Trapping of Ar on well ordered Ar, Kr, and Xe overlayers on Pt(1 1 1) at 30 K

Abstract

The dynamics of Ar trapping on Ar, Kr, and Xe covered Pt(1 1 1) were investigated using supersonic molecular beam techniques at a surface temperature of 30 K. The initial trapping probability of Ar on the clean surface decreases from 0.8 to 0 as the normal incident energy is increased from 1.75 to 28 kJ mol −1, and scales with normal incident energy (E T cos 2θ) , indicating a smooth gas–surface potential [A.F. Carisson R.J. Madix, submitted for publication]. In contrast, the trapping probability on the Ar, Kr and Xe saturated surfaces decreases from 0.9 to only 0.3 in the same energy range and is nearly independent of the angle of incidence. The relative trapping probability on the Ar and Xe covered surfaces is in general accord with the relative energy exchange expected from the simple Baule model, but the trapping probability on the Kr covered surface appears further enhanced by potential corrugation. Experimental results for Ar trapping on Pt(1 1 1)–Ar are in qualitative agreement with a recoil effect predicted by Head-Gordon and Tully for Ar trapping on Ar covered Ru(0 0 1) [Surf. Sci. 268 (1992) 113]. At high incident energies on the gas covered Pt(1 1 1) surface, the adsorption probability is nearly independent of angle of incidence between 0° and 30°, but it then increases rapidly between 30° and 60°. Similar effects are observed for Ar trapping on Pt(1 1 1) saturated by Kr and Xe. The dependence of the trapping probability on coverage is quantitatively described by the modified Kisliuk model [J. Chem. Phys. 92 (1990) 1397; J. Phys. Chem. 95 (1991) 2461], which allows for an extrinsic precursor to adsorption as well as a direct channel. The probability of Ar trapping increases with Ar uptake to different degrees on the Ar, Kr and Xe covered surfaces.