Relaxation of Adsorbate Excitations

An adsorbed atom or a molecule being in its excited state is characterized by a finite lifetime which is determined by the reciprocal of the decay rate of this state. The finiteness of the lifetime leads to a broadening of the lines in the optical spectra of the adsorbate. Besides spontaneous emission which occurs also for free atoms and molecules, adsorbed species have other specific channels of relaxation, conditioned by their proximity to the surface. Any relaxation process must obey the conservation law of energy and therefore it takes place only if there is a substrate excitation which can accept the energy that the excited adsorbate releases. Therefore, possible decay mechanisms are determined by the energy spectrum of the substrate and thus generally are different for metals, semiconductors and dielectrics. They can be broadly classified as being mediated by photons, phonons, electron-hole pairs and conduction electrons. 

Dephasing is another important broadening process for spectral lines of adsorbates. Elastic collisions of phonons and conduction electrons with adsorbed atoms or molecules disrupt the phases of their induced dipole moments and thus provide surface-specific pathways for phase relaxation. If an adsorbed particle can be considered as a two-level system, both the lifetime of its excited state, T, and the dephasing time, T, contribute to the spectral linewidth 7 as  

In the following we shall consider the different relaxation pathways in some 

The radiative lifetime ranges from 10 to 10 s for atomic transitions and from 10 to 0.1 s for vibrational transitions in molecules. Therefore, radiative decay is entirely negligible in comparison with the other pathways on metal and semiconductor surfaces. However, it can play a role in relaxation of electronically excited states of adsorbates on dielectrics. 

This mechanism concerns the relaxation of vibrationally excited molecules near surfaces. It depends strongly on the ratio between the vibrational frequency of a molecule, coq, and the Debye frequency of the substrate, cup, which determines the upper limit of the phonon spectrum. If a o cod, relaxation through creation of a single phonon is possible. Usually the corresponding decay rates are of the order of lO -lO s . For (n — 1)cud o o ncup, the relaxation is accompanied by the generation of n phonons. The probability of an n-phonon process to occur rapidly decreases with order n. Typical values for the two-, three- and four-phonon decay rates are 10 -10 10 -10 and 10 -10 s, respectively (Zhdanov and Zamaraev 1982). 

---