Surface Vibrational and Electron Spectroscopy - Experiments and first-principle calculations

Sammanfattning: This work deals with interpretation of experimental surface spectroscopic data with the aid of first-principle calculations. Surface infrared reflection-absorption spectra as well as spectra from the electron spectroscopic methods photoemission and X-ray absorption are considered. In all cases the surface is modeled by a metal cluster with the same (frozen) geometry as part of the surface. The work on surface vibrational spectra has aimed at including anharmonic effects using first-principles calculations. Anharmonicity influences surface adsorbate vibrations in two ways. First, the frequencies are lowered as compared to the harmonic frequencies. Second, anharmonic resonances mix C-H stretch fundamentals and binary C-H deformation overtones and combinations and therefore redistribute the harmonic intensity only found in the C-H stretch fundamental. We show that by using a sophisticated model for anharmonicity based on density-functional electronic structure calculations and 2nd order perturbation theory both these effects can be included without scaling the harmonic frequencies. In order to save computational time anharmonic effects are calculated for a corresponding free molecule and excellent agreement between experiments and calculations is demonstrated for a few adsorbates adsorbed on Cu(100). Frequencies, relative intensities and the number of observed modes are all reproduced. Photoemission and x-ray absorption data have been modeled using electronic structure calculations for the density-functional method B3LYP on metal clusters including only a single metal atom. For the system studied this simple model reproduces virtually all experimental results, such as binding energy shifts, valence photoemission spectra and x-ray absorption spectra and thus allows for easy and detailed spectral interpretation.