Desorption Stimulated by Collective Electron Excitation

Abstract

In recent years laser-induced desorption processes have found a continuously growing interest [1–3]. Desorption stimulated with laser light can take place by different mechanisms. Most frequently it is simply promoted by thermal heating. Light is absorbed at the substrate surface, the temperature rises and the particles desorb. Bond breaking at the surface can also be stimulated by vibrational excitation of the adsorbate molecules with infrared laser light. Coupling of the vibrational excitation to the phonon bath of the substrate is the dominant channel for energy relaxation. As a result, the surface temperature rises causing desorption. This means that the energy is absorbed by a resonant process initially, but the desorption is thermal, a phenomenon denoted as “resonant heating”. It is responsible for the lack of molecular or isotopic selectivity in such experiments. In addition to vibrational excitation laser induced photodesorption can also be accomplished by electronic excitation. Even though the study of such processes is a new field and the number of experiments is still small compared to that of classical DIET processes [4–6] stimulated by electron bombardment or synchrotron radiation, different types of excitation have been observed ana different mechanisms established. Electronic excitation may either be accomplished in the adsorbate, in the substrate or in the adsorbate-substrate complex. The majority of experiments have been performed with high intensity pulsed lasers, where the available energy of the photons usually does not exceed about 6 eV.