Synchrotron laboratories

Figure 11.3 XANES spectra of pure Cr203 (a) and K2Cr04-4H20 (b). Note the characteristic pre-edge peak in the latter representing the 100% Crvi endmember, which is virtually absent in the 0% Crvl endmember in (a). The data were collected at the Hamburg Synchrotron Laboratory (DESY-HASYLAB).

Figure 7.4 shows the experimental set-up of a typical XAS experiment. For most EXAFS experiments the source is a synchrotron. Laboratory sources are rare and usually do not exhibit enough intensity to get usable EXAFS spectra. 

This increase in X-ray intensity, together with the development of synchrotron laboratories with facilities dedicated to measurement of XAS, has allowed XAS to develop into a relatively routine analytical tool. It is straightforward to measure XAS spectra for transition metal solutions with concentrations greater than 1 mM (ca. 50 ppm) and it is possible, using the most intense synchrotron sources and the most sensitive detectors, to measure high-quality XAS spectra for samples containing less than 1 ng of the metal of interest. Typical sample volumes range from one mL to one pL and with microfocused X-ray beams (see below) even smaller volumes can be studied. 

Fig. 1 A typical ESRF bending magnet spectrum from a 0.8 Tesla dipole magnet. For different synchrotron laboratories this curve can be shifted in both the vertical (intensity) as well as the horizontal (energy) direction. The important feature is that the spectrum is monotonous.

A mini review of the development of the experimental possibilities and the scientific questions that were driving this catalysis oriented research at the different synchrotron laboratories in the first decade since the introduction of combined XAS-XRD can be found in a 1999 manuscript By G. Sankar and J.M. Thomas. Further, and more recent reviews, oriented towards catalysis research have also been published." ... 

Ultraviolet photoelectron spectroscopy (UPS) is a variety of photoelectron spectroscopy that is aimed at measuring the valence band, as described in sectionBl.25.2.3. Valence band spectroscopy is best perfonned with photon energies in the range of 20-50 eV. A He discharge lamp, which can produce 21.2 or 40.8 eV photons, is commonly used as the excitation source m the laboratory, or UPS can be perfonned with synchrotron radiation. Note that UPS is sometimes just referred to as photoelectron spectroscopy (PES), or simply valence band photoemission. 

Unlike traditional surface science techniques (e.g., XPS, AES, and SIMS), EXAFS experiments do not routinely require ultrahigh vacuum equipment or electron- and ion-beam sources. Ultrahigh vacuum treatments and particle bombardment may alter the properties of the material under investigation. This is particularly important for accurate valence state determinations of transition metal elements that are susceptible to electron- and ion-beam reactions. Nevertheless, it is always more convenient to conduct experiments in one s own laboratory than at a Synchrotron radiation focility, which is therefore a significant drawback to the EXAFS technique. These focilities seldom provide timely access to beam lines for experimentation of a proprietary nature, and the logistical problems can be overwhelming. 

The advantages of SEXAFS/NEXAFS can be negated by the inconvenience of having to travel to synchrotron radiation centers to perform the experiments. This has led to attempts to exploit EXAFS-Iike phenomena in laboratory-based techniques, especially using electron beams. Despite doubts over the theory there appears to be good experimental evidence that electron energy loss fine structure (EELFS) yields structural information in an identical manner to EXAFS. However, few EELFS experiments have been performed, and the technique appears to be more raxing than SEXAFS. 

Table 1 lists core levels and their BEs for elements commonly used in technology, which are sufficiendy sharp and intense, and which are accessible to laboratory He I or He II sources (21.2-eV or 40.8-eV photon energy) or to synchrotron sources (up to 200 eV or higher). The analytical approaches are the same as described in the XPS article. For example, in that article examples were given of Si 2p spectra obtained using a laboratory A1 Ka X-ray source at l486-eV photon energy. The... 

Tonner et al. have taken scanning XPS microscopies at the Advanced Light Source Synchrotron Radiation Center of Lawrence Berkeley National Laboratory [2.6]. They investigated a polished and sputter-cleaned surface of mineral ilmenite with the nominal composition FeTi03, and used the Fe 3p and Ti 3p lines for imaging. Using synchrotron radiation they demonstrated spatial resolution of approximately 0.25 p,m. 

In both cases, laboratory X-ray sources may be used and the X-ray measurements taken in 0-29 geometry. For weakly scattering systems synchrotron radiation is helpful. 

The financial support of the CICYT (project no. IN89-0066), DGICYT (project no. PB94-1529), and Consejerfa de Educacidn de la Comunidad de Madrid is gratefully thanked. We also acknowledge the support of NATO (grant CRG 920094) and the assistance of Dr. W. Bras and the Daresbury Laboratory (UK) in the synchrotron experiments. 

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