Electron-stimulated desorption, surface

An electron or photon incident on a surface can induce an electroiuc excitation. When the electroiuc excitation decays, an ion or neutral particle can be emitted from the surface as a result of the excitation. Such processes are known as desorption induced by electroiuc transitions (DIET) [82]. The specific teclmiques are known as electron-stimulated desorption (ESD) and photon-stimulated desorption (PSD), depending on the method of excitation. 

The most common ions observed as a result of electron-stimulated desorption are atomic (e. g., H, 0, E ), but molecular ions such as OH", CO", H20, and 02" can also be found in significant quantities after adsorption of H2O, CO, CO2, etc. Substrate metallic ions have never been observed, which means that ESD is not applicable to surface compositional analysis of solid materials. The most important application of ESD in the angularly resolved form ESDIAD is in determining the structure and mode of adsorption of adsorbed species. This is because the ejection of positive ions in ESD is not isotropic. Instead the ions are desorbed along specific directions only, characterized by the orientation of the molecular bonds that are broken by electron excitation. 

The observations of complex dynamics associated with electron-stimulated desorption or desorption driven by resonant excitation to repulsive electronic states are not unexpected. Their similarity to the dynamics observed in the visible and near-infrared LID illustrate the need for a closer investigation of the physical relaxation mechanisms of low energy electron/hole pairs in metals. When the time frame for reaction has been compressed to that of the 10 s laser pulse, many thermal processes will not effectively compete with the effects of transient low energy electrons or nonthermal phonons. It is these relaxation channels which might both play an important role in the physical or chemical processes driven by laser irradiation of surfaces, and provide dramatic insight into subtle details of molecule-surface dynamics. 

The bombardment of surfaces with electrons causes the dissociation of molecular surface complexes. Some of the fragments enter the gas phase and others remain attached to the solid. The first process is called Electron-Stimulated-Desorption (ESD) and has been widely investigated. There have been several recent reviews ... 

Many of the mechanisms discussed in Sect. 2.2.6.1 with regard to ions may also apply to chemical reactions enhanced by electron bombardment. A discussion of that type will not be repeated in this section. However, a mechanism for the electron-enhanced etching of SiOj can be suggested on the basis of processes which are known to occur. It is known, for example, that electron bombardment of SiOj causes oxygen to be desorbed into the gas phase, i.e., electron stimulated desorption occurs . The silicon which remains upon the surface can now be attacked by the XeFjfgas) to produce SiF4(gas). In this manner both oxygen and silicon are removed from the SiOj lattice and the material is etched. The chemistry involved is likely to be more complex, but this simple model illustrates a possible mechanism. 

The ejection of atoms or molecules from the surface of solid in response to primary electronic excitation is referred to as electronically stimulated desorption (ESD) or desorption induced by electronic transitions (DIET). Localization of electronic excitations at the surface of RGS induces DIET of atoms both in excited and in ground states, excimers and ions. Most authors (see e.g. Refs. [8,11,23,30] and references therein) discuss their results on DIET from RGS in terms of three different desorption mechanisms namely (i) M-STE-induced desorption of ground-state atoms (ii) "cavity-ejection" (CE) mechanism of desorption of excited atoms and excimers induced by exciton self-trapping at surface and (iii) "dissociative recombination" (DR) mechanism of desorption of excimers induced by dissociative recombination of trapped holes with electrons. 

A diatomic molecule has to be adsorbed parallel to the surface in order to dissociate. The more favourable adsorption complex for molecules like CO on a group 8-10 metal surface is that in which the molecular axis stands perpendicular to the surface. It has been demonstrated by ESDIAD (electron stimulated desorption, ion angular dependence) that the molecular axis vibrates with regard to the surface normal and that the amplitude of vibration increases with increasing temperature this is shown in Fig. 4.41. 

Observation of the ion angular distribution after electron stimulated desorption of chemisorbed species (ESDIAD) can provide direct quantitative information on the orientation of adsorbed molecules on surfaces. Electrons incident on the surface can excite chemical bonds into non-bonding states, causing molecular decomposition. The excess energy can be converted into kinetic energy, which accelerates an ionic fragment of the molecule along the axis... 

---