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