Electron- and photon-stimulated desorption: Benefits and pitfalls

Electron beam irradiation of solids often results in damage-producing events along with information-producing events. In the present paper we explore mechanisms of beam damage in solids, as well as examples of the benefits of electron- (and photon-) stimulated processes to study molecules at surfaces. Information about the geometrical structure of adsorbed species can be obtained from measurements of the angular distribution of ions released by electron- or photon-stimulated desorption. The directions of ion emission are directly related to the orientations of the surface bonds which are ruptured by the excitation. The method of Electron-Stimulated Desorption Ion Angular Distributions (ESDIAD) has proven particularly useful in characterizing local molecular structure at surfaces in the absence of long-range order; recent measurements of bonding configurations stabilized by impurities or by lateral interactions are discussed. Photon-stimulated desorption (PSD) studies using synchrotron radiation are providing new insights into fundamental electronic excitation processes at surfaces. Mechanisms for the excitation and desorption of ions are examined (valence, shallow core level and deep core level excitations), and examples include ion desorption from adsorbed monolayers, as well as ion formation and desorption processes in condensed molecular films.