Photoelectron solid surfaces

Prior to the advent of electron spectroscopy there were no experimental facilities available that could provide, even at the most elementary level, a qualitative analysis of the chemical composition of a solid surface. Surface chemists therefore find acceptable estimates of absolute concentrations which might only be accurate to no better than +20%. There is, however, substantial evidence to suggest that photoelectron spectroscopy can provide data that are at least within these limits and where relative concentrations are being considered, the accuracy is somewhat better. 

The significance of the development of photoelectron spectroscopy over the last decade for a better understanding of solid surfaces, adsorption, surface reactivity, and heterogeneous catalysis has been discussed. The review is illustrative rather than exhaustive, but nevertheless it is clear that during this period XPS and UPS have matured into well-accepted experimental methods capable of providing chemical information at the molecular level down to 10% or less of a monolayer. The information in its most rudimentary state provides a qualitative model of the surface at a more sophisticated level quantitative estimates are possible of the concentration of surface species by making use of escape depth and photoionization cross-section data obtained either empirically or by calculation. 

Fig. 4.9. Schematic of photoemission experiments, A beam of incident photons with energy ftto induces electrons to emit from the sample. The photoelectrons are collected by the velocity analyzer and the electron detector at angles 9 and

Solid metal hydrides specifically have been reviewed here, but XPS and UPS can serve as tools to study vapors or volatile liquids. Much of the original work with these two methods involved organic molecules only later were solid surfaces studied. Therefore, they should always be considered as helpful analytical instruments for examining the bonding chemistry of organometallic compounds. This symposium covered mainly organometallic hydrides, and they are prime candidates for photoelectron spectroscopy study. 

Some of the techniques described in this chapter used most widely today are Auger electron spectroscopy, X-ray photoelectron spectroscopy, electron-probe micro-analysis, low energy electron diffraction, scanning electron microscope, ion scattering spectroscopy, and secondary ion mass spectroscopy. The solid surface, after liberation of electrons, can be analyzed directly by AES, XPS, ISS, and EPMA (nondestructive techniques), or by liberation of ions from surfaces using SIMS (involving the destruction of the surface). Apart from the surface techniques, reflectance-absorbance infrared (RAIR) spectroscopy has also been employed for film characterization (Lindsay et al., 1993 Yin et al., 1993). Some... 

Analytical surface techniques such as Auger Emission Spectroscopy (AES) and X-ray Photoelectron Spectroscopy (XPS) analysis are extremely useful in identifying the chemistry of the solid surfaces (Buckley, 1981 Briggs and Seah, 1990). Table 5.7 is a summary of the XPS spectra data for rollering surfaces in oil containing dibenzyl disulfide under various conditions ... 

Photoelectron spectroscopy is a simple extension of the photoelectric effect involving the use of higher-energy incident photons and applied to the study not only of solid surfaces but also of samples in the gas phase. Equations (8.1) and (8.2) still apply but, for gas-phase measurements in particular, the work function is usually replaced by the ionization energy I,2 so that Equation (8.2) becomes... 

The third problem also concerns the choice of whether to leave out certain material. In a book of this size it is not possible to cover all branches of spectroscopy. Such decisions are difficult ones but I have chosen not to include spin resonance spectroscopy (NMR and ESR), nuclear quadrupole resonance spectroscopy (NQR), and Mossbauer spectroscopy. The exclusion of these areas, which have been well covered in other texts, has been caused, I suppose, by the inclusion, in Chapter 8, of photoelectron spectroscopy (ultraviolet and X-ray), Auger electron spectroscopy, and extended X-ray absorption fine structure, including applications to studies of solid surfaces, and, in Chapter 9, the theory and some examples of lasers and some of their uses in spectroscopy. Most of the material in these two chapters will not be found in comparable texts but is of very great importance in spectroscopy today. 

X-ray photoelectron spectroscopy (XPS) has been recognized as one of the best analytical methods for probing composition and electronic structure of solid surfaces and interfaces. However, it is based on monitoring emitted photoelectrons, which imposes a limitation on the operating pressures consequently, substantial time has passed before this technique was applied in reaction experiments in the near-atmospheric pressure range. 

The susceptibility of solid surfaces to contamination often results in a requirement for an ultrahigh vacuum (UHV) chamber for preparation and observation of particular samples. For many materials, including metals such as platinum and nickel, adsorption of hydrocarbons and chemisorption of oxygen are quite fast at atmospheric pressure, and the surface must be isolated in UHV to prevent rapid degradation. In addition, a sample in UHV may be subjected to surface analytical techniques such as X-ray photoelectron and Auger spectroscopy to verify or corroborate Raman results. As a result, much of the early and well-characterized surface Raman experiments were carried out in UHV chambers operating below 10 torr (12). 

Study of the modification of solid surfaces requires, preferably, surface sensitive methods. Spectroscopic techniques, for example X-ray photoelectron spectroscopy (XPS) and FTIR spectroscopy are excellent tools for gathering information on the chemical surface composition and the kind and number of functional surface groups. The fact that the carbon and nitrogen containing organic phase is only introduced during the adsorption procedure and locally fixed on the outside of the particles allows the use of established methods for polymer and solid-state characterization, particularly NMR and solid-state NMR spectroscopy (e.g. 13C CP MAS NMR). 

The molecular spectroscopy of adsorbed cations and anions has two principal subdivisions (1) invasive methods (such as X-ray photoelectron or secondary-ion mass spectrometry) that require sample desiccation and high vacuum and (2) noninvasive methods that require little or no alteration of a sample from its received condition. Invasive methods have an important role to play in the characterization of solid surfaces (27), but to use them for resolving surface speciation on particles in aqueous systems simply begs the question. 

Secondary ion mass spectrometry (SIMS) is a relatively new technique for surface chemical analysis compared with Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS). SIMS examines the mass of ions, instead of energy of electrons, escaped from a solid surface to obtain information on surface chemistry. The term secondary ion is used to distinguish primary ion that is the energy source for knocking out ions from a solid surface. The advantages of SIMS over electron spectroscopy are ... 

Delamar, M., Correlation between the isoelectic point of solid surfaces of metal oxides and X-ray photoelectron spectroscopy chemical shifts, J. Electron Spectrosc., 53, Cll, 1990. 

During this study, we have found that laser intensity is one of the important factors that control laser surface chemistry. At a small laser intensity, molecules adsorbed on solid surfaces dissociate into atoms and radicals. Some of these atoms or radicals react with atoms of the solid substrates. At a large laser intensity, atoms are photoablated from the solid surfaces to react with the molecules adsorbed or in the gas phase. Hence, we describe in this paragraph a) the dynamical study of UV laser photodissociation of halogen or metal-containing molecules on solid surfaces, b) reactions of atoms generated in the photodissociation of an adsorbate with solid surfaces, and c) reactions of molecules in the gas phase with the photoelectrons or metal atoms generated on intense laser irradiation of solid surfaces. 

X-ray photoelectron diffraction (XPD) is a UHV method that exploits the masking of photoelectron scattering paths by atoms in surface layers as photoelectrons escape from a solid surface to determine surface structure (Williams et al. 1979 Holland et al. 1980 Fadley, 1992 Fadley et al. 1994, 1997). In one of the few applications of XPD to low temperature geochemical or environmental samples, Thevuthasan et al. (1999) determined the structure of the clean surface of hematite (0001). 

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