Anharmonic Adlayer Vibrations on the Si(111):H Surface

Using infrared absorption spectroscopy, the frequency and linewidth of the H-stretching vibration on the Si(111):H-(1x1) surface has been measured over a temperature range of 14-350K. An ab initio calculation of the temperature dependence of the anharmonic frequency shift and the intrinsic linewidth of this mode has been carried out, which fully accounts for the quantum nature of the atomic vibrations by using interacting phonon theory. The theoretical results show that at temperatures greater than 200K, both line shift and linewidth of the stretching mode are primarily due to strong anharmonic coupling to the bending modes, which suffer decay into substrate modes via cubic anharmonicity. At low temperatures, direct coupling to various phonon modes of the substrate dominates the temperature dependence of the line shift. In addition, predictions are made for the frequency shift due to zero-point motion of the atoms. A strong sensitivity of the frequency on the shape of the Si-H potential has been found which makes quantitative predictions of the influence of zero-point motion very difficult.