying substrate with a selective solvent before
spin-casting of the diblock solution. When using this procedure we also
see, using atomic force microscopy, a unique alignment effect which occurs
in the cylinder-forming microdomains of PS-b-PMMA annuli without the aid
of an external alignment field. These aligned microdomains, with
controlled nanometer scale spacing and coherence on the order of microns,
facilitate the ongoing exploration of self-organizing nanofabricated
surfaces.
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In this paper we demonstrate that externally applied tensile and compressive
stresses can systematically modify the electrochemical surface reactivity of pure
and alloyed metals. Atomic force microscopy is used to statistically characterize
the extent and nature of interface change for nickel and aluminum alloy 2024-T3
subjected to electrochemical conditions under various levels of stress. Statistical
analysis of AFM images reveals that the extent of electrochemical reactivity is
significantly enhanced when subjecting the sample to tensile as opposed to
compressive stress; this enhancement increases monotonically as the level of applied
stress is systematically increased. Surface morphologies differ on the pure nickel
and alloyed aluminum samples, with the nickel interfaces exhibiting facetted features
which are aligned 120° from one another while the surface features on aluminum alloy
2024-T3 are circular pores. These results unambiguously indicate that the kinetics
for electrochemical metallic processes, which nucleate at surface defects and grain
boundaries, can be significantly modified by the presence of external stress fields.
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Novel cell surface topography was revealed on cocci from a glycopeptide-intermediate Staphylococcus aureus (GISA) clinical strain by using atomic force microscopy (AFM).
The GISA isolate and its revertant had two parallel circumferential surface rings. One
equatorial surface ring was observed in control strains. In vancomycin-susceptible
strains, additional rings were formed in the presence of vancomycin. Ring depth
measurements also revealed striking differences between the GISA strain and susceptible
strains grown with or without vancomycin.
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Time-sequenced atomic force microscopy (AFM) studies of ultrathin films of
cylinder-forming polystyrene-block-polymethylmethacrylate (PS-b-PMMA) copolymer are
presented which delineate thin film mobility kinetics and the morphological changes
which occur in microphase-separated films as a function of annealing temperature.
Of particular interest are defect mobilities in the single layer (L thick) region,
as well as the interfacial morphological changes which occur between L thick and
adjacent 3L/2 thick layers, i.e., structural changes which occur during multi-layer
evolution. These measurements have revealed the dominant pathways by which
disclinations and dislocations transform, annihilate, and topologically evolve during
thermal annealing of such films. Mathematical combining equations are given to
better explain such defect transformations and show the topological outcomes which
result from defect-defect encounters. We also report a collective, Arrhenius-type
flow of defects in localized L thick regions of the film; these are characterized by
an activation energy of 377 kJ/mol. These measurements represent the first direct
investigation of time-lapse interfacial morphological changes including associated
defect evolution pathways for polymeric ultrathin films. Such observations will
facilitate a more thorough and predictive understanding of diblock copolymer thin
film dynamics, which in turn will further enable the utilization of these nanoscale
phase-separated materials in a range of physical and chemical applications.
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The low-energy surface vibrational structure of the (11.5´Ö3)
striped phase of 1-decanethiol (C10H21SH) chemisorbed on a
reconstructed Au(111) surface has been studied using a high momentum- and energy-resolution
helium atom scattering apparatus. Energy-transfer spectra for this system exhibit a
dispersionless inelastic feature at 8 meV. We assign this to the frustrated translation
of the entire molecule vibrating with polarization perpendicular to the surface. These
results are in contrast to the absence of inelastic peaks in the time-of-flight spectra of
ordered shorter chain alkanethiols chemisorbed on copper surfaces. Differences in the
phonon spectra between these systems are attributed to the longer chain length, the chain
orientation, and the weaker S-Au interaction. These results further the understanding of
the forces that govern nanoscale self organization.
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Time-lapse scanning tunneling microscopy (STM) has been used to observe the
oxygen induced reconstruction behavior of Ni(977), a stepped metallic surface.
Previous studies using helium atom diffraction resolved the macroscopic kinetics for
the reversible step-doubling and -singling of this vicinal surface. Sequential STM
imaging recorded at elevated temperature has now elucidated atomic-level mechanistic
details for the merging of steps in the presence of small amounts of adsorbed oxygen,
less than 2% of a monolayer. Point contact between neighboring steps decorated with
chemisorbed oxygen facilitates rapid step coalescence by means of zippering. An
optimal oxygen concentration of step edge saturation was found to enable the step
merging to proceed most rapidly. Excess oxygen was found to hinder the coalescence
of neighboring steps through the possible growth of overlayer structures on the
terraces. At sufficiently high temperatures, the surface is driven back to single
steps due to oxygen dissolution. The departure of oxygen from the surface through
dissolution, as well as the associated presence of oxygen in the selvedge region,
may both play a role in destabilizing the double steps. Local step density
influences the coalescence behavior by defining the number of available step edge
sites. The microscopic details made available by time-resolved STM imaging
illuminate some of the mechanistic steps related to the initial stages of metallic
oxidation, and the sensitivity of surface morphological transformations to local
surface structure and adsorbate coverage.
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Step dynamics induced by the chemisorption of oxygen on Ni(977) at low coverages
and elevated temperatures have been studied using scanning tunneling microscopy
(STM). Previous experiments using both local probe and scattering techniques have
assessed the surface reconstruction behavior and structural transformations for this
vicinal system. At temperatures above 390 K, the surface is capable of undergoing
step doubling or merging if exposed to small amounts of oxygen. These double steps
exist up to 565 K where step-adsorbed oxygen dissolves into the nickel lattice
destabilizing this morphology. Real-time STM measurements have been made on the
behavior of individual merging events as a function of local step density, and hence
local oxygen concentration at step edges, at 465 K. Results indicate that there is
an optimal oxygen coverage, corresponding to complete titration of the single step
density, that enables fast step merging to occur. An areal sweep rate of ~60
Å2 s-1 was found for step doubling under these conditions.
For oxygen coverages greater than the single step density in which four adjacent
single steps are embedded in an otherwise doubled local environment, step merge
motion was punctuated in time. We attribute this observation to local energetics,
in which specific structural fluctuations including adsorbate step decoration and
local step and kink configurations enable the doubling transition. Moreover, under
these same conditions, strong spatial and temporal correlations were observed for
the coalescence of adjacent pairs of steps. These time-lapse STM studies advance
our understanding of the atomic-level mechanisms which contribute to the initial
stages of oxidation and facetting for metallic surfaces.
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We have studied the effect of an extended array of steps on the two-dimensional
phase behavior of chemisorbed oxygen overlayers on a vicinal nickel surface using low
energy electron diffraction (LEED), Auger electron spectroscopy (AES), and scanning
tunneling microscopy (STM). Phase behavior of oxygen on the vicinal Ni(977) surface
was examined and compared with that for oxygen adsorbed on the flat Ni(111) surface.
There are two significant differences in the phase diagrams for these two surfaces.
On Ni(111) at q = 0.25 ML, oxygen forms a
p(2´2) structure that disorders to a lattice gas at 440 K
and remains disordered until it is ultimately dissolved into the bulk above 500 K.
Surface defects, such as the steps on Ni(977), substantially modify this phase
progression. On Ni(977), the p(2´2) phase still disorders
at 440 K, but a second ordered phase, which can be designated as
Ni[8(111)´(100)]-2(1d)-O in microfacet notation, exists
between room temperature and above 500 K when the oxygen is finally incorporated into
the bulk. This adsorbate phase is step-stabilized and can be generated by dosing the
surface with a small amount of oxygen or as a result of partial dissolution of oxygen
from the higher coverage p(2´2) phase. Moreover, anisotropic
disordering effects are evident due to the presence of the steps as indicated by the
increasingly oblate shape of diffraction spots as the p(2´2)
disorders. The process of oxygen dissolution is also qualitatively altered by the
presence of regular steps.
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Scanning tunneling microscopy (STM) has been used to observe oxygen induced microfaceting
of Ni(977) in the temperature range of 390-565 K. Step doubling occurs on this surface
provided the step-edges are locally decorated with oxygen. In this Letter, time-lapse images
of this process have been used to resolve two key steps of the coalescence mechanism.
Merging of steps is initiated by the bulging of one step in the downstairs direction towards
its neighbor. This rate limiting step is followed by the second mechanistic process, namely
zippering of adjacent steps. Merging step contact angles have been analyzed to extract
information on the energetics of step-step interactions. These results give a real-space
view of the atomic-level surface structural changes which accompany the initial stages of
metallic oxidation of interfaces containing extended surface defects.
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In order to make effective use of the extreme density of nanoscale elements that form
spontaneously in self-assembling architectures, one must address the associated issue of
minimizing defect creation during the formation of such structures. In this paper we
examine the competing roles that nucleation kinetics and two-dimensional growth processes
play in nanostructure formation and defect minimization. We employ oxygen-induced step
doubling of vicinal Ni(977) surfaces as our physical system, using elevated temperature
scanning tunnelling microscopy and Monte Carlo simulations to extract the desired details
of interface evolution. Two interesting topological defect features are observed on the
surface after doubling reaches its asymptotic limit: (i) "frustrated ends," which form when
two counter-propagating step-doubling events that have a single step in common intersect,
leaving a stable topological defect, and (ii) residual "isolated single steps," which form
when a single step is unable to partner with an adjacent step. This latter defect occurs
when a single step is surrounded on both sides by previously doubled structures. In an
attempt to understand and control these results, Monte Carlo simulations were performed
which simulated our experimental findings. These simulations indicate that experimental
control of the delicate and competing interplay of nucleation kinetics and two-dimensional
growth kinetics is the key to the formation of more perfect interfaces. In this instance
this corresponds to using a small initial oxygen exposure and reduced substrate temperature
to achieve a doubled surface of higher perfection. Such optimized interfaces can act as
templates for guiding the hierarchical assembly of nanowires and other nanoscale molecular
assemblies.
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High-quality epitaxial b-SiC films have been successfully grown on
Si(100) at substrate temperatures considerably lower than those used during conventional CVD
growth. This has been achieved using translationally-energetic and spatially-directed
methylsilane delivered via seeded supersonic molecular beams. Methylsilane kinetic energy
was found to affect dramatically both film morphology and growth behavior, as well as the
enhancement of growth efficiency in the substrate temperature range 830-1030 K. Films
obtained from thermal beams (0.079 eV) grow only through the facile mechanism involving the
reaction of out-diffused silicon atoms with precursor species, identical to the growth of
so-calleed "buffer layers" via the reactive conversion of the silicon surface. At moderately
higher kinetic energies (0.45 eV), a second growth mechanism opens which operates in addition
to the silicon out-diffusion process. Growth at the higher incident energy can grow thicker films,
i.e., is not thickness-limited, and occurs with essentially the same rates with or without a buffer
layer. The morphological evolution of films grown on bare substrates proceeds through a pitted
buffer or transitional layer which allows for the relaxation of strain due to lattice mismatch.
The continuous, void free films eventually obtained exhibit the doubly-degenerate domain structure
characteristic of cubic epitaxial material growing nearly two-dimensionally. Furthermore,
remarkable square-pyramidally shaped and azimuthally-aligned isolated three-dimensional features
are observed which grow simultaneously with the two-dimensional film. Films grown below 900 K,
though also epitaxial b-SiC, do not show these isolated three-dimensional
features, and are much rougher than films grown above 900 K. These results emphasize that new,
enhanced growth regimes for electronic materials deposition can be achieved by using high-intensity
and velocity-tuned supersonic molecular beams to deliver kinetically accelerated neutral molecules
for use as efficient growth precursors. These experiments also suggest that lower substrate thermal
ranges may, for favorable cases, become accessible for growing high-quality films when using
supersonic molecular beam epitaxy (SMBE) deposition methods.
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We have studied the effect of adsorption of a low-density alkanethiol monolayer on the state
of the Au(111) reconstruction. It is commonly believed that the substrate deconstructs
following formation of a thiolate self-assembled monolayer, but our results suggest this is
not always the case. Helium diffraction from 1-decanethiol and 1-octanethiol striped phase
monolayers is exploited to establish the surface nearest-neighbor spacing and to illustrate a
unit cell corresponding to the long dimension of the (23´Ö3)
reconstruction. Using our observed 0.198 Å-1 peak spacing and the
(11.5´Ö3) unit cell reported in the literature, we measure a substrate
nearest-neighbor spacing of 2.76 Å along the [11¯0] direction, which represents the atomic
spacing of the uniaxially-compressed, reconstructed gold surface. Moreover, ½-order peaks in
the diffraction from decanethiol/Au(111) demonstrate a distinction between neighboring thiolate
dimers. These peaks are not observed for the octanethiol/Au(111) system. Therefore, the
½-order peaks are not an inherent characteristic of alkanethiol SAMs. The most likely
explanation for these peaks is a reconstructed substrate. Complementary scanning tunneling
microscopy data are also presented that show persistence of the reconstruction during growth
of a decanethiol striped phase monolayer and no evidence for vacancy islands typically
associated with deconstruction. Our model involving a still-reconstructed substrate is
consistent with all of the available data, while alternative models fail to explain the results
presented in this article.
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We have observed that the reconstruction dynamics for stepped Ni(977) are influenced by the
oxygen dissolution history of the crystal. Using a complementary approach incorporating both
real- and reciprocal-space techniques, it is found that the upper end of the thermal range over
which this stepped metal surface transforms from single to double steps increases with selvedge
or sub-surface oxygen concentration. These results enhance our understanding of how adsorbate
dissolution, and hence oxygen exposure history, modify energetic pathways for metallic oxidation.
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We report a simple method to align lying-down cylindrical domains of PS-b-PMMA in the trough
regions of 555 nm deep silicon nitride gratings without the aid of an external orientation field. This alignment is perpendicular to the orientation of the grating lines and essentially spans the width of the grating trough. The proposed mechanism involves polymer flow and confinement in small dimensions. Also, regularly spaced bumps of almost uniform size can be formed on the crest regions of the support grating.
The thickness dependence of PS-b-PMMA thin film surface morphology is also utilized to generate
micron-scale periodic structures on 115 nm gratings. These experiments to align microdomains
with controlled nanometer-scale spacing can be extended to generate self-organizing nanofabricated
surfaces.
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In this work the initial stages of disilane Si2H6 reactive deposition on Ni(977) to form silicon nanowires have been examined using scanning tunnelling microscopy. The effects of disilane exposure and dosing rate on the length distributions of the resulting silicon nanowires are discussed. Creating such nanoscale structures using reactive deposition on stepped metal templates has potential applications in “bottom-up” nanofabrication technologies, especially in the preparation of massively parallel, aligned, and high aspect ratio structures.
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The chemical reactivity and morphological evolution of highly-
ordered pyrolytic graphite upon exposure to hyperthermal (~ 5 eV) O(3P) atoms
have been investigated. The erosion rate and surface roughness exhibit a
profound dependence on the temperature of the sample during the experiment.
Atomic force microscopy images of eroded surfaces exposed to atomic oxygen at
298 K reveal embedded cylindrical erosion pits which span nanometer through
micrometer length-scales. In contrast to this, very rough matted surfaces are
observed following exposure at a surface temperature of 493 K. The erosion
rate triples upon this increase in sample temperature. These data are compared
to those of experiment EOIM-III, collected in low-earth orbit on-board the
spacecraft Atlantis during STS-046.
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A study of the energy accommodation of neon colliding with a crystalline self-assembled 1-decanethiol monolayer adsorbed on Au(111) is presented. The intensity and velocity dependencies of the scattered neon flux as a function of incident angle and energy were experimentally measured. Scattering calculations show good agreement with these results, which allows us to examine the detailed dynamics of the energy and momentum exchange at the surface. Simulation results show that interaction times are at most a few picoseconds. Even for these short times, energy exchange with the surface, both normal and in-plane, is very rapid. An important factor in determining the efficiency of energy exchange is the location at which the neon collides with the highly corrugated and structurally dynamic unit cell. Moreover, our combined experimental and theoretical results confirm that these are truly surface collisions in that neon penetration into the organic boundary layer does not occur even for the highest incident energies explored, 560 meV.
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The energy transfer dynamics associated with Ne-atom collisions with a n-hexylthiolate self-assembled monolayer (SAM) surface are studied in chemical dynamics simulations by using both harmonic/separable and anharmonic/coupled surface models and considering a SAM in both its classical potential energy minimum and containing a 293 K energy distribution. For the anharmonic surface, excitation of higher energy potential energy minima arising from different intermolecular conformations of the alkyl chains and intramolecular vibrational energy redistribution (IVR) between the surface modes during the collision enhances energy transfer from Ne-atom translation to the surface. IVR is efficient for the alkyl chain's torsional modes and intramolecular-chain bending modes and occurs on the time-scale of the Ne-atom collision. It does not occur during the collision for other modes, such as the higher frequency intra-chain CCC bends. This IVR between surface modes during the collision increases the number of modes coupled to the Ne-atom translational motion and enhances energy transfer to the surface. Whether modes promoting or not-promoting IVR are initially excited depends on the surface site at which the Ne-atom collides. The simulation indicates the presence of these different energy transfer dynamics for different surface sites is the origin of the bimodal energy transfer distribution function P(Ef). It is suggested that a large number of surface modes and chains coupled by IVR, for collisions at some surface sites, may create a sufficiently large bath to form a Boltzmann-like component in P(Ef).
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A study of the scattering of Ar from a well-ordered standing-up phase of 1-decanethiol adsorbed on Au(111) at surface temperatures from 110-185 K is presented. The final energies and intensities were measured as a function of incident polar and azimuthal angles using incident energies from 60-600 meV. These experimental results are compared to classical trajectory calculations. Scattering shows two distinct exit channels. The higher energies are due to direct inelastic scattering and have the greatest intensities at glancing incident and final angles. The lower energy channel is due to trapping-desorption; it has a Maxwell-Boltzmann energy distribution at the surface temperature and a cosine angular intensity profile. The simulations show that the timescale for normal momentum accommodation is very fast. The parallel momentum accommodation takes slightly longer, dependent on the initial conditions, but is still complete within only a few picoseconds. The result is that much of the Ar undergoes trapping-desorption, and the promptly scattered direct inelastic component, which interacts with the surface for ~1 picosecond, retains more of its parallel than perpendicular momentum, leaving the surface preferentially at glancing polar angles. Another interesting observation is that the energy exchange between the surface and the directly scattered Ar has a dependence on the incident azimuthal angle. This is, in a sense, another type of structure scattering, where it is the anisotropic elastic response of the surface rather than the corrugation that leads to the angular dependence of the atom scattering.
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The erosion of highly-ordered pyrolitic graphite (HOPG) upon exposure to hyperthermal atomic O(3P) atoms has been explored. Profilometry has been utilized in order to determine that the overall erosion yield is linear with respect to increasing atomic oxygen exposure at a constant sample temperature of 373 K. Atomic force microscopy (AFM) has been employed to image the eroded material that contains numerous nanoscale to microscale cylindrical etch pits. Since there is also a linear relationship between the etch pit diameter and atomic oygen fluence, it is proposed that the largest cylinders are nucleated at or near the topmost sheet of the original graphite material. There is a large distribution of depths for cylinders of a chosen diameter. This suggests that the chemical and physical nature of the nucleation event plays a key role in the final depth of the etch pit.
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Nanoscale diblock copolymer domains are aligned via top-down/bottom-up hierarchical assembly. Grating substrates template cylinder alignment with demonstrated 5000:1 aspect ratio for 100 µm domains extendable to arbitrary
length scales. Depending on trough depth and amount of deposited polymer, aligned domains are (1) confined to the channels or (2) expanded across the grating frequently with (3) a complete absence of defects. This methodology can be exploited in hybrid hard/soft matter systems for electronics, catalysis, and sensors.
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The effect of chain length on the low-energy vibrations of alkanethiol striped phase self-assembled monolayers on Au(111) was studied. We have examined the low-energy vibrational structure of well-ordered, low-density 1-decanethiol (C10), 1-octanethiol (C8), and 1-hexanethiol (C6) to further understand the interaction between adsorbate and substrate. Dispersionless Einstein mode phonons, polarized perpendicularly to the surface, were observed for the striped phases of C10, C8, and C6 at 8.0, 7.3, and 7.3 meV, respectively. An overtone at 12.3 meV was also observed for C6/Au(111). These results, in concert with molecular dynamics simulations, indicate that the forces between the adsorbate and substrate can be described using simple van der Waals forces between the hydrocarbon chains and the Au substrate with the sulfur chemisorbed in the threefold hollow site.
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We report a combined top-down/bottom-up hierarchical approach to fabricate massively parallel arrays of aligned nanoscale domains by means of the self-assembly of asymmetric PS-b-PEP diblock copolymers. Silicon nitride grating substrates of various depths and periodicities are used to template the alignment of high aspect ratio cylindrical polymer domains. Alignment is nucleated by PS preferentially wetting the trough sidewalls and is thermally extended throughout the polymer film by defect annihilation. Topics discussed include a detailed analysis of the capacity of this system to accommodate lithographic defects and observations of alignment beyond the confined channel volumes. This graphoepitaxial methodology can be exploited in hybrid hard/soft condensed matter systems for a variety of applications.
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Ultrathin diblock copolymer films have been offered as promising candidates for bottom-up templates in nanotechnological applications. Their natural tendency to self-organize into laterally alternating domains with a length scale tunable in the range of 10-100 nm is fundamental to their potential in this arena. However, having arbitrary control over the orientation of these domains is equally crucial and, until now, largely unrealized. We will present a novel lithographically assisted self-assembly approach that leads to low defect density domains of mesoscopic dimensions spanning 0.2-2 microns in width, 100 microns in length, and with nanoscopic features down to 20 nm. Potential applications extend from fundamental polymer science to sensor technology, electronics/spintronics, and optics.
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The spatially anisotropic kinetics involved in the chemical reaction
between highly ordered pyrolytic graphite (HOPG) and a beam containing
hyperthermal (~8 km s-1) O(3P) atomic oxygen and molecular oxygen yields unique
surface morphologies. Upon exposure at moderate sample temperatures (298-423
K), numerous multilayer circular pits embedded in the reacted areas have been
observed with the use of atomic force microscopy (AFM) and scanning tunneling
microscopy (STM). These cavities have diameters spanning nanometers to
micrometers and depths from a few to tens of nanometers. The most striking
characteristic of these pits is the convex curvature of the pit bottoms, where
the highest point on the pit bottom is at the center and the lowest point
occurs around the peripheral edge. Such structure arises by the interplay
between kinetics of pit nucleation, the spatially anisotropic kinetics involved
in the lateral and downward reactivity of HOPG, and the fluence of atomic
oxygen. These kinetics, which are also influenced by the high reactivity of
the translationally hot impinging oxygen atoms, govern the overall
morphological evolution of the surface.
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We have utilized time-resolved high-temperature atomic force microscopy (AFM) to investigate the mechanism by which topographic templates induce alignment of cylinder-forming diblock copolymer thin films. By tracking the same sample spot during thermal annealing, we observed that the structural evolution and alignment of thin films in confiement involve an intermediate state with disordered morphology and the evolution and annihilation of disclination quadrupoles guided by the channel edges, which ultimately lead to the essentially perfect alignment of cylindrical microdomains.
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A cylindrical-phase diblock copolymer ultrathin film is modified with vacuum UV light to selectively remove one of the surface domain components. The corrugated film then serves as a template for the self-organization of colloidal magnetic nanoparticles. This hierarchical methodol. is a general route to the nanoscale assembly of functional materials. This work has ramifications for potential future bit-patterned magnetic-storage media.
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A study of the interaction of hyperthermal Xe with a well-ordered standing-up phase of 1-decanethiol adsorbed on Au(111) is presented. Exptl., double-differential measurements were made of the postcollision Xe kinetic energy as a function of incident and final angles. These expts. are compared to classical trajectory calcns. The results show the two expected channels: direct-inelastic scattering from the surface and accommodated Xe due to trapping-desorption. There is also evidence of a further interaction mechanism. This involves the penetration of the atom deep into the channels between the aligned chains of the monolayer. When the collision energy has been dissipated, the implanted Xe is expelled as the chains return to their equil. positions. The expelled Xe leaves the surface with an energy much higher than expected for trapping-desorption, and with an angular-intensity distribution peaked close to the direction of the 1-decanethiol chain orientation. For this reason, we call this new scattering mechanism directed ejection.
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We have examined the low-energy single-phonon vibrations of disordered mono- and bilayers of sulfur hexafluoride physisorbed on Au(111) with inelastic helium atom scattering. At monolayer coverages, SF6 exhibits a dispersionless Einstein mode at 3.6 ± 0.4 meV. We observed two distinct overtones of this vibration as both creation and annihilation events at 7.1 ± 0.7 meV and 10.9 ± 1.4 meV, respectively. The overtones are harmonic multiples of the fundamental Einstein oscillation. Bilayers of SF6 exhibit a softer fundamental vibration with an excitation energy of 3.3 ± 0.3 meV. This softening, due to the weaker SF6 binding, also results in reduced overtone energies of 6.6 ± 0.7 meV and 9.8 ± 0.6 meV. The disordered bilayer does not exhibit dispersion, indicating that the molecules are still behaving like Einstein oscillators and not beginning to act as bulk crystalline SF6. The results have improved our understanding of the adsorbate-substrate and interadsorbate interactions which govern the properties of this model molecular physisorption system.
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Using molecular beams, we studied the reaction of O(3P) with 1- and 2-butene on the surface of Rh(1 1 1) and ice, and propene on the surface of Au(1 1 1), in vacuum at cryogenic temperatures. Unlike in the gas phase, and similar to reactions in the condensed phase, only the oxygen addition products were observed. The surface serves as a sink for the excess energy of this highly exoergic process, stabilizing the adduct product channels.
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Supersonic molecular beams have been used to determine the yield of CO from the partial oxidation of CH4 on a Rh(111) catalytic substrate, CH4+(1/2)O2-->CO+2H2, as a function of beam kinetic energy. These experiments were done under ultrahigh vacuum conditions with concurrent molecular beams of O2 and CH4, ensuring that there was only a single collision for the CH4 to react with the surface. The fraction of CH4 converted is strongly dependent on the normal component of the incident beam's translational energy, and approaches unity for energies greater than ~1.3 eV. Comparison with a simplified model of the methane-Rh(111) reactive potential gives insight into the barrier for methane dissociation. These results demonstrate the efficient conversion of methane to synthesis gas, CO+2H2, are of interest in hydrogen generation, and have the optimal stoichiometry for subsequent utilization in synthetic fuel production (Fischer-Tropsch or methanol synthesis). Moreover, under the reaction conditions explored, no CO2 was detected, i.e., the reaction proceeded with the production of very little, if any, unwanted greenhouse gas by-products. These findings demonstrate the efficacy of overcoming the limitations of purely thermal reaction mechanisms by coupling nonthermal mechanistic steps, leading to efficient C–H bond activation with subsequent thermal heterogeneous reactions.
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Collisional energy transfer at the surface of poly(methyl methacrylate) thin films on SiOx/Si was investigated using low-energy neutral helium atom scattering. Analysis of spectra in two scattering regimes yields results consistent with the hypothesis that thinner films are stiffer, suggesting that for highly nanoconfined films, polymer-substrate interactions influence vibrational dynamics at the polymer-vacuum interface. Specifically, thinner films are found to have lower mean-square displacements and decreased annihilation events as compared to thicker films. The scattering spectra are fit well by a semiclassical scattering model, though deviations arise at sample temperatures near the bulk glass transition. We have found helium atom scattering to be a sensitive probe of the vibrational dynamics of the polymer thin-film surface. This technique holds promise for the exploration of glassy dynamics of polymer thin films.
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We have examined the adsorption behavior at ~110 K of NO on NiO(111) overlayers prepared on a Ni(111) substrate. High-resolution electron-energy-loss spectroscopy shows fundamental changes in the vibrational spectrum for the beam dosed surface in comparison with the background dosed surface. Three vibrational peaks are observed after beam dosing, two of which are not observed after conventional background dosing. The peaks can be assigned to NO stretches for a previously observed NO state, a new NO bonding geometry, and a new NO2 surface species, previously unobserved under NO dosing. The difference is accounted for by increased NO uptake due both to kinetically activated adsorption and to increased exposure.
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He diffraction has been used to investigate changes in the surface morphology of reconstructed Au(111) when small quantities of O atoms are adsorbed. It is proposed that the electronegative oxygen removes charge from the surface, which causes the surface to revert to the (111) structure. The extent of this deconstruction is dependent on the initial O coverage and the surface temperature. These results further delineate and emphasize the delicate interplay of adsorbate coverage and surface structure for the oxygen-gold system, a topic of current high interest due to the remarkable and technologically relevant catalytic properties of gold interfaces and clusters spanning atomic through nanoscale dimensions.
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The photodesorption and photoreactivity of the molecular beam dosed NO/NiO(111)/Ni(111) system has been examined using high-resolution electron energy loss spectroscopy (HREELS). The molecular beam dosed surface exhibits three major vibrational peaks, which we attribute to a linear bonding NO species at 230 meV, a bent bonding NO species at 197 meV, and a stretching mode of NO2 at 160 meV. UV photon irradiation causes the attenuation of these peaks accompanied by simultaneous emergence of a fourth vibrational peak at 215 meV. The emergence of this peak is explained by a mechanism of selective desorption of the original, linear-bound NO species and a reaccommodation of the beam-induced, bent NO structure to a new state on the lower coverage surface. The measured desorption cross sections and wavelength dependence are consistent with those of other studies of photochemical processes on such interfaces, indicating that photoinduced electron transfer from the substrate to the adsorbate is the mechanism responsible for the observed behavior. Back to publications, Back to Sibener Group Home Page
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| last modified: 16-May-08 |