Resonant UV Laser Excitation of Physisorbed Molecules: The Competition between Their Desorption, Dissociation, and Ionization

摘要

The photochemistry of physisorbed dimethylcadmium molecules resonantly excited by the powerful radiation of an excimer KrF laser (248 nm) has been studied using desorption mass spectrometry. The desorption of the parent molecules, their one-photon fragmentation up to Cd atoms and CH3 radicals, and three-photon dissociative ionization leading to generation of these ionic fragments have been observed. Depending on the strength of the dimethylcadmium bond to the surface, either its dissociation or desorption predominantes. Photolysis and ionization of the molecules desorbed due to heating of a quartz substrate by a pulsed CO2 laser have also been considered. The collisions between desorbed species near the surface have been found to play an important role in the processes studied. These collisions result in both partial stabilization of unstable photolysis intermediate such as CdCH3 and ionization of the parent molecules.