Department of Chemistry, Wayne State University, Detroit, Michigan 48202
Abstract
Collision-induced desorption (CID) is used to selectively probe the well depth of one particular adsorbate–surface potential energy surface in the presence of multiple adsorbates. Ethylene adsorbed at saturation coverage on Pt{111} precovered with atomic oxygen represents a system with three adsorbates: oxygen and two different adsorption forms of ethylene. Both the p-bonded ethylene (p-C2H4) and di-s-bonded ethylene (di-s-C2H4) species are formed at coverages of 0.25 ML preadsorbed atomic oxygen and 0.15 ML ethylene on Pt{111} at 100 K. Deconvolution of Al Ka x-ray photoelectron (XP) spectra suggests that C(1s) XP binding energy is 283.1 eV for p-C2H4 and is 283.7 eV for di-s-C2H4. The C(1s) XP spectra together with the CID result reveal that the coverage ratio of p-C2H4 and di-s-C2H4 at saturation at 100 K is ~1:1. The selectivity of CID is demonstrated by desorbing only p-C2H4 using a neutral Xe atomic beam with translational energies ranging from 2.1 to 4.1 eV. In this translational energy range, di-s-C2H4 remains intact on the O/Pt{111} surface. The threshold energy for CID of p-C2H4 was determined by extrapolation to the Xe energy where the CID cross section is equal to zero. The threshold of 2.4±0.1 eV can be related to the p-C2H4–Pt binding energy using classical collision mechanics. An upper limit for the p-C2H4 binding energy on O/Pt{111} is calculated to be 0.95±0.04 eV from the CID threshold measurement, using a Pt effective mass of 1.5 Pt atom. ©1996 American Institute of Physics.