SURFACE-DIFFUSION OF XENON ON PT(111)

The surface diffusion of xenon on the Pt (111) surface was investigated using laser induced thermal desorption (LITD) and temperature programmed desorption (TPD) techniques. The surface diffusion coefficient at 80 K decreased dramatically from D = 8 X 10(-7) cm2/s at theta = 0.05theta(s) to approximately D = 2 X 1 0(-8) cm2/s at theta = theta(s), where theta(s) denotes the saturation coverage at 85 K, corresponding to a commensurate monolayer coverage of 5.0 X 10(14) xenon atoms/cm2. This coverage dependence was consistent with attractive interactions between the adsorbed xenon atoms and the existence of two-dimensional condensed phases of xenon on Pt (111). The kinetic parameters for surface diffusion at theta = theta(s) were E(dif) = 1. 3 +/- 0. 1 kcal/mol and D0 = 1. 1 X 10(-4 +/- 0.2) cm2/s. The magnitude of E(dif) at theta = theta(s) represented the combined effect of the intrinsic corrugation of the adsorbate-surface potential and attractive interactions between the adsorbed xenon atoms. LITD experiments at theta = 0.25 theta(s) revealed diffusion kinetic parameters of E(dif) = 1.2 +/- 0.2 kcal/mol and D0 = 3.4 X 10(-4 +/- 0.5) cm2/s. The constant E(dif) at low and high coverage was attributed to the ''breakaway'' of xenon atoms from the edges of condensed phase xenon islands. The coverage dependence of the surface diffusion coefficient for Xe/Pt (111) was explained by a multiple site diffusion mechanism, where collisions with xenon islands limit diffusional motion. Thermal desorption kinetics for xenon on Pt(111) were determined using TPD experiments. Using the variation of heating rates method, the desorption parameters were E(des) = 6.6 +/- 0.2 kcal/mol and nu(des) = 1. 3 X 10(13 +/- 0.4) s-1, in good agreement with previous studies. The xenon TPD peak shifted to higher temperature versus initial coverage at a fixed heating rate, providing further evidence for attractive interactions between the adsorbed xenon atoms.