Over the past few years, we have used ab initio calculations to aid in the interpretation of ultraviolet photoemission data. The approach was first used successfully in our Ge2H6 studies. We were able to confirm the molecular orbital assignments for physisorbed Ge2H6 by first calculating the molecular orbital energies for gas phase Ge2H6 and then comparing the calculated energy levels with the experimentally-measured spectrum of the condensed phase. We then extended the calculations to large substituted silanes and germanes as models for adsorbates on Si surfaces. The geometries of these models are then optimized and the molecular orbital energies are determined. Following a population analysis, we are then able to use the calculated results to help assign spectral features to a given molecular orbital.
Recently, we have used density functional theory to explore the structure of adsorbed unsaturated cyclic organic molecules at germanium surfaces. These calculations are used to aid the experimental studies examining the cycloaddition of molecules at semiconductor surfaces. The surface is modeled by a germanium cluster as shown below.
The results of the DFT calculations are used to help in the analysis of experimental data. We have had good success in using the calcualtions in support results from ultavioletphotoelectron spectroscopy, high resolution electron energy loss spectroscopy, and infrared spectroscopy. As an example, below are the calcualted vibrational modes of adsorbed ethyl groups on a silicon surface compared with the measured HREELS spectrum.
