Computation of adsorbate vibrational frequencies: methyl, methylidyne and methoxy adsorbed on the Al(111) surface

A general strategy for computing adsorbate vibrational frequencies using cl uster calculations is illustrated for methyl, methylidyne and methoxy adsor bed on an Al(111) surface. The method uses energy second derivatives rather than the numerical fitting procedures often used in cluster calculations a nd avoids having to make guesses at the form of the normal modes for the vi brating polyatomic adsorbate. Using energy derivatives also facilitates per forming a convergence check on the adsorbate frequency values. The methyl a nd methylidyne computed frequencies are in good agreement with vibrational frequencies for the CH3I dissociation product observed by high resolution e lectron energy loss spectra (HREELS). The computed methoxy adsorbate freque ncies are also consistent with HREELS experiments, but the optimized methox y structure, while agreeing with chemical intuition, differs from the metho xy adsorption at the hcp threefold site recently determined by normal incid ence standing X-ray wavefield absorption (NISXW) experiments. (C) 1999 Else vier Science B.V. All rights reserved.