Desorption photon induced

Laser desorption/ ionization LDI Photon induced desorption/ ionization Nonvolatile atomic and molecular ions Isotope ratio Trace analysis... 

Matrix-assisted laser desorption/ ionization MALDI Photon induced desorption/ ionization Nonvolatile molecular ions Soft method Large molecules... 

Desorption of impurities adsorbed on the first wall and limiters may significantly contribute to the total impurity influx into the plasma during a discharge. In addition to thermal desorption, one must consider ion and neutral atom impact desorption, electron induced desorption and photon induced desorption. Since thermal desorption is a relatively well understood process it will not be further discussed here although it is obviously of great importance to Tokamak operation. 

In the area of photon-induced desorption the following additional studies are required ... 

Desorption yields for the three above mentioned processes can be combined with the expected particle fluxes to give estimates of the impurity influx for these desorption processes. The corresponding impurity influx from deuterium ion impact desorption is estimated to be about two orders of magnitude larger than that expected from either electron or photon induced desorption. Therefore, the impurities released into the plasma during a short discharge ( 1 s) will be dominated by deuterium impact desorption while electron and photon induced desorption will occur only to a much more limited extent. 

The photon-induced dissociation and desorption of methyl bromide from a LiF surface was monitored [113]. Discuss the evidence that the photon energy was absorbed directly by the molecule adsorbed on the alkali halide surface. Would you expect the same photon-induced dissociation behavior if methyl bromide was chemisorbed on a transition metal surface ... 

Multilayers of glycine, adsorbed on Ti02 (110), were studied with synchrotron radiation-based UV light [428]. Ultraviolet photoemission spectroscopy (UPS) and XPS data showed the multilayers as formed by glycine molecules in polar zwit-terionic form (NHs CH2COO ). Photon induced damage of multilayers occurs fast (produces a first order desorption of zwitte-rionic molecules with total cross section). 

DIET Desorption induced by electronic transitions [147a] General class of desorption and reaction phenomena induced by electron or photon bombardment Same as ESD and PSD... 

Femtosecond lasers represent the state-of-the-art in laser teclmology. These lasers can have pulse widths of the order of 100 fm s. This is the same time scale as many processes that occur on surfaces, such as desorption or diffusion. Thus, femtosecond lasers can be used to directly measure surface dynamics tlirough teclmiques such as two-photon photoemission [85]. Femtochemistry occurs when the laser imparts energy over an extremely short time period so as to directly induce a surface chemical reaction [86]. 

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. 

Some recent advances in stimulated desorption were made with the use of femtosecond lasers. For example, it was shown by using a femtosecond laser to initiate the desorption of CO from Cu while probing the surface with SHG, that the entire process is completed in less than 325 fs [90]. The mechanism for this kind of laser-induced desorption has been temied desorption induced by multiple electronic transitions (DIMET) [91]. Note that the mechanism must involve a multiphoton process, as a single photon at the laser frequency has insufScient energy to directly induce desorption. DIMET is a modification of the MGR mechanism in which each photon excites the adsorbate to a higher vibrational level, until a suflBcient amount of vibrational energy has been amassed so that the particle can escape the surface. 

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... 

The theoretical description of photochemistry is historically based on the diabatic representation, where the diabatic models have been given the generic label desorption induced by electronic transitions (DIET) [91]. Such theories were originally developed by Menzel, Gomer and Redhead (MGR) [92,93] for repulsive excited states and later generalized to attractive excited states by Antoniewicz [94]. There are many mechanisms by which photons can induce photochemistry/desorption direct optical excitation of the adsorbate, direct optical excitation of the metal-adsorbate complex (i.e., via a charge-transfer band) or indirectly via substrate mediated excitation (e-h pairs). The differences in these mechanisms lie principally in how localized the relevant electron and hole created by the light are on the adsorbate. 

In addition to thermal desorption, gas desorption has been found to result from electron, ion and photon bombardment of surfaces. Therefore, simultaneous particle and photon bombardments can be expected to alter desorption rates, as well as the nature and charge distribution of the desorbed species. Furthermore, simultaneous bombardment of a surface by neutrons and ions could affect diffusion processes, e.g., by radiation-induced segregation. In turn, desorption processes can be influenced by altering the diffusion of species from the bulk to the surface. The type, energy, and angular distribution of particles expected to strike neutral beam injector dump areas (such areas can represent 1/9 of total first wall area) can cause synergistic effects on gas desorption which can be quite different from those expected from the interaction of plasma radiations with the first wall. 

Laser-induced desorption of CO and CO+ from Pt(l 11) is observed by Fukutani et al. [12]. Upon laser irradiation on the CO-saturated Pt(l 1 1) surface at X = 193 nm with a laser fluence of 4mJ/cm2 for 10, 20, and 30 min, the on-top CO decreases gradually in RAIRS intensity, as shown in Fig. 12b-d, respectively. The effects of laser irradiation are clearly demonstrated in the difference spectra shown in Fig. 12. On-top CO decreases, while bridge CO remains unchanged. Neutral CO desorbed from the surface is clearly observed by the REMPI method. In addition to neutral species, CO+ ion desorption is also observed. Figure 19 displays the evolution of the desorption yield as a function of accumulated incident photon numbers at X = 193 nm with a laser fluence of 5.6 mJ/cm2 after a CO exposure of 2 L. Decay rates of the desorption intensity represented by an exponential decay are identical for neutral and ion species within the experimental error and the cross section s is 3 x 10 19 cm2. This result suggests that neutral CO molecules and CO+ ions are desorbed from the same adsorption species. 

The experimental results obtained by Fukutani et al. [8, 11] for NO and CO desorption from Pt(l 11) are summarized in Table 3. These results show clearly that the desorption is via a non-thermal process and induced by an electronic transition. The experimental results for NO desorption from Pt(00 1) at various photon energies observed by Fukutani et al. [5] are similar to those from Pt( 1 1 1) at A. = 352nm. Both rotational and vibrational temperatures from Pt(l 11) at X = 193 nm are higher than those at A. = 352 nm. Thus, the desorption from Pt(l 11) at X = 193 nm might proceed by a different path. 

The range of adsorption processes that can occur on metal oxide surfaces is very broad these will be discussed in many of the other chapters in this book. Chapter 5 considers in detail the atomic positions of adsorbed moieties on several different oxide surfaces. The use of vibrational spectroscopies as a complement to electronic techniques is discussed in Chap. 13. Chapter 15 considers desorption from oxide surfaces induced by incident electrons or photons. 

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