Abstract
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 �s, 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, Deltaepsilon, of 245�15 meV and a preexponential,
nu, of 14<sup> + 24</sup><sub> - 8</sub> �1011 s-1. In contrast,
a similar plot for rates which are controlled by the presence of
defect sites gives Deltaepsilon=410�40 meV and nu=9<sup> + 40</sup><sub>
- 8</sub> �1015 s-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. The Journal of Chemical Physics is copyrighted
by The American Institute of Physics.
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