Electron-stimulated Desorption ESD

Electron-stimulated Desorption (ESD).—The desorption of neutrals and ions as a result of electron bombardment of adsorbate layers is well known both as an inconvenience in experiments such as LEED and AES and as an experimental technique for investigation of the adsorbed state. In general, only the desorbed 

Clearly, the ESD effect must be minimized in analytical techniques using incident electron beams (e.g., AES), otherwise information will be lo.st. ESD is a complex process and can include various mechanisms, as described by Hoflund [3]. Many variables are involved in ESD. For a given surface the desorbing particle current, I d, can be expressed as 

A total cross section for BSD, Qtotai, can be defined by the equation 

Ingelbert and J. F. Hennequin, in Secondary Ion Mass Spectrometry SIMS III, (A. Benninghoven, J. Giber, J. Laszlo, M. Riedel and 

Netzer, J. E. Houston, D. M. Hanson and R. Stockbauer, in Desorption Induced by Electronic Transitions DIET /, (N. H. Tolk, M. M. Traum, J. C. Tully and T. E. Madey, eds.). Springer-Verlag, Berlin. 1983, p. 120. 

in Inelastic Particle-Surface Collisions, (E. Taglauer and W. Heiland, eds.). Springer-Verlag, Berlin, 1981, p. 80. 

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The nature of reaction products and also the orientation of adsorbed species can be studied by atomic beam methods such as electron-stimulated desorption (ESD) [49,30], photon-stimulated desoiption (PDS) [51], and ESD ion angular distribution ESDIAD [51-54]. (Note Fig. VIII-13). There are molecular beam scattering experiments such... 

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. 

Electron Stimulated Desorption (ESD) and ESD Ion Angular Distribution (ESDIAD)... 

Anion Desorption Measurements. Reactive scattering by DEA fragment ions in condensed media was first noted in the form of OH electron-stimulated desorption (ESD) yields from O2 embedded in multilayer alkane films [246] and subsequently for aniline physisorbed on top of O2 solids [215]. The anion produced following DEA to N2O has also been observed to react with other N2O molecules within an Ar/N20 matrix to generate a desorbed yield of NO and NO2, among other products. Part of the H2 ESD yield observed from multilayer films of H2O, at incident electron energies below 10 eV, has also been attributed to proton abstraction by fragments produced by DEA, viz., -I- H20 H30 ... 

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 ... 

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. 

Another novel method described recently involves the use of the electron-stimulated desorption (ESD, EID) of species from a surface. Lichtmann and Campuzano [239] used a scanning electron beam to induce desorption of F+ ions from a fluoridised molecule and obtained traces of F+ current (measured in a mass spectrometer) versus distance scanned across the sample. Only qualitative results were obtained but this again illustrates the strength of the scanning technique for boundary diffusion studies. 

The second application is to the direct measurement of adsorption-desorption processes using the Auger peak height of the particular element as a monitor. The principal limitation is the possible influence of the electron beam on the adsorbate, which can result in beam-induced desorption, adsorption or dissociation. The basis of electron-stimulated desorption (ESD) was established some time ago independently be Menzel and Gomer [38] and Redhead [39]. Electron impact causes Franck—Condon transitions of bound electrons in the adsorbed species into excited states. There is, therefore, a probability of dissociation with subsequent desorption of the particular species involved. As an example of these effects on semiconductor surfaces, Joyce and Neave [40] have reported results on silicon, while Ranke and Jacobi [41] have discussed the electron-stimulated oxidation of GaAs. 

Electron stimulated desorption (ESD) - See Techniques for Materials Characterization, page 12-1. 

FIGU RE 4.7. The Antoniewicz model for electron-stimulated desorption (ESD) of neutral particles [39]. 

Low energy electrons are directed at the surface and can cause an atom or molecule on the surface to be desorbed. By measuring the intensity of the desorbed species as a function of take-off angle information regarding how the molecule is oriented on the surface can be determined (i. e. bond directionality). Electron stimulated desorption ESD usually occurs by electronic energy transfer and not by direct momentum transfer. 

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). 

Electron-stimulated desorption ESD 5 A Mass spectrum < 1 monolayer e beam Poor sensitivity 109... 

An electron emitted by either EE mechanism mentioned above can strike a surface and stimulate the emission of an ion. In surface physics, electron-stimulated desorption (ESD) is a widely studied phenomenon that has recently been extended to organic adsorbates (28). The energetics of this mechanism would demand that electrons strike the surface with a minimum energy of several eV, but this is conceivable considering the charging of the fracture surfaces that frequently occurs and the EE energy distributions observed. If this latter mechanism is correct, the flux of EE measured-i-the flux that... 

The static charge of the fracture surfaces leads to acceleration of the emitted electrons, modifying their energy distributions. A large portion of the EE is pulled back to the surface resulting in a self-bombardment process. This results in the emission of ions (NIE and PIE)(8.9,2S,30) and excited neutrals( o) (NE ) via electron stimulated desorption (ESD).(6i)... 

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