Surfaces spectroscopy

X-ray photoelectron spectroscopy (XPS) is based on the photoelectric effect an atom absorbs a photon of energy hv next, a core or valence electron with binding energy Eb is ejected with kinetic energy (Fig. 10.6)  

Photoelectron peaks are labelled according to the quantum numbers of the level from which the electron originates. An electron coming from an orbital with main quantum number n, orbital momentum l (0,1,2,3,..indicated as s, p, d,f,..) and spin momentum s (+1/2 or -1/2) is indicated as nl +s. Examples are Rh 3ds/2, Rh 3 3/2,0 Is, Fe 2p3/2, Pt 4/7/2. For every orbital momentum l 0 there are two values of the total momentum / = 1 + 1/2 and j = 1-1/2, each state filled with 2j + 1 electrons. Hence, most XPS peaks come in doublets and the intensity ratio of the components is (/ +1)//. In case the doublet splitting is too small to be observed (as in practice with Si 2p, A12p, Cl 2p), the subscript / + s is omitted. 

Because a set of binding energies is characteristic for an element, XPS can be used to analyze the composition of samples. Almost all photoelectrons used in laboratory XPS have kinetic energies in the range of 0.2 to 1.5 keV, and probe the outer layers of the catalyst. The mean free path of electrons in elemental solids depends on the kinetic energy. Optimum surface sensitivity is achieved with electrons at kinetic energies of 50-250 eV, where about 50% of the electrons come from the outermost layer. 

One can use the Zr/Si intensity ratio to investigate the thermal stability of the catalysts. As Fig. 10.8b shows, the impregnated catalysts show a decrease in Zr/Si ratio at relatively low calcination temperatures, indicating that the zirconium phase looses dispersion. The catalyst prepared via the new ethoxide route is much better resistant to sintering. Dispersion values can be calculated from the 

Zr/Si ratio, by using a model, for example the one published by Kuipers et al. [22]. When applied to the spectra of Fig. 10.8, Kuipers model indicates that the ZrC 2 dispersion of the three impregnated and calcined catalysts is 5 to 15%, whereas the dispersion of the catalyst from ethoxide is around 75%. 

We are particularly concerned about assumption (3), as surface spectroscopy has recently shown that in addition to growing chains a substantial amount of carbidic carbon develops on the catalyst surface (35-37). While only part of it might be reaction intermediate under steady state conditions, it would be converted almost completely to methane and small amounts of higher hydrocarbons when the surface is exposed to hydrogen in the absence of CO (37). 

The approach by Dautzenberg et al. emphasizes in our opinion a valid and hitherto hardly exploited inroad to the FT kinetics. In retrospect, with the knowledge now available from surface spectroscopy, an experimental verification of the complicating factors mentioned appears desirable. This would be valuable in particular because the main conclusion, viz. a high coverage of steady-state catalysts with growing chains, has not been confirmed by the in. situ IR studies, discussed in the next section. 

The vast amount of work performed with surface sensitive spectroscopic methods, notably X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS), has changed drastically our notion of the preferred modes of chemisorption of carbon monoxide on transition metals. Less than one decade ago Ford (38) in his authorative review still stated tungsten to be unique among the transition metals in being able to 

In the present context attention will be focused on those studies performed with surface spectroscopy that dealt with hydrocarbon-producing systems. Broadly speaking these studies use either electron spectroscopy (UPS, XPS, and AES) or in situ IR as central technique, and accordingly the review is subdivided in two sections. In Section IV,C an integrating discussion will be attempted. 

Electron spectroscopy involves the detection of electrons escaping from a catalyst surface under photon or electron bombardment. The conventional applications of these techniques therefore require the specimen to be situated in a vacuum ofl0 Pa(s 10 Torr) or even lower pressure. The catalytic reaction thus has to be interrupted prior to spectroscopic analysis, so the information is confined to stable, that is, evacuation-resistant adsorbed layers present on the catalyst after interruption of the FT synthesis. 

In contrast to the previous section where we considered infrared radiation that passed through a thin film and was partially absorbed, we now consider emission from a thin film when we bombard it with radiation. Therefore, we now consider a variety of excitation sources and an equally large number of emitted or backscattered particles or photons. For excitation sources, we have  

For all of the techniques to be discussed, the process has to be carried out under high vacuum conditions. For this reason, they are not useful for routine process control. 

A slightly different approach uses a resonant nuclear reaction. For example, N incident on hydrogen produces a high-energy y-particle via the following reaction when the nitrogen beam is at the resonance energy, 6.385 MeV  

A wide range of other nuclear reactions have been used in material science for the depth-profiling analysis of a number of different elements, some of which are tabulated by Feldman and Mayer (1986). Whether these techniques will in the future prove useful for polymers remains to be seen. 

Our purpose here is to provide an overview of these techniques and the information they can provide. The reader who needs to know in more depth about the use of the techniques is referred to one of the many books and review articles on the subject, for example Briggs (1990) and Sabbatini and Zambonin (1993). 

A commercial x-ray photoelectron spectrometer uses a fixed anode x-ray source, typically producing magnesium Ka or aluminium Ka radiation, which directs a beam at the sample surface. The sample chamber is held at ultra-high vacuum and the resulting photoelectrons are collected and their energy analysed in an electrostatic analyser such as a cylindrical mirror analyser. 

The basis of the x-ray spectroscopy technique is of course the photoelectric effect an incident photon of fi-equency v can result in the photoemission of an electron with kinetic energy that is related to the binding energy of the electron Eq by 

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The data above were collected in UHV environment to achieve the most pristine surface. Spectroscopy in air is usually more difficult to interpret due to contamination with oxides and other species, as is the case with all surface-sensitive spectroscopies. 

Several features of ISS quantitative analysis should be noted. First of all, the relative sensitivities for the elements increase monotonically with mass. Essentially none of the other surface spectroscopies exhibit this simplicity. Because of this simple relationship, it is possible to mathematically manipulate the entire ISS spectrum such that the signal intensity is a direct quantitative representation of the surface. This is illustrated in Figure 5, which shows a depth profile of clean electrical connector pins. Atomic concentration can be read roughly as atomic percent direcdy from the approximate scale at the left. 

Surface Spectroscopy of Platinum-Cadmium SuUide-PerfluorosuUbnate Polymer Systems... 

KAKUTA ET AL. Surface Spectroscopy of Pt-CdS-Perjluorosulfonaie Polymer 567 Experimental... 

One major interest in vibrational surface spectroscopy is the ability to directly probe lipid layers. Similarly to the previous case, the structure of the alkyl chains of phospholipids is readily determined from the ratio of the magnitude of the CH2 and CH3 symmetrical stretching modes [136,137]. At the D2O-CCI4 interface, a layer of... 

J. W. Linnett. There were 11 papers with theoretical inputs but with more emphasis given to new developments in experimental methods including structural (LEED and electron microscopy) and surface spectroscopies. LEED provided crucial evidence for the role of surface steps at platinum single crystals in the dissociation of various diatomic molecules, while electron microscopy revealed the role of dislocations as sites of high reactivity of... 

In this chapter, we have chosen from the scientific literature accounts of symposia published at intervals during the period 1920 1990. They are personal choices illustrating what we believe reflect significant developments in experimental techniques and concepts during this time. Initially there was a dependence on gas-phase pressure measurements and the construction of adsorption isotherms, followed by the development of mass spectrometry for gas analysis, surface spectroscopies with infrared spectroscopy dominant, but soon to be followed by Auger and photoelectron spectroscopy, field emission, field ionisation and diffraction methods. 

Ammonia oxidation was a prototype system, but subsequently a number of other oxidation reactions were investigated by surface spectroscopies and high-resolution electron energy loss spectroscopy XPS and HREELS. In the case of ammonia oxidation at a Cu(110) surface, the reaction was studied under experimental conditions which simulated a catalytic reaction, albeit at low... 

Table 2.1 Surface chemistry mediated via oxygen transients evidence from surface spectroscopy.

Oxygen activation of molecules at metal surfaces was first established in the 1970s by surface spectroscopies (XPS and UPS) over a wide temperature range (80-400 K). Furthermore, the distinction was made between the reactivity of partially covered surfaces and the relative inactivity of the oxide monolayer. 

Since ion beams (like electron beams) can be readily focussed and deflected on a sample so that chemical composition imaging is possible. The sputtered particles largely originate from the top one or two atom layers of a surface, so that SIMS is a surface specific technique and it provides information on a depth scale comparable with other surface spectroscopies. 

Electrochemical processes are always heterogeneous and confined to the electrochemical interface between a solid electrode and a liquid electrolyte (in this chapter always aqueous). The knowledge of the actual composition of the electrode surface, of its electronic and geometric structure, is of particular importance when interpreting electrochemical experiments. This information cannot be obtained by classical electrochemical techniques. Monitoring the surface composition before, during and after electrochemical reactions will support the mechanism derived for the process. This is of course true for any surface sensitive spectroscopy. Each technique, however, has its own spectrum of information and only a combination of different surface spectroscopies and electrochemical experiments will come up with an almost complete picture of the electrochemical interface. XPS is just one of these techniques. 

Hutter H, Brunner C, Nikolov SG, Mittermayr C, Grasserbauer M (1996) Image surface spectroscopy for two and three dimensional characterization of materials. Fresenius J Anal Chem 355 585... 

Zachara J.M., Kittrick J.A, Harsh J.B. Solubility and surface spectroscopy of zinc precipitates on calcite. Geochim Cosmochim Acta 1989 53 9-19. 

Hamers RJ (1989) Atomic-resolution surface spectroscopy with the scanning tunneling microscope. Ann Rev Phys Chem 40 531-559... 

Polarization Fourier transform infrared surface spectroscopy makes use of light beams polarized in two mutually perpendicular directions (Fig. 3.2), surface-parallel (s-polarization) and surface-normal (p-polarization). 

Nomarski differential in plants with light-stressed foliage Reveals edges in biological microscopy Scanning Surface topography, surface spectroscopy,... 

Ultrahigh vacuum surface spectroscopies can provide far greater breadth and depth of information about surface properties than can yet be achieved using in situ spectroscopies at the aqueous/metaI interface. Application of the vacuum techniques to electrochemical interfaces is thus desirable, but has been plagued by questions of the relevance of the emersed, evacuated surfaces examined to the real electrochemical interfaces. This concern is accentuated by surface scientists observations that in UHV no molecular water remains on well-defined surfaces at room temperature and above (1). Emersion and evacuation at room temperature may or may not produce significant changes in electrochemical interfaces, depending.on whether or not water plays a major role in the surface chemistry. 

All electrochemical techniques measure charge transferred across an interface. Since charge is the measurable quantity, it is not surprising that electrochemical theory has been founded on an electrostatic basis, with chemical effects added as a perturbation. In the electrostatic limit ions are treated as fully charged species with some level of solvation. If we are to use UHV models to test theories of the double layer, we must be able to study in UHV the weakly-adsorbing systems where these ideal "electrostatic" ions could be present and where we would expect the effects of water to be most dominant. To this end, and to allow application of UHV spectroscopic methods to the pH effects which control so much of aqueous interfacial chemistry, we have studied the coadsorption of water and anhydrous HF on Pt(lll) in UHV (3). Surface spectroscopies have allowed us to follow the ionization of the acid and to determine the extent of solvation both in the layer adjacent to the metal and in subsequent layers. 

One other operational detail merits brief mention before applications to surface spectroscopy are considered. Infrared sources decline markedly in intensity at longer wavelengths and therefore PA spectra must be source intensity normalized before peak heights can be ascribed any quantitative significance. It has sometimes been mistakenly supposed that the PA spectrum of graphite could be used to normalize infrared PA spectra. 

This book deals only with the chemistry of the mineral-water interface, and so at first glance, the book might appear to have a relatively narrow focus. However, the range of chemical and physical processes considered is actually quite broad, and the general and comprehensive nature of the topics makes this volume unique. The technical papers are organized into physical properties of the mineral-water interface adsorption ion exchange surface spectroscopy dissolution, precipitation, and solid solution formation and transformation reactions at the mineral-water interface. The introductory chapter presents an overview of recent research advances in each of these six areas and discusses important features of each technical paper. Several papers address the complex ways in which some processes are interrelated, for example, the effect of adsorption reactions on the catalysis of electron transfer reactions by mineral surfaces. 

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