Photoelectron instrumentation

This chapter is directed toward both new students and established researchers in organometallic chemistry whose expertise is not in the area of photoelectron spectroscopy. It is recognized that relatively few laboratories have direct access to photoelectron instrumentation, but that many are interested in the information that can be obtained. This chapter will briefly describe the photoelectron experiment and general sample requirements, and the principles for understanding the information contained in the data. The presentation of some "case studies" will illustrate to the reader... 

These principles extend to the study of more complex or-ganometallic molecules and clusters, and current improvements in gas phase and surface photoelectron instrumentation promises to make the use of these techniques more routine than has been the case in the past. Photoelectron spectroscopy is taking a place alongside crystallography, theoretical calculations, and other common techniques for characterization of new organometallic molecules. 

Time-of-flight mass spectrometers have been used as detectors in a wider variety of experiments tlian any other mass spectrometer. This is especially true of spectroscopic applications, many of which are discussed in this encyclopedia. Unlike the other instruments described in this chapter, the TOP mass spectrometer is usually used for one purpose, to acquire the mass spectrum of a compound. They caimot generally be used for the kinds of ion-molecule chemistry discussed in this chapter, or structural characterization experiments such as collision-induced dissociation. Plowever, they are easily used as detectors for spectroscopic applications such as multi-photoionization (for the spectroscopy of molecular excited states) [38], zero kinetic energy electron spectroscopy [39] (ZEKE, for the precise measurement of ionization energies) and comcidence measurements (such as photoelectron-photoion coincidence spectroscopy [40] for the measurement of ion fragmentation breakdown diagrams). 

Energy Spectrometry (EDS) uses the photoelectric absorption of the X ray in a semiconductor crystal (silicon or germanium), with proportional conversion of the X-ray energy into charge through inelastic scattering of the photoelectron. The quantity of charge is measured by a sophisticated electronic circuit linked with a computer-based multichannel analyzer to collect the data. The EDS instrument is... 

Consider what happens if, for example, an ensemble of carbon atoms is subjected to X rays of 1486.6 eV energy (the usual X-ray source in commercial XPS instruments). A carbon atom has 6 electrons, two each in the Is, 2s, and 2p orbitals, usually written as C Is 2s 2p. The energy level diagram of Figure la represents this electronic structure. The photoelectron process for removing an electron from the... 

X-ray Photoelectron Spectroscopy analysis of the samples was performed with a Surface Science Instruments spectrometer (SSI 100) with a resolution (FWHM Au 4f7/2) of 1.0 eV. The X-ray beam was a monochromatised AlKa radiation (1486.6 eV). A charge neutraliser (flood gun) was adjusted at an energy of 6 eV. As the Cls spectra of these compounds were very complex, the binding energies were referenced to the binding energy of Ols, considered experimentally to be at 531.8 eV [8). 

For probing the nature of the acid sites by X-ray photoelectron spectroscopy (XFS), the samples were evacuated before gaseous pyridine was adsorbed. Excess pyridine was desorbed at 1S0°C, and then samples were pressed onto a sample stub imder Nj and loaded into the SCIENTA ESCA-300 instrument without exposure to air. Sample charging was minimized by using a Qood gun while acquiring the experimental data. 

The energies of the Auger electrons leaving the sample are determined in a manner similar to that employed for photoelectrons already described in chapter 2 Section 4. Modern instruments nearly always incorporate cylindrical mirror analysers (CMA) because their high transmission efficiency leads to better signal-to-noise ratios than the CHA already described. 

Modem instrumentation has improved substantially in recent years, which has enabled the measurement of XPS spectra of superior resolution necessary to reveal the small BE shifts present in highly covalent compounds such as those studied here. In a laboratory-based photoelectron spectrometer, a radiation source generates photons that bombard the sample, ejecting photoelectrons from the surface that are transported within a vacuum chamber to a detector (Fig. 2). The vacuum chamber is required to minimize the loss of electrons by absorption in air and, if a very high quality vacuum environment is provided (as is the case with modem instruments), the surface contamination is minimized so that the properties of the bulk material are more readily determined. 

The photoelectron spectrometer is an instrument which scans the range 0 < <... 

The work of Siegbahn s group who, in the 1950s, improved the energy resolution of electron spectrometers and combined it with X-ray sources. This led to a technique called electron spectroscopy for chemical analysis (ESCA), nowadays more commonly referred to as X-ray photoelectron spectroscopy (XPS) [6]. Siegbahn received the Nobel Prize for his work in 1981. Commercial instruments have been available since the early seventies. 

Turner, N. H. (1997). X-ray photoelectron and Auger electron spectroscopy. In Analytical Instrumentation Handbook, ed. Ewing, G. W., New York, Marcel Dekker, pp. 863-914 (2nd edn.). 

X-ray photoelectron spectroscopy is indeed quite informative, but requires the use of expensive instrumentation. Also, the detection of photoelectrons requires the use of ultrahigh vacuum, and therefore can mostly be used for ex situ characterization of catalytic samples (although new designs are now available for in situ studies [146,147]). Finally, XPS probes the upper 10 to 100 A of the solid sample, and is only sensitive to the outer surfaces of the catalysts. This may yield misleading results when analyzing porous materials. 

During the last several years, a number of new instrumental surface techniques have been developed that are quite effective in detecting changes in the surfaces of minerals that have undergone chemically induced or natural geologic alteration. These techniques are quite sensitive (approximately 0.1-0.5% atomic concentration for x-ray photoelectron and Auger spectroscopy, for example), and they make it possible to monitor very small amounts of elements that may be present in the near surface material. Any change in the surface with respect to chemical composition may readily be measured qualitatively... 

Surface characterization studies by X-ray photoelectron spectroscopy (XPS) were conducted using DuPont 650 and Perkin Elmer 5300 instruments. Samples were prepared by placing solid material on double stick adhesive tape, or by allowing solvent to evaporate from an acetone dispersion of a suspension placed on a stainless steel probe. A magnesium anode was used as the X-ray source (hv 1253.6 eV). The temperature of samples during the analysis was approximately 30-40°C and the vacuum in the analysis chamber was about 10 torr. Potential... 

Lifetime instruments using a streak camera as a detector provide a better time resolution than those based on the single-photon timing technique. However, streak cameras are quite expensive. In a streak camera, the photoelectrons emitted... 

Some instrumental methods have been used for the investigation of sulphide mineral-thio-collector system such as infra-red (IR) spectroscopy (Mielezarski and Yoon, 1989 Leppinen et al., 1989 Persson et al, 1991 Laajalehto et al, 1993 Zhang., et al., 2004a) and X-ray photoelectron spectroscopy (XPS) (Pillai et al, 1983 Page and Hazell, 1989 Grano et al, 1990 Laajalechto et al, 1991). These surface sensitive spectroscopic techniques can be applied for the direct determination of the surface composition at the conditions related to flotation. 

We must also take into account two further factors. First, the fact that the transmission efficiency of the analyzer is a fimction of the kinetic energy (K.E.) of the photoelectrons in the ESCA-3 Vacumn Generators instrument the transmission is inversely proportional to the K.E. of the electrons (3a). Second, photoelectron yields must refer to total yield from a particular ionization process and this need not, for example, be just the area of the relevant peak. Account must be taken of all processes that divert electrons from the primary peak, e.g., shake-up, shake-oflF, and plasmon peaks. In some cases, e.g., emission from the Cu 2P3/2 level, the contribution of additional processes is small but in others, and emission from the Al(2p) shell is an example, the no-loss peak is substantially less than the true Al(2p) emission. 

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