Gas-phase Photoelectron Spectroscopy

BY R. G. EGDELL

1 Introduction

The present Report aims to provide a comprehensive review of work in gas-phase photoelectron spectroscopy (p.e.s.) and related areas for the period 1977-early 1980, although overlap with the previous article in this series' is avoided as far as possible. The field has now reached a level of considerable maturity, with relatively few new innovations in experimental or theoretical techniques. But the reviewer is unaware of any decline in the general level of activity in the field.

The usefulness of photoelectron spectroscopy as a means of illustrating a variety of concepts involved in a discussion of electronic structure at an elementary level is reflected in the introduction of p.e.s. in general textbooks on physical and inorganic chemistry. More specialized undergraduate texts on and the theory of chemical bonding and symmetry also give prominence to the technique. A useful monograph by Ballard attempts a development of molecular orbital theory, drawing on the results of p.e. experiments to illustrate basic ideas. A similar approach is adopted in an Open University Course Unit which presents a more limited discussion of molecular valence-level p.e.s. in relation to the theory of electronic structure. The Unit also deals with core-level and solid-state p.e.s. Nonetheless there still remains a clear need for a generally available undergraduate text dealing in a unified way with valence- and core-level p.e.s. of solids and gases. An enigmatically titled volume published in India comes close to satisfying this need but is difficult to obtain in Western Europe or the U.S.A. As regards more advanced books we draw attention to a number of authoritative articles on various aspects of p.e.s. in Volumes 2 and 3 of the series edited by Baker and Brundle. Special mention should also be made of a recent book by Berkowitz. This provides a thorough account of the dynamic aspects of p.e.s., including the theory of photoabsorption, photoelectron angular distributions, and fragmentation of molecular ions. This comprehensive text includes some previously unpublished experimental results. Finally we note accounts of p.e.s. given in Volume 9 of Comprehensive Analytical Chemistry and in a book on instrumental analysis.

The recognition of the importance of p.e.s. in contemporary physics and chemistry is reflected in the appearance of articles concerned with historical aspects of development of the subject. A volume edited by Carlson contains reprints of key papers in X-ray photoelectron spectroscopy (X-p.e s.). Looking to the future, Bock has addressed himself to the question of what will become of p.e.s. He concludes that the technique is of value in analysis and optimization of gas-p hase reactions, although the reviewer can foresee other areas of application.

Turning now to review articles of various sorts, we again commend the terse but comprehensive and timely articles by Baker and co-worker. These provide coverage of both X-p.e.s. and u.v.-p.e.s. for the periods 1976-1977 and 1978-1979. Reviews of a general nature but with much more limited scope have been given by Fellner-Feldegg, Turner! Larkins: Leckey: and Siegbalahn. Instrumentation in molecular p.e.s. has been authoritatively reviewed by Eland, whilst Schwartz has provided a useful survey of techniques used to calculate ionization energies. We also draw attention here to a burgeoning body of literature of an introductory or review nature published in foreign journals without international circulation. The extensive series of articles in Volume 16 of the Japanese journal Kagaku Sosetu deserve special mention, but we have also noted surveys of p.e.s. from Japan, China, Hungary, Poland, Russia, and Uganda. Whilst of limited general interest these articles are clearly of great value to workers in the respective countries.

The largest body of review articles is concerned with application of p.e.s. in restricted areas of chemistry or physics. The impact of electron spectroscopy on the study of the electronic structure of atoms (particularly in its dynamic aspects) is reflected by the appearance of excellent surveys of the area by Wuilleumier and co-workers, Codling, and Samson. The basic theory of atomic photoionization and its application to specific problems has been dealt with by Manson. As regards more chemically oriented articles, Jolly has presented a timely review which deals critically with the use of X-p.e.s. in the study of solid-state and molecular inorganic systems. Elsewhere, Jonathan and co-workers have surveyed the application of p.e.s. to the study of transient species. A number of articles deal with p.e.s. of metal complexes. Furlani and Cauletti presented the first comprehensive review of He-I p.e.s. of transition-metal complexes, dealing for the most part with papers published before 1977. A review by Cowley covers some more recent material but adopts a similar comprehensive approach. By contrast, Hillier has surveyed the area in a more personalized manner, dealing with both core- and valence-level spectra and emphasizing the relationship between molecular orbital calculations and photoelectron spectra. In the published proceedings of a NATO Advanced Study Institute devoted to the organometallic chemistry of the f-block elements, Fragala has discussed p.e. spectra of actinide complexes, including in his paper some previously unpublished results. Other reviews concerned with electron spectra of specific groups of inorganic compounds include those on organotin compounds, organophosphorus compounds, and organosulphur compounds. The u.v.-p.e.s. of organic compounds has been surveyed comprehensively by Ra0, whilst Gleiter deals with the more restricted topic of p.e.s. and bonding in small ring hydrocarbons. A selective and critical article by Sandorfy explores the relationship vacuum U.V. absorption and p.e. spectra of organic systems. The 'impact' of photoelectron spectroscopy on biology has been discussed by McGlynn. Although a number of molecules of biological importance have now been studied by p.e.s., it is unclear whether these measurements have had a significant influence on biological science.

Finally we note that several conferences have been concerned with p.e.s. The meeting held in Uppsala in 1977 was mentioned in the previous report, although the proceedings were not discussed in detail. We remedy this deficiency in the present report. A second major conference on electron spectroscopy was held in...