SELECTIVE COLLISION-INDUCED DESORPTION - MEASUREMENT OF THE PI-BONDED C2H4 BINDING ENERGY ON PT(111) PRECOVERED WITH ATOMIC OXYGEN

摘要

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 pi-bonded ethylene (pi-C2H4) and di-sigma-bonded ethylene (di-sigma-C2H4) species ale formed at coverages of 0.25 ML preadsorbed atomic oxygen and 0.15 ML ethylene on Pt{111} at 100 K. Deconvolution of Al K alpha x-ray photoelectron (XP) spectra suggests that C(1s) XP binding energy is 283.1 eV for pi-C2H4 and is 283.7 eV for di-sigma-C2H4. The C(1s) XP spectra together with the CID result reveal that the coverage ratio of pi-C2H4 and di-sigma-C2H4 at saturation at 100 K is similar to 1:1. The selectivity of CID is demonstrated by desorbing only pi-C2H4 using a neutral Xe atomic beam with translational energies ranging from 2.1 to 4.1 eV. In this translational energy range, di-sigma-C2H4 remains intact on the O/Pt{111} surface. The threshold energy for CID of pi-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 pi-C2H4-Pt binding energy using classical collision mechanics. An upper limit for the pi-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. (C) 1996 American Institute of Physics. [References: 45]