Metastable metal surface

Most of the publications dedicated to the interaction between the RGMAs and a solid surface refer to the rare gas - metal system. The secondary electron emission that occurs in the system allows one to judge of the mechanism that deactivates metastable atoms on a metal surface, as well as to evaluate the concentration of metastable atoms in the gaseous phase. 

Oliphant and Moon theoretically considered the possibility of electron emission by resonance ionization of metastable atoms near a metal surface. Shekter [122] investigated the Auger-neutralization of ions on a metal surface. Hagstrum [124, 125] carried out an generalized analysis of metastable atoms with a metal surface. 

Fig. 5.18. Energy diagrams of possible mechanisms for deactivation of rare gas metastable atoms on a metal surface [126]...

This relationship of the metastable atom deactivation mechanisms is valid for atomically pure metal surfaces and is proved true in a series of works [60, 127, 128]. Direct demonstrations of resonance ionization of metastable atoms near a metal surface are given by Roussel [129]. The author observed rebound of metastable atoms of helium in the form of ions from a nickel surface in the presence of an adsorbed layer of potassium. In case of large coverages of the target surface with potassium atoms, when the work of yield becomes less than the ionization potential of metastable atoms of helium, the signal produced by rebounded ions disappears, i.e. the process of resonance ionization becomes impossible and the de-excitation of metastable atoms starts to follow the mechanism of Auger deactivation. 

Adsorbed layers, thin films of oxides, or other compounds present on the metal surface aggravate the pattern of deactivation of metastable atoms. The adsorption changes the surface energy structure. Besides, dense layers of adsorbate may hamper the approach of metastable atom sufficiently close to the metal to suppress thus the process of resonance ionization. An example can be work [130], in which a transition from a two- to one-electron mechanism during deactivation of He atoms is exemplified by the Co - Pd system (111). The experimental material on the interaction of metastable atoms with an adsorption-coated surface of... 

The results of work [ 135] are of specific interest. The work surveyed the influence of the nature and structure of adsorbed layers upon the mechanism of deactivation of He(2 S) atoms. It has been shown that on a surface of pure Ni(lll) coated with absorbed bridge-positioned molecules of CO or NO, the deactivation of metastable atoms proceeds by the mechanism of resonance ionization with subsequent Auger-neutralization. With large adsorbent coverages, when the adsorbed molecules are in a position normal to the surface, deactivation proceeds by the one-electron Auger-mechanism. The adsorbed layers of C2H4 and H2O on Ni(lll) de-excite atoms of He(2 S) by the two-electron mechanism solely. In case of NH3 adsorption, both mechanisms of deactivation are simultaneously realized. Based on the given data, the authors infer that the nature of metastable atoms deactivation on an adsorbate coated metal surface is determined by the distance the electron density of adsorbate valance electrons is removed from the metal lattice. 

The presence of adsorbed layers also affects the other parameters of the interaction between metastable atoms and a metal surface. Titley et al. [136] have shown that the presence of an adsorbed layer of oxygen on a W( 110) surface increases the reflection coefficient of helium metastable atoms. The reflection is of irregular nature and grows higher when the incidence angle of the initial beam increases. A series of publications [132, 136, 137] indicate that the presence of adsorbed layers causes an increase in the quantum yield of electron emission from a metal under the action of rare gas metastable atoms. 

The most obvious way to raise the sensitivity of sensors to RGMAs is by activating their surface with additives that actively interact with metastable atoms and have some electron coupling with semiconductor. These additives can be microcrystals of metals. As previously shown, the de-excitation of RGMAs on a metallic surface truly proceeds at high efficiency and is accompanied by electron emission. Microcrystals of the metal being applied to a semiconductor surface have some electron coupling with the carrier [159]. These two circumstances allow one to suppose that the activation of metals by microcrystals adds to the sensitivity of semiconductor films to metastable atoms. 

Oxygen chemisorption at cryogenic temperatures provided the clue for the presence of metastable reactive oxygen states at metal surfaces, with XPS... 

The discussion of concentration polarization so far has centred on the depletion of electroactive material on the electrolyte side of the interface. If the metal deposition and dissolution processes involve metastable active surface atoms, then the rate of formation or disappearance of these may be the critical factor in the overall electrode kinetics. Equation (2.69) can be rewritten for crystallization overvoltage as... 

Halley et al. employed a MD method for the simulation of metal/water interfaces.72 They found that the occupancy of on-top binding sites for water in this model as applied to a (1 0 0) surface of copper was very sensitive to potential. They suggested that this may provide an explanation for some previously unexplained features of X-ray data on water structure and noble metal/water interfaces. They also noticed that the strong bonding of water on a metal surface may result in metastable charging of the interface in molecular dynamics timescales. 

However, the deposition of carbon does not take place on metal surfaces, such as platinum. The reason for this effect is the high catalytic efficiency of pure metals for surface recombination of atoms and metastables, which leads to a decrease of the plasma energy near the surface. Consequently, inequality (2a ) is not fulfilled there (cf.28>). 

Clearly, for all metal surfaces, the formation of formate is much more favorable, both thermodynamically and kinetically, the difference between the two activation barriers increasing from 9 to 13 to 37 kcal/mol along the series Fe/W < Ni < Ag. Qualitatively, this model prediction is in full agreement with the fact that a formate species appears to be a ubiquitous metastable intermediate in the decomposition of HCOOH on Mo (122), Ru (723), Rh (124), Ni (725), Pd (726), Pt (727), and Cu (128), but a formyl species has never been observed (102a, 129). 

Electroless plating. This process involves autocatalyzed decomposition or reduction of a few selected metastable metallic salt complexes on substrate surfaces. The reaction should be carefully controlled to avoid potential decomposition of the metal film thus formed and to control the film thickness. The induction period of the reaction can be appreciable and an effective way of reducing the period is to preseed the substrate in advance with nuclei of the metal to be deposited in an activation solution [Uemiya et al., 1990]. 

The model of electron transfer in gas-phase metal-molecule reactions can be extended to more complex systems such as the collisions of metastable rare gas atoms with molecules to produce negative molecular ions [306], In surface chemistry the harpoon model describes the forces between the reagents after the electron transfer has been applied to reactions of molecules with metal surfaces [120]. Another domain, involving the reaction of metal ions with complex systems could be interpreted in the framework of electron transfers in the porphyrin site of the heme within hemoglobin, addition of oxygen to the Fe " " results in an electron transfer from the metal to the oxygen. The dynamics of this attachement and of the photo-induced detachment could be viewed in that perspective. 

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