The potential energy surface, vibrational phase relaxation and the order-disorder transition in the adsorption system Pt{111}-CO

Abstract

We report detailed measurements of the lineshape and intensity of the C-O stretching vibration in the adsorption system Pt{111}-CO using IR reflection-absorption spectroscopy. A simple analysis of the data reveals interesting and useful information about the dynamics of molecules at surfaces. In particular, from the coverage dependence of the intensity for the on-top and bridging species we estimate a difference in binding energy between the two sites of ~ 60 meV. The "entropy" factor is found to be particularly important in determining their relative occupation. A semi-quantitative potential energy curve linking the on-top and bridge sites can be constructed but the operation of lateral interactions in the c(4×2) structure causes it to be modified, producing an elevated minimum (T * ) about 0.4 Å off the on-top site. From an analysis of the temperature dependence of the relative intensities of the C-O stretches the bridge-to-T* excitation energy is determined to be about 80 meV. The change in halfwidth and frequency as a function of temperature up to 250 K can be explained in terms of the relaxation-dephasing model. The dephasing mode for the on-top species is clearly the soft, frustrated translation at 48 cm-1 identified in inelastic helium scattering experiments by Lahee et al. The corresponding mode for the bridging species is at ~ 300 cm-1, supporting a recent reanalysis of the He scattering data which suggests that there are no soft frustrated translations as in the case of the on-top species. Above 250 K the rapid increase in linewidth indicates an order-disorder transition which is confirmed by LEED intensity data. Finally, we speculate as to the relevance of the information obtained here for solving other interesting adsorption problems. © 1989.