High Oxygen Coverages Under UHV Conditions:
(1x1)-O/Rh(111)


    Under UHV conditions, the saturation coverage of dissociatively chemisorbed oxygen on Rh(111) is 0.5 monolayers (ML) when dosing with molecular oxygen. Using a radio-frequency discharge nozzle beam, we were able to dose the crystal with atomic oxygen, and produce a well-ordered 1 ML of adsorbed oxygen.
 
Figure 1 shows some temperature-programmed thermal desorption (TPD) spectra, where the signal of recombinatively desorbing O2 is monitored as a function of surface temperature, which was ramped at 10 K/sec. Figure 1a compares the spectra after dosing with molecular oxygen, which produces 0.5 ML coverage, and oxygen atom dosing, where the desorption signal is twice as large. With continued atomic oxygen dosing, further oxygen is deposited, probably subsurface, as shown in figure 1b.


Figure 2 shows surface diffraction spectra along the two principle symmetry directions taken using elastic He scattering. When dosing with molecular oxygen, saturation coverage 0.5 ML, the surface is composed of rotated (2x1) domains, which gives a (2x2) pattern. The half-order features are indicated by arrows. After twice as much oxygen is deposited using atomic oxygen, the spectra still have strong first-order features, but the half-order features are gone or greatly attenuated.
Figure 3a shows oxygen uptake for submonolayer coverage using oxygen atoms at a surface temperature of 325 K. The circles are the measured data, and the solid line is the fit for a first-order isotherm. The dashed line is a calculated second-order isotherm for molecular oxygen with an initial sticking coefficient of 1. Figure 3b shows the measured oxygen uptake rate for oxygen atoms after 1.0 ML has been deposited. The solid line is for the absorption of oxygen using molecular oxygen dosing.



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