Excitation of vibrational mode in an adatom-substrate system under a resonant laser field

Abstract

We present a general discussion about the excitation of stretch mode and energy transfers in an adatom-substrate system under an infrared resonant laser field. Generally, there are two different mechanisms by which the adatom-substrate bond can be highly excited: the pooling process and the absorption of multi-photon. We find that if the frequency dispersion of the laser is much smaller than the anharmonicity of the selected mode, that is, the laser can be treated as monochromatic, the pooling process is dominant in exciting the stretch mode to high energy states. As the frequency dispersion of the laser increases, the excitation to high energy states by direct photon absorption becomes more and more important. We also find that the dependence of the two mechanisms on frequency are quite different: if the pooling dominates, there is only a single peak in excitation probability; if the photon-absorption dominates, there may be several peaks and the main peak may have a red shift. The hydrogen-covered Si(111) system is used as an example to illustrate our discussion and results. PACS numbers: 68.43.Tj,68.43.Pq,68.35.Ja,33.80.Rv