Shift and broadening of adsorbate vibrational modes

Abstract

The shift and broadening of the vibrational frequency of the adsorbate on a metal surface are calculated on the basis of two different models. The first model is an anharmonic one which includes multiphonon processes to all order in first-order perturbation theory. We show that anharmonic damping due to multiphonon processes is substantial for CO adsorbed on a Ni(100) surface and small for H adsorbed on a W(100) surface. At room temperature multiphonon processes with n > 2 are very important even in cases when the frequency of vibration of the adsorbate lies below twice the maximum phonon frequency of the substrate ( n is the order of the process). For the CNi stretching mode of top-bonded CO on Ni(100) our results are in accord with the experimental data. The second model is an electronic one which includes the electron-hole pair loss mechanism. We show that the observed shift and broadening of the CO stretching vibration mode for a CO molecule adsorbed on Cu(100) and for a H atom adsorbed on a W(100) surface can be explained on the basis of this model. Variation of the shift and broadening of the CO stretching mode with the distance from a CO molecule to Ni(111) surface are calculated.