Theory of the excitation of the vibrational mode of an adatom-substrate system under a resonant laser field

We present a theoretical description of the excitation of a stretch mode and energy transfers among different degrees of freedom within an adatom-substrate system under an infrared resonant laser field. There are two competing mechanisms via which the adatom-substrate bond can be highly excited: The pooling process and the absorption of multiphotons. We show that if the frequency dispersion of the laser is much smaller than the anharmonicity of the selected mode, namely, if the laser can be treated as monochromatic, the pooling process is dominant in exciting the stretch mode to highly vibrational states. As the frequency dispersion of the laser increases, the excitation to highly vibrational states by direct photon absorption becomes more dominant. We further show that the dependences of the two mechanisms on photon frequency are quite different: If pooling dominates, there is only a single peak in the excitation probability; if multiphoton absorption dominates, there may exist several peaks and the main peak may exhibit a redshift. These findings are illustrated using the H/Si(111) system as a prototype example.