Photon-stimulated desorption

PSD Photon-stimulated desorption [149, 162-165] Incident photons eject adsorbed molecules Desorption mechanisms and dynamics... 

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. 

Madey T E 1986 Electron- and photon-stimulated desorption probes of structure and bonding at surfaces Science 234 316... 

A number of surface-sensitive spectroscopies rely only in part on photons. On the one hand, there are teclmiques where the sample is excited by electromagnetic radiation but where other particles ejected from the sample are used for the characterization of the surface (photons in electrons, ions or neutral atoms or moieties out). These include photoelectron spectroscopies (both x-ray- and UV-based) [89, 9Q and 91], photon stimulated desorption [92], and others. At the other end, a number of methods are based on a particles-in/photons-out set-up. These include inverse photoemission and ion- and electron-stimulated fluorescence [93, M]- All tirese teclmiques are discussed elsewhere in tliis encyclopaedia. 

Other techniques in which incident photons excite the surface to produce detected electrons are also Hsted in Table 1. X-ray photoelectron Spectroscopy (xps), which is also known as electron spectroscopy for chemical analysis (esca), is based on the use of x-rays which stimulate atomic core level electron ejection for elemental composition information. Ultraviolet photoelectron spectroscopy (ups) is similar but uses ultraviolet photons instead of x-rays to probe atomic valence level electrons. Photons are used to stimulate desorption of ions in photon stimulated ion angular distribution (psd). Inverse photoemission (ip) occurs when electrons incident on a surface result in photon emission which is then detected. 

Photons, Compton-scattered, 21 312 Photon spectrum, 21 296 Photon stimulated desorption (PSD), 24 74 Photooxidation, 9 385-386 dye-induced, 19 195 in industry, 9 518 reactions, 9 515-518 Photooxidative degradation... 

Fig. 2.19 ( ) Time-of-flight spectrum in photon stimulated field desorption of water using synchrotron radiation, obtained by Jaenike et al.m...

There are four requirements for these experiments a magnetic substrate, adsorbed chiral molecules, a source of ionizing radiation, and a technique to monitor the reactions or products. If the products are emitted into the gas phase then mass spectroscopy may be employed. However, detection of neutral desorbing species is problematic, particularly if the species are a major component of the residual gas. Detection of ions may be employed and often such photon-stimulated ion desorption measurements can reveal a great deal about the surface reactions [112]. 

Induced ion desorption by resonant photon-stimulated desorption has been also observed. The yield of FT desorbed ions as a function of the incident photon energy from CeOa has been studied by the group at Sandia Laboratory. [101] They observed a sharp resonance in PSD of using photon energies near the Ce 4d edge at 110 eV. The resonance is due to autoionisation from excited states from the interaction of the Ce 4d and 4f levels. 

NEXAFS probes the empty electronic levels of the system, which, in the case of HCl, directly reveal if it is dissociated or not (or ionised/molecular). Our measurements show unambiguously the ionisation of HCl on ice at 20 K. This facile ionisation at very low temperature is in good agreement with the idea of a barrier- free ionization of HCl on ice. Furthermore, when measuring the NEXAFS spectra in Photon-Stimulated-Desorption mode (PSD), we greatly enhance the sensitivity to molecular HCl species that could eventually be adsorbed on the surface, especially at low temperature where HCl condensation can occur. Indeed, the PSD spectra show that at 20 K and at low coverage (0.1 L - 0.3 L) a small amount of molecular HCl is condensed at the side of the Cf anions. 

Antoniewicz, P.R. (1980) Model for electron-stimulated and photon-stimulated desorption. Phys. Rev. B, 21, 3811-3815. 

This table was derived from empirical data on conductivity and magnetism in solids. The shaded area maps the first order Mott transition between localised and itinerant behaviour in the solid, and the elements which lie on it have sensitive (e.g. pressure-dependent) properties. However, they are also remarkable in atomic physics for their giant resonances, are noted catalysts, or provide good materials for H storage, and then exhibit photon-stimulated desorption peaks which replicate the giant resonance profiles. Many of these properties seem to depend on the critical localisation of / and d electrons. 

The interrelation between several of these properties is well illustrated by the study of photon-stimulated desorption of H and radicals containing H from a solid surface of Ce02, illustrated in fig. 11.3. When a surface of CeC>2 on which H2 has been adsorbed is exposed to radiation, desorption... 

Fig. 11.3. Photon stimulated desorption from a surface of Ce02, as a function of photon energy, in the range of the Ad — / giant resonance. The top three curves show the desorption signal for several molecular ions containing H, while the bottom curve shows the photoabsorption cross section for comparison (after B.E. Koel et at. [602]).

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