Electron stimulated desorption, neutral species

Neutral Species Electron-Stimulated Desorption from Short Single DNA Strands... 

Electron-Stimulated Desorption (ESD) of Ions and Neutral Species... 

When adsorbed molecules are bombarded with electrons, local heating effects occur that lead to thermal desorption. In addition, there is a small but finite probability that electrons in the chemical bonds that hold the adsorbate to the surface will be excited into a repulsive state, leading to the desorption of that molecule either as a neutral species or as a molecular ion. Desorption of neutral species under electron-beam bombardment is frequently observed in studies of electron-surface interactions. A fraction of the adsorbed molecules will be ionized. These can be detected as positive ions, and the spatial distribution of this ion flux can be imaged on a fluorescent screen. Electron-stimulated desorption ion-angular distribution (ESDIAD) [56, 61, 64, 79-84] is the name of the technique that is used to learn about the site symmetry and orientation of adsorbed molecular species, since the molecular ions are usually emitted in the directions of their chemical bonds with the surface and with an unchanged orientation with respect to the orientation of the molecule when it was adsorbed on the surface. 

Neutral species represent the majority of particles desorbed during electron stimulated desorption (ESD) experiments from solid surfaces or other desorption experiments with other types of ionizing radiation (Eeulner and Menzel 1995). However, the study of emission of these species has received little interest compared to charged particles. The comparative scarcity of information on this subject reflects both the difficulties in measuring the neutral species and the number and complexity of desorption mechanisms (Bazin et al. 2010). In fact, the origins of the neutral species are multiple DEA, electron-hole pair recombination, dipolar excitations, and multihole final states with or without recombination of different particles (Kimmel et al. 1994). Neutral species can also be produced by direct electronic excitation of a molecule to a repulsive state leading to electronic excitation dissociation (EED) (Eigure 16.3). 

The surface species are also desorbed as ions or neutrals through electronic excitation induced by the irradiation of electrons or photons with energies from 10 eV to more than 1000 eV. This is called desorption induced by electronic transition (DIET). Photon-stimulated desorption shows a clear selection rule, and the results are easy to interpret. However, the number of photon energies available in the laboratory is limited. On the other hand, electron energies can be varied easily. Thus, electron-stimulated desorption is more widely used. By measuring the desorption rate through electronic excitations, one obtains information on the atomic geometry at the surface as well as the adsorbed species and the desorption mechanisms. 

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