Gas-phase Photoelectron Spectroscopy

1 Introduction

Previous articles in the Specialist Periodical Reports series dealing with photoelectron spectroscopy (p.e.s.) have adopted a broad approach reviewing all aspects of the technique. To keep this contribution for the extended period 1973 — 1977 within manageable bounds we have restricted our attention to gas-phase p.e.s. While our canvas is clearly defined for ultraviolet p.e.s. there is less distinction between gas- and solid-phase X-ray work; core levels are of a largely atomic nature irrespective of the phase under investigation. This article aims to provide comprehensive coverage of the literature up to the end of 1976 and a more selective discussion of papers published in 1977.

During the period covered by this Report the field has continued to expand and the production of p.e. spectra is now a largely routine procedure in many laboratories. Review articles proliferate and may usefully be divided into three categories. First and least numerous are those that provide a thorough coverage of the literature of a given period. Within this group the u.v.-p.e.s. field from 1973 to 1975 has been covered by Baker et al. and the earlier period from 1971 to 1973 by Betteridge and Williams. The X-p.e.s. work published between 1972 and 1975 has been dealt with by Hercules. The second type of review still has a broad scope but adopts a rather more selective and sometimes personalized approach. Such reviews have been given by Price, DeKock and Lloyd, Jonathan, Carlson, Schweitzer, Bahr, and Siegbahn. In this section we must mention the detailed and regular reviews produced by Siegbahn. While dealing with the general field of p.e.s. these lean heavily upon the substantial contributions made by the Uppsala laboratory.

Finally and forming by far the largest part of this section are the reviews or articles addressing themselves to a specific aspect of p.e.s. Thus :he field of high-temperature inorganic studies is admirably covered by Berkowitz while articles by Heilbronner and Maier deal with organic u.v.-p.e.s. A useful review of instruments commonly used in p.e.s. with particular emphasis on their performance is given by Wannberg et al. while Krause deals with the experimental principles of electron spectroscopy. The close affinities between u.v.-p.e. and Rydberg absorption spectra observed in the vacuum U.V. have been stressed by Sandorfy. A review of gas phase X-p.e.s. carried out at Uppsala has been given by Gelius. This deals with the fine high-resolution work carried out with monochromatic X-rays by the Swedish group. A general article on photoemission from atoms and small molecules has been given by Samson. This dwells particularly on accurate intensity measurements in u.v.-p.e.s. and on the measurement of asymmetry parameters.

Although many of the articles mentioned so far include consideration of photoionization theory and the calculation of binding energies, there are now a number of articles dealing specifically with these topics. These tend to deal either with the process of photoionization (Cooper and Marr) and the associated theory of shake-up and shake-off (Fadley) or with the various approximations used in the calculation of binding energies. For X-p.e.s. this includes the calculation of chemical shifts as covered by Basch, Shirley, Schwartz, and Gelius. As regards valence ionization energies, although there have been significant developments in theoretical techniques (see Section 4) no general review on this topic came to our attention. At an elementary level molecular orbital theory is discussed in relationship to u.v.-p.e. by Bock and M01lère while a more thorough treatment with discussion of various semi-empirical MO models is given by Bock et al.

In addition to the articles so far discussed a number of books devoted to p.e.s. have been published. A text by Carlson deals with Auger spectroscopy as well as X-p.e.s. and u.v.-p.e.s. and a recent volume by Rabalais devoted to u.v.-p.e.s. places particular emphasis on the theoretical aspects of the subject. A most useful general book on gas-phase u.v.-p.e.s. written by Eland deals with the elementary theory and practice of the technique in a very readable manner. Two recent volumes bringing together important articles on a wide range of photoelectron topics are those edited by Baker and Brundle and by Briggs. The former work is Part I of a projected three-volume series.

We should also mention the published proceedings of the series of NATO advanced study institutes which have dealt with various aspects of p.e.s., in particular in relation to other techniques used to study the electronic structure of molecules and solids. In a similar vein p.e.s. has been set against other forms of electron spectroscopy including Auger and energy-loss spectroscopy by Brion and in relation to U.V. absorption measurements in a two-volume work by Robin.

Finally there have of course been several conferences in the period covered by this Report either devoted to or touching on p.e.s. The first of these, held at Namure in 1974, was biased towards X-ray work although it did include a useful review by Frost of developments in u.v.-p.e. at that time. The proceedings of the conference were published as Volume 5 of the Journal of Electron Spectroscopy and Related Phenomena and also in a volume edited by Caudano and Verbist. Photoelectron work also occupied a considerable portion of the proceedings of the Sixth Conference on Molecular Spectroscopy held at Durham, England in 1976. The most recent conference dedicated to p.e.s. was held at Uppsala in 1977 and the proceedings are published in Physica Scripta.

In this introduction we have attempted to mention and in some cases comment on the review material currently available in the p.e. field. It should be noted however that refs. 41-45 contain many excellent articles both of a general and of a specialized nature by established authors. They are, however, too many in number to list here and the reader is advised to consult the original collections of articles for further details.

2 Advances in Experimental Techniques

Since the scope of this Report is essentially limited to gas-phase p.e.s. only developments directly relevant to this aspect of the technique will be discussed. In general one of the three basic electrostatic deflection analysers, i.e. the 127° cylindrical, the cylindrical mirror, or the spherical sector, forms the basis of most spectrometers. A few novel arrangements have been described during the period of this Report, most notably the threshold photoelectron or photoionization resonance spectrometer in its various forms.

As might have been expected the principal growth area for p.e.s. in its simple form (i.e. the energy analysis of photoelectrons) has been in...