The Interaction of Hyperthermal Xe with Ordered 1-Decanethiol/Au(111): Penetration Followed by Directed Re-emission

Knowing the mechanisms involved in energy exchange between solid surfaces and impinging gas molecules is important because of such disparate phenomena as aerodynamic drag and surface reactions.  To help elucidate these mechanisms, we have undertaken experiments and simulations involving the collision of Ne, Ar, and Xe with an ordered 1-decanethiol monolayer.  Rare gases are ideal for studying energy disposal because of the lack of internal structure, well known interaction potentials, and the absence of any subsequent chemical reaction with the surface.  In this page, we briefly present some interesting results for Xe.

All of the data was taken along the direction. To the left are example time-of-flight (TOF) spectra and fits taken after Xe collision with the surface.  The incident conditions were E=3600 meV and Θ=20º.  Final conditions are noted on the plots. The circles represent the data, dashed lines are the fits. The three fitting functions are shifted Maxwell-Boltzmann distributions for the two fast, or unaccommodated, components, and a Maxwell-Boltzmann at the surface temperature of 135 K. This latter, the trapping-desorption component, represents atoms that have fully accommodated with the surface.  The solid line through the data is their sum.

To the right is a summary of the average kinetic; energy (open circles) and intensities (solid symbols) for the incident conditions of E=3600 meV and a surface temperature of 135 K. Incident polar angles are noted in the insets. Squares represent the trapping-desorpion component, and the dashed line is the expected cosine angular intensity dependence. Circles represent the total unaccommodated portion. Of particular note is the change in the energy and intensity distributions of the unaccommodated Xe.  At glancing incident angles, they peak towards glancing final angles, but at more normal incident angles, this situation reverses.

To the left is shown the final azimuthal and polar angle dependence of the simulations for unaccommodated Xe with E=3500 meV and an incident polar angle of 20º at the indicated incident azimuthal angle.  What is clear is that the Xe leaves the surface preferentially in the direction of the chain tilt and rotation relative to the Au substrate.  In fact, these Xe atoms penetrate well below the surface into the interstices between the chains.  Here is a movie for one of these trajectories, movies/penetration.gif.  The atom in this trajectory leaves the surface at 30º with an energy of 250 meV.

At more glancing incident angles, or lower projectile mass or energy, the rare gas scatters from the surface.  This results in the energy and intensity distributions peaking at glancing final angles.  For the high energy Xe at near normal angles, there is penetration followed by the directed re-emission of the atom.  The average kinetic energy of these expelled atoms is well above what is expected for thermal acccommodation with the surface.  It is possible that one of the two fast components in the experimental TOF spectra shown above is due to direct-inelastic surface scattering, and the other is due to penetration followed by directed re-emission.

For Xe simulations with an incident angle of 20º and E=3500 meV, there are also a few trajectories that scatter from the surface movies/surface.gif, and some that are trapped movies/trapped.gif.

100. "Experimental and Simulation Study of Neon Collision Dynamics with a 1-Decanethiolate Monolayer"
N. Isa, K. D. Gibson, T. Yan, W. L. Hase, and S. J. Sibener, J. Chem. Phys. In Press (2003) Abstract

101. "Role of Surface Intramolecular Dynamics in the Efficiency of Energy Transfer in Ne-Atom Collisions with a N-hexylthiolate self-assembled monolayer"
Tianying Yan, N. Isa, K. D. Gibson, S. J. Sibener, and William L. Hase, Parmenter Festschrift of the Journal of Physical Chemistry A In Press (2003) Abstract

103. "Experiments and Simulations of Ar Scattering from an Ordered 1-Decanethiol/Au(111) Monolayer"
K. D. Gibson, N. Isa, and S. J. Sibener, J. Chem. Phys. In Press (2003) Abstract