MEASUREMENT OF XE DESORPTION RATES FROM PT(111) - RATES FOR AN IDEAL SURFACE AND IN THE DEFECT-DOMINATED REGIME

The rate of thermal desorption of Xe from a Pt (111) surface has been measured over a range of 7 orders of magnitude using a combination of molecular-beam techniques. Rates up to ∼ 104 s-1, corresponding to residence times as short as 100 μs were extracted from the time-of-arrival distributions for atoms leaving the surface after short Xe beam pulses were applied. Rates as low as 10-3 s-1 were measured using a time-delayed flash-desorption technique. For intermediate rates, the transient decay of the desorbing Xe was recorded directly following the closing of a beam shutter. Temperature programmed desorption (TPD) spectra show first-order desorption kinetics and also reveal the presence of "defect" sites with substantially higher Xe binding energy which dominate the desorption kinetics at low coverages (below 0.005 Xe monolayers). These defects can be specifically saturated with CO molecules, permitting the measurement of rates characteristic of an ideal Pt(111) surface. An Arrhenius plot of these desorption rates is found to be linear over the entire range covered (80-160 K), giving an adsorption energy, Δε, of 245 ±15 meV and a preexponential, v, of 14-8+24× 1011 s-1. In contrast, a similar plot for rates which are controlled by the presence of defect sites gives Δε = 410 ± 40 meV and v = 9-8+40×1015s-1. A desorption model including the effect of defects is developed which relates the desorption rate to the microscopic behavior of Xe atoms on terraces and at defects. This model leads to an estimate for the preexponential factor for defect-dominated desorption which is quite consistent with the very large measured value, and allows the simulation of TPD spectra and isothermal coverage decay curves, which involve coverages both higher and lower than the defect-site density. © 1990 American Institute of Physics.