Xenon diffusion on a stepped Pt(11,11,9) surface

Abstract

The surface diffusion of xenon was studied on a stepped Pt(11,11,9) surface. Laser-induced thermal desorption (LITD) techniques were employed to examine the coverage dependence and anisotropy of the Xe diffusion. The coverage dependence of the Xe desorption rate was also measured using isothermal LITD and isothermal mass spectrometric methods. Both the diffusion and desorption rates of Xe on Pt(11,11,9) were independent of Xe coverage. These coverage-independent results provide no evidence for Xe trapping on step sites and are inconsistent with the existence of Xe islands. The coverage-independent desorption kinetics were similar to the kinetics on Pt(111) at low coverage with Edes=5.7 kcal/mole and νd=3.7×1010 s−1. The Xe diffusion was anisotropic and overwhelmingly parallel to the step edges. The diffusion kinetic parameters parallel to the step edges were Edif=2.8 kcal/mole and D0=50 cm2/s. This diffusion barrier is much larger than earlier theoretical predictions. The diffusion pre-exponential is also much larger than the values predicted by transition state theory. These experimental results suggest that Xe diffuses on the stepped Pt(111) surface with a multiple-site hopping mechanism. The temperature dependence of this multiple-site hop length may dominate the measured diffusion activation energy. Consequently, in accordance with recent calculations, the experimentally measured diffusion barrier may not directly reflect the underlying potential energy surface.