Ion and electron spectroscopy

Literature reports on interfaces are mainly limited to metallic solids while little is known on ceramic materials, which are mainly ionic solids of nonstoichiometric compounds. The reason for the scarcity of literature reports on ceramic interfaces results from the substantial experimental difficulties in studies of these compounds. Even the most advanced surface-sensitive techniques have experimental limitations in the surface studies of materials. Most of these techniques are based on ion and electron spectroscopy, such as XPS, SIMS, LEED, AFM, and LETS, and are still not adequate to characterize the complex nature of compounds. Namely, these surface techniques require an ultra-high vacuum and therefore may not be applied to determine surface properties during the processing of materials which takes place at elevated temperatures and under controlled gas phase composition. Consequently, the resultant experimental data allow one to derive only an approximate picture of the interface layer of compounds. 

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. 

We use laser photofragment spectroscopy to study the vibrational and electronic spectroscopy of ions. Our photofragment spectrometer is shown schematically in Eig. 2. Ions are formed by laser ablation of a metal rod, followed by ion molecule reactions, cool in a supersonic expansion and are accelerated into a dual TOE mass spectrometer. When they reach the reflectron, the mass-selected ions of interest are irradiated using one or more lasers operating in the infrared (IR), visible, or UV. Ions that absorb light can photodissociate, producing fragment ions that are mass analyzed and detected. Each of these steps will be discussed in more detail below, with particular emphasis on the ions of interest. 

Solutions of alkali metals in liquid ammonia have been studied by many techniques. These include electrical conductivity, magnetic susceptibility, nuclear magnetic resonance (NMR), volume expansion, spectroscopy (visible and infrared), and other techniques. The data obtained indicate that the metals dissolve with ionization and that the metal ion and electron are solvated. Several simultaneous equilibria have been postulated to explain the unique properties of the solutions. These are generally represented as follows ... 

B.M. If all the Ti ions in the sample had formed superoxo species upon interaction with H2O2, the effective magnetic moment should have been 1.73-1.78 B.M. The concentration of superoxo-Ti species is, thus, about 45% of the total Ti, comparable to the values found by EPR (55%) and electronic spectroscopies. The remaining fraction is, presumably, the diamagnetic hydroperoxo-/peroxo-Ti species. 

Direct observation of point defects in metals has been possible by field ion microscopy. Impurity point defects may be usefully investigated by electron microscopy in combination with electron diffraction and electron spectroscopy. Direct observation of the dopant environment in fluorites has been attempted by Catlow et al. (1984) by employing EXAFS in conjunction with computer simulation. 

J. Ullrich, R. Moshammer, A. Dorn, R. Domer, L.Ph.H. Schmidt, H. Schmidt-Bocking, Recoil-ion and electron momentum spectroscopy Reaction-microscopes, Rep. Prog. Phys. 66 (2003) 1463. 

A major effort was devoted to the characterization of these catalysts by vibrational and electronic spectroscopies [28i]. Use of IR and Raman spectroscopies confirmed that the monolayer consisted of vanadyl complexes, exhibiting both Lewis and Bronsted sites, and suppressing nucleophilic sites on the support. According to UV-VIS spectra, most of the vanadium in the calcined catalysts is in the Vv state, but part is easily reducible. Evidence for electronic interaction between vanadium and titanium ions was also presented. The monolayer is effective in suppressing Raman bands due to the anatase support, and an explanation for this effect has been offered [30]. 

Static Secondary Ion Mass Spectrometry and Electron Spectroscopy for Chemical Analysis... 

During the last 25 years X-ray spectroscopy has been intensively developed for plasma diagnostics. Since the first application of X-ray spectrometers on the early fusion devices such as PLT and TFR, it has been used to determine basic plasma parameters such as the temperature of ions and electrons. It is now frequently being applied not only to low density plasmas in tokamaks and astrophysical objects [1], but also to laser-produced plasma [2]. It has been shown, that the precision of plasma parameters as obtained from X-ray spectroscopy is competitive to the standard methods for plasma diagnostics, such as Thomson scattering and charge exchange spectroscopy for electron and ion temperature, respectively [3]. 

Practical Surface Analysis, D. Briggs, M.P. Seah, Eds. Vol. 1, 2nd ed. (1990) Auger and X-ray Photoelectron Spectroscopy, Vol. 2 (1992), Ion and Neutral Spectroscopy. Wiley. (A standard book with emphasis on techniques and applications. Vol. 1 electron spectroscopy. Vol. 2 ion spectroscopy.)... 

It is important to consider the connection between the two types of studies. One often refers to the "pressure gap" that separates vacuum studies of chemisorption and catalysis from commercial catalytic reactions, which generally run above —often well above — atmospheric pressure. There is simply no way to properly simulate high pressure conditions in a surface analysis system. Reactions can be run in an attached reaction chamber, which is then pumped out and the sample transferred, under vacuum, into an analysis system equipped for electron, ion and photon spectroscopies. However, except for some optical and x-ray methods that can be performed in situ, the surface analytical tools are not measuring the system under reaction conditions. This gap is well recognized, and both the low- and high-pressure communities keep it in mind when comparing their results. 

More recent experiments by Walker et al. [37] using linearly and circularly polarised light and ion TOP spectroscopy confirmed the existence of the knee but, more importantly, electron-ion and electron-electron coincidence experiments were performed. Using the electron-ion coincidence technique, it was found that the great majority of the Xe ions were produced in a sequential process. Moreover, direct double ionization was not observed in these electron-electron coincidence experiments. Charalambidis et al. [38] considered this long-standing question of direct double ionization in Xe and also concluded that it does not occur. That does not mean that correlated double emission cannot occur under appropriate conditions. 

For many years, a lively controversy centered over the actual existence of nonclassical carbocalions. " The focus of argument was whether nonclassical cations, such as the norbornyl cation, are bona fide delocalized bridged intermediates or merely transition states of rapidly equilibrating carbenium ions. Considerable experimental and theoretical effort has been directed toward resolving this problem. Finally, unequivocal experimental evidence, notably from solution and solid-state C NMR spectroscopy and electron spectroscopy for chemical analysis (ESCA), and even X-ray crystallography, has been obtained supporting the nonclassical carbocation structures that are now recognized as hypercoordinate ions. In the context of hypercarbon compounds, these ions will be reviewed. 

The method is based on the ability of TCNQ to react with bonded nitrogen compounds (secondary, tertiary amines) to form stable ion-radical salts (IRS) under mild conditions and with high yields. Completeness of formation of stable IRS has been monitored by ESR and electronic spectroscopy. One of the most important features of this method is the formation of two-dimensional phase by TCNQ anion-radicals on the surface, if bonded donor molecules are fixed at a small distance between their anchoring points, i.e. when separation between points of fixation is in the range 1-1.5 nm. ESR spectra of such samples contain an exchange-narrowed singlet with AH = 0.5 — 1.1 Gs, and g-factor 2.0025. Such spectra are typical for crystalline samples of IRS of TCNQ [25]. If bonded donor molecules are separated by as much as 2 nm or more, a distinct phase of TCNQ IRS is not formed. ESR spectra of such samples reveal either dipole-dipole broadening or hyperfine structure. 

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