Direct calculation of experimentally measurable chemisorptive parameters from theoretical quantum chemical cluster models

Abstract

OAl 3, OAl 4, and two different OAl 6 clusters are used to model O chemisorption in the threefold sites of the (111) face of Al metal. A gaussian-based model potential theory for valence electrons only allows the ready calculation of the equilibrium O position and vibrational frequency, and of the electron distribution. These studies show that the O atom has nearly constant character in all clusters/states studied, with edge effects of little importance. Calculated AlO bond lengths are accurate when compared to critically discussed experimental results deduced from EXAFS measurements. Use of a real all-electron O atom gives accurate calculated O ls binding energies when compared to ESCA measurements and other calculations. O-surface vibrational frequencies are predicted to be around 400 cm -1, clearly different from predicted values around 525 cm -1 for a bridge site and 625 cm -1 for an overhead site; these can be subject to EELS experimental study. The theory thus seems able to predict and interpret experimental quantities, and a few related theoretical results for NO on Al and O on Zn are given as examples of the further utility of the theoretical models and techniques.