Electron spectroscopy
A hybrid version of photoemission spectroscopy and vibrational spectroscopy is electron energy loss spectroscopy (EELS, or HREELS, where HR denotes high resolution) in which the energy loss suffered by a beam of electrons is monitored when they are reflected from a surface. As in Raman spectroscopy, the spectrum of energy loss can be interpreted in terms of the vibrational spectrum of the adsorbate. High resolution and sensitivity are attainable, and the technique is sensitive to light elements (to which X-ray techniques are insensitive). Very tiny amounts of adsorbate can be detected, and one report estimated that about 48 atoms of phosphorus were detected in one sample. As an example, Fig. 25.9 shows the EELS result for CO on the (111) face of a platinum crystal as the extent of surface coverage increases. The main peak arises from CO attached perpendicular to the surface by a single Pt atom. As the coverage increases the neighbouring smaller peak increases in intensity. This peak is due to CO at a bridge site, attached to two Pt atoms, as in (1). A very important technique, which is widely used in the microelectronics industry, is Auger electron spectroscopy (AES). The Auger effect is the emission of a second electron after high energy radiation has expelled another. The first electron to depart leaves a hole in a low-lying orbital, and an upper electron falls into it. The energy these releases may result either in the generation of radiation, which is called X-ray fluorescence (Fig. 25.10a) or in the ejection of another electron (Fig. 25.10b). The latter is the secondary electron of the Auger effect. The energies of the secondary electrons are characteristic of the material present, so the Auger effect effectively takes a fingerprint of the sample. In practice, the Auger spectrum is normally obtained by irradiating the sample with an electron beam of energy in the range 1–5 keV rather than electromagnetic radiation. In scanning Auger electron microscopy (SAM), the finely focused electron beam is scanned over the surface and a map of composition is compiled; the resolution can reach below about 50 nm.
Fig. 25.9 The electron energy loss spectrum of CO adsorbed on Pt (111). The results for three different pressures are shown, and the growth of the additional peak at about 200 meV (1600 cm−1) should be noted. (Based on spectra provided by Professor H. Ibach.)
Fig. 25.10 When an electron is expelled from a solid (a) an electron of higher energy may fall into the vacated orbital and emit an X-ray photon to produce X-ray fluorescence. Alternatively, (b) the electron falling into the orbital may give up its energy to another electron, which is ejected in the Auger effect.
