Photoelectron Spectroscopy

1 Introduction

Photoelectron spectroscopy remains an active field of research at both the fundamental spectroscopic level and also as regards its applications to problems in chemistry and physics. The scope of the technique as applied to a variety of chemical problems has been discussed in some review articles. These cover both gas-phase studies using U.V. exciting radiation ('u.v. molecular photoelectron spectroscopy') and solid state work using soft X-radiation ('X-ray photoelectron spectroscopy').

The proceedings of a major conference on electron spectroscopy in general, held at Asilomar, California, in September 1971, have recently been published. The contents provide a most illuminating perspective of the subject. Also now generally available are the proceedings of a conference on molecular spectroscopy (including photoelectron spectroscopy), held at Brighton, also in September 1971. These two publications will be fully reviewed in next year's Report, though we shall below make reference to any work germane to the 1971 literature.

Special mention should also be made of a recent review by Brundle on 'The Application of Electron Spectroscopy to Surface Studies'. A particularly valuable feature of this article for our purposes is the discussion of various branches of electron spectroscopy, such as Auger spectroscopy, which are not dealt with in the present Report.

2 Ultraviolet Molecular Photoelectron Spectroscopy

Instrumental. Few major advances have been reported this year, although there have been a number of interesting developments. High-quality work continues to appear from many types of analyser: however, it is worthy of note that in practice, equally high resolution may be obtained from the simpler analysers as from the more complex designs. For example, Price et al., using a conventional 10 cm radius 127° cylindrical analyser, have resolved the spin-orbit splitting in the first p.e. band of O2, first reported by Edqvist et al. in 1970; the latter used a variable retarding field with a 10 cm spherical analyser. A very simple and inexpensive spectrometer design, combining a variable retarding field with a very small deflection analyser, has been described. The results from this instrument, which is capable of 25 meV resolution, suggest that for many applications a relatively unsophisticated system is quite adequate. The use of a multi-channel analyser with a simple spherical-grid retarding spectrometer has been described by Delwiche et al. Although the resolution, ca. 30meV at best, is remarkable for this type of electron analyser, the sensitivity of the equipment described is very much less than is readily attainable using a conventional deflection analyser and counting system at much lower total cost.

The application of a type of analyser new to u.v.–p.e. work, the cylindrical mirror, has been described by Berkowitz, who also incorporated a high-temperature molecular beam generator in his instrument. The system in this mode, however, gives a performance which seems inferior to that of the new Perkin-Elmer heated inlet system on a more traditional instrument (see below). Studies on HF and DF also failed to establish any significant advantage for this analyser over the conventional type previously used by Brundle to study the same molecules. Moreover, the combination of a focussing lens system feeding a spherical analyser has been shown by Heddle to be superior theoretically (in terms of étendue) to the cylindrical mirror, although it has to be admitted that such an arrangement is inherently much more complex than a simple deflection analyser.

The variation of sensitivity with electron kinetic energy in deflection analysers has been discussed by Berkowitz and Guyon: they point out that the 'l/KE' factor frequently used to correct experimental intensities is not universally applicable. Moreover, in retarding analysers it now appears that whether the sensitivity is constant or declines with increasing electron kinetic energy depends on the optics of the retarding system used, and the position here is thus less straightforward than was implied in last year's Report.

Research by Perkin-Elmer Ltd. has now produced a simple and very effective method for running less-volatile materials. Using the lamp plasma as a source of heat, target chamber temperatures of up to ca. 500°C have been attained, although only 250° C is guaranteed in the current PS 18 model incorporating the device. This development should greatly extend the scope of vapour-phase u.v.–p.e. spectroscopy in the immediate future. As an alternative to a separate u.v.–p.e. instrument, the manufacturers of the established commercial X–p.e. equipment (vide infra, p. 22) all now offer accessory U.V. sources for gas-phase work. Varian, for example, claim 1OOOOO counts s-1 at 21 meV resolution (Ar 2P3/2 line) and 45 000 counts s-1 at 14 meV. No sample pressure is specified, but this performance would seem to be competitive with that of the purpose-built Perkin-Elmer PS 16–18 series as far as work at room temperature is concerned.

An increasing number of He-II spectra are now being reported. Although lamps giving virtually 100%, He-II radiation have been produced experimentally, using both windowed and windowless configurations (Figure l), the experimental difficulties continue to prevent their widespread application and many workers are still using photon fluxes of ca. 1 — 2% of the concomitant He-I output. The principal difficulties appear to centre on the problem of maintaining adequate helium purity at the very high current densities necessary for generation of the 304 Å, line in high intensity.

A Compilation of U.V. –P.E. References. — The inorganic molecules whose u.v. –p.e. spectra. have been reported or discussed in 1971 are listed below (Table 1). Some important related 1972 references have also been included, although comprehensive coverage of the 1972 literature has not been attempted. These latter references are enclosed in brackets. References to 1970 papers have been included in Table 1 only if no mention was made of them in last year's Report. Papers including He-II spectra are marked with a superscipt b: these papers usually include He-I spectra as well. A few organic molecules of particular spectroscopic or inorganic interest have been included, but we have not attempted a comprehensive survey of organic u.v.–p.e. work. Work concerned solely with angular distributions is considered separately below, and has not been included in Table 1.

It will be noted that this table is substantially shorter than last year's : nevertheless, it is our impression that there is no diminution of activity in the u.v.–p.e. field. Last year's Report was...