3.3Inelastic scattering techniques: chemical and electronic state information

Figure 3.8. N(E) (bottom) and dN(E)/dE (top) spectra as a function of glancing incidence angle for a bulk sample of Cd (Janssen et al. 1977, reproduced with permission).

1981. Finally there is electron energy loss spectroscopy, which comes in two varieties (EELS and (high resolution) HREELS), the latter being used primarily for studies of surface and adsorbate vibrational structure (Ibach and Mills 1982, Ibach 1994, Lüth 1993/5).

The various analyzers also have acronyms. The magnetic sector spectrometer may be familiar from analysis using EELS in conjunction with TEM or STEM. It has very good energy resolving power, but collects electrons only over a small angular range; this is well suited to the strongly forward peaked scattering which occurs at TEM energies (.100 keV), as illustrated schematically here in ®gure 3.1(d). The retarding ®eld analyzer (RFA) is the same arrangement as used for LEED, ®gure 3.3; the only diVerence is that one ramps, and may modulate the retarding voltage V on the grid (or the sample), collecting all the electrons with energy E.eV, i.e. the RFA is a high pass ®lter. The advantage of the RFA is simplicity and availability, plus the very large angular collection range; the disadvantage lies in the poor signal to noise ratio inherent in diVerentiating the collected signal (once or twice) to get the spectrum of interest.

One can appreciate these points by drawing a spectrum such as ®gure 3.7 and convincing yourself that the signal intensity collected, I, collected by an RFA corresponds to

78 3 Electron-based techniques

Figure 3.9. Electron energy analyzers: (a) and (b) the cylindrical mirror analyzer (CMA), (a) in the normal orientation using a concentrically mounted electron gun; (b) in an oV-axis geometry to accommodate a bulky ®nal magnetic lens (after Venables et al. 1980); (c) the concentric hemispherical analyzer (CHA), with pre-analyzer lenses allowing retardation and variable energy resolution, and/or multichannel detection; (d) a display analyzer used for synchrotron radiation (SR) research, with a multi-channel plate (MCP) parallel recording detector (after Daimon et al. 1995, reproduced with permission).

so that an N(E) spectrum corresponds to diVerentiating the signal once, and a dN/dE spectrum to diVerentiating twice. These two types of spectrum are shown in ®gure 3.8, ignoring the diVerence between N(E) and E´N(E), which is discussed next.

The electron energy analyzers in common use are the cylindrical mirror analyzer (CMA) and concentric hemispherical analyzer (CHA), shown in ®gure 3.9. These are both band pass ®lters, passing a band of energy (DE) at a pass energy E, typically by adjusting a slit width (w) to change the energy resolution DE/E. These analyzers can be operated with retardation, so that the pass energy is less than the energy of the electron being analyzed; this is easier for the CHA, with retarding lenses in front of the analyzer, and can lead to a high energy resolution in the resulting spectrum. If the analyzer is retarded to a constant pass energy, then the spectrum re¯ects N(E), which is often peaked at low energies, since secondary electron emission is strong. If there is no retardation, or if the pass energy is a constant fraction of the analyzed energy, then the spectrum re¯ects E´N(E).

The goal of energy analysis is to combine high energy resolving power r5(E/DE),

Figure 3.10. Energy levels and density of states of aluminum as (a) an atom, (b) a metal and

(c) an oxide (after Bauer 1975, and Chattarji 1976; reproduced with permission).

with a high collection solid angle V. It is not very surprising that nature doesn't like you doing that: it suggests getting something for nothing. So the various analyzers have been optimized by all the tricks one can think of, such as second order focusing, where changing the angle of incidence to the optic axis a, produces aberrations of order a2 or higher. The net result is that the energy resolution looks like

where L is a characteristic size of the analyzer, and A, B and n,2 are constants for the equipment (Roy & Carette 1977, Moore et al. 1989). If one can detect neighboring energies in parallel, so much the better; this can be done relatively easily with the CHA, but is more diYcult with the CMA.

3.3.2Photoelectron spectroscopies: XPS and UPS

A comparison of the three main analytical techniques which use electron emission can be understood in relation to ®gure 3.10. UPS uses ultra-violet radiation as the probe, and collects electrons directly from the valence band, whereas XPS excites a core hole with X-rays. The core line is often split by spin-orbit interactions, whereas the valence

803 Electron-based techniques

line is wider because of band broadening, indicated by the N(E) distributions in ®gures 3.10(b,c). An outline of photoemission models is given by Lüth (1993/5 chapter 6). The wide range of applications can be appreciated from the early case studies compiled by Ley & Cardona (1979) and the text by Hüfner (1996).

The third technique illustrated directly in ®gure 3.10 is AES, which can be excited by (X-ray) photons or, more usually, electrons. The basic Auger process involves three electrons, and leaves the atom doubly ionized. In general, XPS and AES are used for species identi®cation, and core level shifts in XPS can also give chemical state identi®- cation. AES is routinely used to check surface cleanliness. UPS, especially ARUPS, is the main technique for determining band structure (of solids, not just the surface) and can also identify surface states. The surface sensitivity depends primarily on the energy of the outgoing electron.

Some details about the X-ray sources and monochromators used are given by Lüth (1993/5, panel 11) where the importance of synchrotron radiation sources to current research is emphasized. These sources have high intensity over a range of energies and very well de®ned direction, so that they are well suited to AR-studies; such studies form a substantial part of the wide-ranging programs at synchrotron facilities such as the Advanced Light Source (ALS) in the USA, the Daresbury Synchrotron Radiation Source (SRS) in the UK, HASYLAB or BESSY in Germany, the ESRF in France or the SPring 8 in Japan, to name only a few. Much useful information on these and other programs can be obtained directly via the internet, as indicated in Appendix D.

Until one has visited one of these installations, it is diYcult to grasp the scale and complexity of the operation. Although the end product research overlaps strongly with that coming out of small scale laboratories, the tradition derives more from large budget particle physics, with the consequent need for substantial long range planning and technical backup. By the time one gets to the individual researcher/user, who is typically based at a university or industrial laboratory located some distance away, and who has a limited amount of `beam time' allotted on a particular `station', one is into social structures in addition to science. Safety training, where to (and how much) sleep, group organization and continuity are all extremely important factors in¯uencing whether good work is produced. Stress is important as a spur to achievement in science, but sometimes it can get out of hand. As one who has never actually worked in such a facility, I can imagine that it takes a bit of getting used to, and strategies for eVective working need to be thought about explicitly. Nonetheless, the upside is that all this wonderful equipment, and expert help, is available to help you produce the results you need!

There have been several attempts to develop display analyzers, where the angular information is displayed in parallel at a given energy, which is swept serially (or vice versa). All these analyzers are technically demanding attempts to utilize the beam time and low counting rate eYciently, and have typically been constructed for a synchrotron environment (Eastman et al. 1980, Leckey et al. 1990, Daimon 1988, Daimon et al. 1995). This last spectrometer is shown in ®gure 3.9(d), indicating that several ®nely fashioned grids are required to keep the ®elds in the diVerent regions of the spectrometer isolated from each other. Designing a usable wide angle, gridless analyzer design remains quite challenging (e.g. Huang et al. 1993), but there are always some projects