Photon-stimulated desorption and fragmentation processes: formic acid on Si(111)7 x 7

A photon-stimulated desorption (PSD) process consists of the production of ions and neutrals upon irradiation with light of proper wavelength. Here, we present a short review on the PSD mechanisms and the parameters influencing it, such as the adsorbate-substrate coupling, the light polarization, the surface orientation, the temperature, the mass of the leaving fragments as well as the co-adsorption effects. Furthermore, as an example, we will present a PSD investigation of formic acid on Si(111)7 x 7 in the energy ranges including the C 1s and the O 1s thresholds. D+, C+, CD+, O+', CO+, DCO+ ions are produced in both energy ranges. The excitation in the C 1s energy range offers a less efficient substrate-mediated quenching with respect to the excitation in the 0 Is one, with a consequent more structured photoexcitation spectrum. In the energy range including the O 1s the transitions to pi*((C=O)) and sigma*((C-D)) levels produce selectively either D+ or CDO+. O+ is produced both by a transition to a pi*((C=O)) level on through a shake-off process, and the remaining fragments are generally produced through secondary electrons. (C) 2002 Elsevier Science B.V. All rights reserved.