Measurement of Xe desorption rates from Pt(111): Rates for an ideal surface and in the defect-dominated regime

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 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, ν, of 14+24−8 ×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 ν=9+40−8 ×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.

Bridging the structure gap: Chemistry of nanostructured surfaces at well-defined defects

“…A third peak at higher temperature had been found already by Rettner et al [150] and is present also for Xe/Pt(557) and Xe/Pt(112) [151]. The results of points (1) and (2), on the contrary, were firstly reported by Widdra et al [149] and demonstrate that Xe desorption off the size limited Pt(997) terraces is different from that off extended (111) terraces.…”

Interaction of the Pt(111) surface with adsorbed Xe atoms

“…Desorption data fix the minimum of the potential between 255 (Ref. 4) and 277 meV, 5 while the curvature at the minimum is given by the measured perpendicular vibration energy of 3.7 meV; 6 the attractive lateral Xe-Xe interaction was measured to be 43 meV per atom (14 meV per bond); 7 and from the difference between the isosteric heats of adsorption for commensurate and incommensurate phases, the lateral corrugation of the potential was estimated to be around 30 meV. 5 In spite of this rather complete picture of the interaction potential, dynamical simulations ultimately based on Pt-Xe and Xe-Xe pair potentials have had limited success in reproducing the observed structural phases and phase transformations of the adsorbed Xe layers.…”

A comparative kinetic study of xenon adsorption on a flat Pt(111) and stepped Pt(557) and Pt(112) surfaces

“…The implications ofthese results will be discussed later. Using molecular beam techniques, Rettner et al 17 were able to measure Ed at very low Xe coverage «()<0.OO5 ML) on a Pt(111) surface where defect sites (mostly atomic steps) dominate the desorption kinetics. In such a "defect-dominated" regime, an Ed value of 9.46 ± 0.92 kca1lmol was obtained.…”

“…Since Xe physisorption is known to be sensitive to step defect density, the two stepped surfaces were chosen with (different) nominal step densities ofO. 17( 1/6) and 0.33 ( 1/3), respectively. Temperature programmed desorption (TPD) was carried out under ultrahigh vacuum (UHV) conditions to obtain information on the coverage dependence of the desorption activation energy Ed' the preexponential factor v d' and the relative sticking coefficient for all the three surfaces.…”