Resonance from adsorbed atom

Figure 26. Comparison of the resonance scattering from H atoms or H+ obtained by fitting the Fano line shape in HCIO4 (open squares) and H2SO4 (closed squares) with the adsorbed hydrogen coverage (closed circles) and sulfate adsorption (open circles) obtained by cyclic voltammetry. (Reproduced with permission from ref 50. Copyright 2001 The Electrochemical Society, Inc.)...

Fig. 2.5 An ion kinetic energy distribution of field desorbed He ions taken with a pulsed-laser time-of-flight atom-probe. In pulsed-laser stimulated field desorption of field adsorbed atoms, atoms are thermally desorbed from the surface by pulsed-laser heating. When they pass through the field ionization zone, they are field ionized. Therefore the ion energy distribution is in every respect the same as those in ordinary field ionization. Beside the sharp onset, there are also secondary peaks due to a resonance tunneling effect as discussed in the text. The onset flight time is indicated by to, and resonance peak positions are indicated by arrows. Resonance peaks are pronounced only if ions are collected from a flat area of the...

The electromagnetic mechanism is necessary to observe the SERS effect from adsorbed molecules, providing enhancement factors of more than 10". The chemical enhancement mechanism usually gives rise to factors only up to 10, but however it is important because both position and relative intensity of the SERS bands closely depend on this effect [9]. The latter is due to the formation of complexes between ligand and metal active sites, which represent atomic-scale defects of the surface. Hence, the chemical SERS effect can be considered mainly due to the resonance between the incident radiation and the electronic excitation of the ligand-metal adduct. 

It is common to nearly all optical techniques that the physical interpretation of measured signals and even of the changes of physical parameters evalrrated from the primary signal is difficult. As far as adsorption induced changes of clean surface properties are concerned, the information obtained from optical techniques is therefore rather indirect. In some cases spectral resonance featrrres associated with the surface (interface) may be detected, and their changes with adsorbing atoms or molecules may be usefirl to follow the kinetics of this process but fundamental information on the properties of the adsorbed species is rarely obtained. 

Atomic hydrogen is small and adsorbs near to the surface. H modes can directly couple to surface states and resonances, such as image potential states, that may overlap in the near-surface region. An empirical verification of this phenomenon, observed on several metals (Pd, Pt, Rh, Ru) is the enhancement of surface resonances by adsorbed H, and the enhancement of H vibrational modes at primary energies which correspond to the population of the surface resonance with electrons from the incident HREELS beam. In Figure 17, the reflectivities of the Pd(lll) and (100) surface with and without adsorbed hydrogen are shown. The relative intensities of the H frustrated translation and rotation (perpendicular and parallel) modes are shown in Figure 18. 

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 evaluation of amount of silver atoms desorbed from resonator during 80 h operation was made applying the known relations [36] for silver atoms adsorbed on the sensor surface in charged form accounting for the fraction of atoms transferred from the source (resonator) to the target (sensor). The estimates indicate that intensity of the flux of silver atoms from the surface of resonator in this experiment was about 1.5-10 3 m 2-s S i.e. during 75 h operations approximately 10% of the surface silver atoms left resonator (under condition that the amount of surface silver atoms is about 4-10 m ) explaining the shift of resonant frequency from its nominal value 10 MHz by 11 Hz. 

Obviously, chemisorption on d-metals needs a different description than chemisorption on a jellium metal. With the d-metals we must think in terms of a surface molecule with new molecular orbitals made up from d-levels of the metal and the orbitals of the adsorbate. These new levels interact with the s-band of the metal, similarly to the resonant level model. We start with the adsorption of an atom, in which only one atomic orbital is involved in chemisorption. Once the principle is clear, it is not difficult to invoke more orbitals. 

The importance and effect of tip electronic states were discussed by many authors (Tersoff, 1990 Tersoff and Lang, 1990 Doyen et al., 1990 Behm, 1990 Lawunmi and Payne, 1990 Sacks and Noguera, 1991). Doyen et al. (1990) made a first-principles calculation of a realistic W tip. They found that the electronic structure of the W tip exhibits a 54= resonance near the Fermi level, which is the most possible origin of atomic resolution. Sacks and Noguera (1991) noted that. v and d states dominate the DOS of the W surface near the Fermi level, and p states could arise from an adsorbed foreign atom. They have also derived the necessary formalism to account for the effect of p... 

In the process of photocatalysis, the electrons and holes produced on photoirradiated Ti02 powders are trapped at the particle surface to form unpaired-electron species (step (4) in Fig.D.3). Photocatalytic reactions are actually the reactions of these radicals with reactant molecules at the Ti02 surface. Electron spin resonance (ESR) spectroscopy has been used for the detection of the photoproduced radicals on Ti02 at low temperatures such as 77 K. It has been reported that photoproduced electrons are trapped at various different sites titanium atoms on the surface or inside the particles, or oxygen molecules adsorbed on the surface. On the other hand, photoproduced holes are trapped at lattice OAygen atoms near the particle surface or at surface hydroxyl groups. We analyzed these radical species for several Ti02 photocatalysts that are commercially available, and found that the differences in the photoproduced radicals resulted from different heat-treatment conditions and the reactivity with several molecules.17)... 

The reversible adsorption of H2 and of D2 on Cr203 at liquid-air temperatures, with initial heats of adsorption of 5.1 and 5.4 kcal./mole respectively (105), are active in the exchange of H2 and D2 and we must, consequently, assume that both gases are adsorbed in the atomic form. We may think of a sharing of electrons between Cr3+ ions and H atoms, while also an electron transfer from the H atom to a Cr3+ ion, forming a Cr2+ ion, may be considered. Both possibilities may contribute to the real situation, the bond being a resonance between them ... 

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