Electron Spectroscopy: Ultraviolet Photoexcitation D. Betteridge and M. A. Williams University College Swansea, SA2 8PP, U.K .
The period covered in this review is from late 1971 to late 1973. During that time, the Journal of Electron Spectroscopy and Related Phenomena has been introduced and become established. A non-mathematical guide to the technique and simplified approach to photoelectron spectroscopy (19), as well as a massive and invaluable conference proceedings (262) have been published. Many of the papers in the conference proceedings have been published separately, in which case we give the journal reference with a crospreference to the proceedings. A recent discussion of the Faraday Society was devoted to photoelectron spectroscopy. There have been comparatively few general reviews (33, 73, 103, 175, 197), but that of Hamnett, Orchard, and Cox is outstanding (161). I t is detailed, critical, and deals with difficulties which are often glossed over or ignored, but unfortunately its coverage of the literature is only through 1970. A review of inorganic aspects has been prepared by DeKock and Lloyd ( 11I ) . In general, i t has been a period of consolidation during which many earlier ideas (especially those of D. W. Turner) have been re-examined and exploited, and many more compounds have been run. Taking a bird’s eye view of the publications since the last review (32) we note the following trends: Several new instruments have been described and others are in the advanced design stage. Most permit heating of the sample and so many more compounds are accessible to the technique. The various branches of electron spectroscopy which were developed separately are now being used in combination. The Vacuum Generators’ ESCA 3 has been specifically designed to allow the same sample to be examined readily in situ by UPS, X-ray photoelectron spectroscopy, Auger spectroscopy, and LEED. UPS has been applied to solids and surfaces. It has become increasingly common for UPS studies of a molecule to be complemented by several types of MO calculation. Simplified methods of interpretation of spectra based on the identification of characteristic localized orbitals have been critically examined and found valuable. The angular distribution of photoelectrons has attracted a lot of attention. On the negative, side, we feel that there has been an undue concentration of a few groups of compounds-uit., substituted benzenes, alkyl alcohols and halides, and simple silicon compounds. In several of these studies, there is a great deal of overlap and lack of reference to published work. A number of papers have been published in which the spectral data are limited to either a portion of the spectrum, showing just one or two bands, or a table of ionization potentials. Such limited information is rarely able to support the author’s arguments.
TECHNIQUE Instrumentation. There are a number of commercially available UV photoelectron spectrometers. Perkin-Elmer has developed the Turner design from the PS 15 prototype to the PS 18, which enables samples to be heated. The ESCA instruments of AEI, McPherson and Vacuum Generators, can be adapted to UPS, the last of these being the most easily converted. A number of new instruments have been described. Two are relatively simple to construct and are quite versatile. That of Evans, Orchard, and Turner (133) is virtually a compact form of the basic instrument introduced by Turner and manufactured by Perkin-Elmer. In the modification the analyzer plates are part of the main instrument block, so the whole can be heated. It is small, but has a useful resolution and has been used for organome-
tallics. Betteridge et al. (34) have constructed an instrument with a magnetic analyzer, and coupled it to a GLC. The GLC-PES link-up has been achieved without a molecular separator. The spectrometer can be used as a selective GC detector and the GLC se’rves both as a heated inlet system for the spectrometer and as a check on sample purity (35). Typical sample sizes are 5 pl for a complete spectrum and 0.1 pl for detection of one peak. Both of these instruments operate a t a fixed analyzer field while the electrostatic potential applied to the exit slit ,is swept. This results in a linear scan and spectral peak intensities which are not affected by the analyzer error, which causes a fall off in intensity with decreasing electron energy. Consequently, peaks with IPS in the 15to 21-eV region are much more intense and the spectrum is more distinctive. Cylindrical mirror analyzers have been incorporated into several instruments. They require careful machining and present a considerable problem of sample introduction. The sample is ideally a point source in the middle of two concentric cylinders, which are under high vacuum. Berkowitz (26, 27) has solved the problem by using molecular beams. Allen et al. ( 3 ) have used a COz laser to heat the sample which is placed inside the analyzer. The instruments are able to handle salts such as thallium(1) and potassium halides, but contamination of the instrument by the sample is a problem. The resolution on Allan et al’s instrument is ca. 60 meV, on Berkowitz’s, it is 40-20 meV depending on slit adjustment. A molecular beam inlet system for a different analyzer has been briefly described by Evans (126). Gardner and Samson have used a spectrometer with a cylindrical mirror analyzer for gaseous samples (146). An isolated ion chamber is incorporated in the inner cylinder. The resolution can be improved from 45 to 23 meV by retarding the electrons and operating a t a fixed potential. It is possible that the constructional modification of Bishop et al., who replace full cylinders in the analyzer with half cylinders (40), will prove of use for UPS. So far, the cylindrical mirror analyzer has not led to the improved resolution and sensitivity predicted by theory. However, these pioneering studies have shown that the engineering problems are not insuperable, so we may expect improvements. Schweitzer’s group have carried out preliminary investigation of the pill box analyzer, which is little more than a box capable, they compute, of a resolution of a few meV on 10-eV electrons (5). They have also performed the first UPS Hadamard transform ( 4 ) . Two-hundred channels were used for the transform and a resolution on argon of ca. 60 meV was achieved. Lindau et al. have designed and constructed a novel retarding field analyzer (198). It is effectively a planar retarding field grid backed by a monochromator, which enables a differential spectrum to be obtained directly. On the argon 3/2Ppeak it gives a count rate of 60,000 cps a t a resolution of 50 meV from 8 X Torr sample pressure. There is some peak distortion characteristic of retarding grid analyzers, but for most purposes this would not be serious. The 127” cylindrical electrostatic analyzer has been the subject of a detailed theoretical (2, 114, 242, 244, 245) and experimental (243) study. Lloyd has made a hemispherical analyzer from spun-copper hemispheres. In theory, the tolerances are so low that poor spectra are expected. In practice, satisfactory resolution and sensitivity have been obtained (199). Lenses which improve the resolution of given analyzers have been designed by Weiss (294) and Preston et al. (231). Weiss computes that his system should achieve 6-meV resolution on 1-eV electrons. Asbrink and Rabalais have achieved sufficiently good resolution to measure ro-
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tational fine structure on water and water-Dr (11). While most workers have been trying t o heat the sample, Thomas e t al. have modified the Perkin-Elmer PS 15 target chamber to permit cooling of the sample vapor. Their object has been to establish the effect of hydrogen bonding on the appearance of t h e PE spectrum (276). For carboxylic acids, they found that the essential features of the spectrum of the monomer are retained in t h a t of the dimer, but the lone pair oxygen peak shifted by about 0.5 eV . Solids a n d Surfaces. An important development of UPS is its application t o solids and surfaces. The sort of information which may be gained has been discussed in relation t o that expected from field emission (FES) and ion-neutralization spectroscopy (INS) by Hagstrom (157). He points out that UPS and INS may give complementary information because of the different depths sampled by the two methods and shows how important it is to examine the sample by several techniques. Mostly, INS samples the surface and UPS the layer below-Le., the selvedge (158).
There are practical difficulties in obtaining the spectrum of a solid, besides the obvious one of providing a good vacuum. First it is desirable to use a sealed source to keep excess helium from the target chamber. One with a n aluminum window has been fully described (186) and a number of others have been mentioned briefly. In all cases, the window reduces the photon flux. A windowless system suitable for surface work has also been reported (77). Second, there is the problem of calibration. In XPS, the sample is mixed or coated with a standard compound which serves as a calibrant. Such a procedure is not well suited to UPS, because the valence-shell bands of the solid are not well separated and spectra of sample and standard will overlap. Further, the addition of the standard may shift the Fermi level, which is taken as the reference level for solids. One approach is to take the o b served onset of the spectrum as the Fermi level and beg the question of whether it has shifted. Another is to calibrate the spectrometer. Poole et al. have given a procedure and noted that the zinc 3d peak a t 9.46 eV provides a useful check on calibration (224). The procedure effectively entails the measurement of the transmission functicn of the spectrometer, which is discussed by the same group (225) and Aksela ( 1 ) . Much of the UPS of solids has been carried out on instruments with a n XPS capability and then the calibration can be extended from the inner levels to the outer. The depth probed varies with the sample, and the energy of the ejected electron. It is less for heavy metals than organic compounds. The penetration of ionizing radiation is far greater than the escape depth of the photoelectrons. The depth sampled falls within the range 5-20 A, which compares with >lo0 A for XRF (73,278). The most basic studies so far carried out have been of metals (121, 263, 264) metal oxides (157, 169, 230) or chalcogenides under conditions of ultrahigh vacuum (284). For nickel, anomolously strong peaks have been shown to arise from adsorbed cesium and sulfur (279). Brundle et al. have embarked on the study of adsorbed species by UPS ( 6 6 ) . For some experiments they started with tungsten (13) or gold (67) a t ca Torr and introduced carbon monoxide and water, respectively, measuring the UPS spectrum a t different sample pressures. The coverage of the metal surface can be followed, and it is found that the technique is not quite so sensitive as Auger spectroscopy. When there are about 100 monolayers of water, the gold peaks are barely observable and the adsorbed water spectrum is very similar to the gas-phase spectrum. The difference in ionization potentials is almost the work function of gold. The limit of detection of adsorbed water is approximately 5 X molecule cm-’ which is much less favorable than that with metals where 2% of a monolayer has been detected. Spectra have been obtained of crystals of naphthacene (172) and perylene (261). They resemble the gas phase spectra shifted by a constant amount, with the addition of a few extra bands which are independent of incident photon energies. It seems certain that there will be a rapid increase in the study of surfaces and solid samples by UPS. 126R
Combined Techniques. There are obvious advantages to examining the same sample by different techniques. Hagstrom (157) describes an instrument with parts for XPS, UPS, INS, and Auger spectroscopy (AS). The Vacuum Generators’ ESCA 3 has ports for XPS, AS, and either LEED, UPS, or nuclear quadropole mass spectrometry. Other companies are offering similar options. Eland has devised a coincidence circuit and spectrometer which permits the detection of the photoion and its associated photoelectron (125). Ames et al. ( f and Schneider and Smith (255) have measured the molecular ion fluorescence which results from photoionization. Several groups have studied the angular dependence of intensities and these will be dealt with in the section on interpretation. INTERPRETATION It is generally assumed that Koopmans’ theorem holds, so that the measured ionization potentials of a molecule are the negative of the molecular orbital energies. The problems of interpretation lie in (a) the practical difficulties of recognizing discrete ionization potentials in a typical spectrum, and (b) the task of deducing from theory the correct ordering and energy of the molecular orbitals within a molecule. We can recognize three broad approaches to the solution of these problems. First, there are the attempts to devise theories which are both accurate and computer-compatible. Second, there are the simplified methods in which specific peaks in the spectrum are identified with localized orbitals of the molecule. Then, shifts in peak positions are associated with electronic. or structural changes within the molecule. The peaks may also be used to identify atoms or molecular groupings within the molecule. This is analogous to the simplified interpretation of infrared and NMR spectra. Third, there is the application of information from analysis of fine structure, associated ionization phenomena, intensities of peaks, angular distribution of electrons, etc. This may lead to the definite assignment of a few peaks in the spectrum. In any given interpretation and in many of the best [e.g., ( 5 9 ) ] , all of these approaches may be used, and the catgories are not hard and fast. Nevertheless, we will use them as guidelines. Exact Methods. Theoretical chemists have not been slow to use UPS to evaluate the various theoretical methods. Most used are CND0/2, MINDO/2, extended Huckel theory (EHT), and ab initio calculations. Of these, E H T seems to be widely applicable, relatively simple, and consistently gives results in agreement with the spectra. Ab initio calculations are also fairly reliable, but they are expensive and their application has been limited. CNDO/2 calculations are relatively simple, but the agreement with spectral results is usually poor. Good, representative discussions of the various calculative approaches are the papers by Cox et al. (phosphorus halides, oxyhalides and other simple inorganic compounds) (93), Batich et al. (cyclooctatetraene and hydrogen derivatives) (21) and Clark, Brogli, and Heilbronner ( norbital energies of the acenes) (84). Another valuable comparison of the various methods is provided by the controversy over the ordering of the bands in silicon fluoride. Jonas et al. (176) on the basis of CXD0/2 calculations disputed a n earlier assignment of Lloyd and Bassett (200). Hall et al. (160) replied by showing that both simplified methods and ab inltio calculations supported the original assignment. The exchange underlines the importance of using several approaches to interpretation. Spectral interpretations have been assisted by ab initio calculations of azoles (98), alcohols (239), azabenzenes (61, methyl manganese pentacarbonyl (159), E H T calculations of hydrocarbons (50, 57j, chloroethylenes (189), azines (266), diademane (I&), pyrene and covenene ( 5 1 ) , CNDO/ 2 calculations of cyanic acid (193), halomethanes (181, 280) and substituted benzenes (190, 191, 267). Simple HMO models which account for n orbital IPS of many compounds have been devised (190). INDO has been used by some workers (18, 85) and MINDO/2 by others (12, 21, 47, 50, 51, 162, 164). It has been modified to SPINDO, by inclusion of parameters obtained from PES, by Fridh, Asbrink, and Lindholm (9, 10, 139, 140). The agreement
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D. Betteridge graduated from the University of Birmingham with a BSc in 1957 and a PhD in 1960 after studying under R. Belcher and T. S. West. He was then successively Research Fellow at the University of Virginia (1960-61) with John H. Yoe, the University of Arizona (1961-62) with Henry Freiser, and the Atomic Energy Research Establishment, Harwell (1962-63) with A. A. Smales. Since 1963 he has been at the University College of Swansea (Wales) where he is now a senior lecturer. In addition to PES, his research interests are the study of the chemistry of analytical chemistry, separations, and the history of the teaching of analytical chemistry. He has served on the Council of the Society for Analytical Chemistry since 1968, has been an assistant editor of Talanta since 1964, and is one of the founder members of the Editorial Board of the Journal of Electron Spectroscopy.
Margaret A. Williams graduated from Imperial College of Science and Technology, London, with a BSc in 1971 and is now a graduate student at the University College of Swansea.
between calculated eigenvalues and experimental IPS is better than that obtained with INDO and MIND0/2. The equivalent orbital method has also been applied to various hydrocarbons (145, 216, 254) as has the Floating Gaussian Orbital method (FGOM) (93, 280) but with only moderate success. There are other theoretical studies of unsaturated 3-membered rings (238), boron trifluoride (286), charge transfer transitions in transition metal complexes (283), and diazanaphthalenes (282). A computational break-through has been achieved by Clementi (85), which, if it proves generally applicable, may make ab initio calculations so rapid that semi-empirical methods will lose their attraction. Koopmans’ theorem fails when the molecular ion which results from ionization, differs significantly in shape or electronic structure from the parent molecule. Despite its well known limitations, it has been widely used to interpret P E spectra, with most of the interpretive problem arising in cases where the molecular ion is severely distorted. Now it has been shown that the process of electronic relaxation, which helps to stabilize the ion, can seriously invalidate the straightforward application of the theorem to UPS. Rohmer and Veillard (241) calculated that in bis-(A-ally1)nickel the energy of the 9a, orbital is -18.3 eV, while in the molecular ion it is -7.9 eV in good agreement with the observed IP of 7.85 eV (201). Brogli et al. have noted that the IP’s of both the la1 and 2bl A orbitals of a series of fulvenes vary with the nature of the substituents, although only the 2bl orbital is predicted to be so affected (54). The unexpected shift, which is up to 0.6 eV, is readily explained when the effects of electronic relaxation are taken into account. It is noted that Koopmans’ theorem will not hold when ionization is from localized orbitals. These findings have serious implications for those who seek to correlate calculated eigenvalues with experimental IP’s, and will ensure that simplified interpretations are taken with an even larger grain of salt. Simplified iMethods. The simplified interpretation of UV photoelectron spectra is based on certain generalizations concerning features of the spectra. These have been discussed in more detail elsewhere (19, 36). An important simplification is that molecular orbitals can often be considered as being localized on an atom or a group of atoms within a molecule-e.g., the lone-pair orbitals of the halogens, 0, N, s, P, etc. These usually give rise to relatively sharp, symmetric P E peaks which are readily identifiable.
Nevertheless, it can be very difficult, if not impossible,
to identify ionization bands on the basis of an isolated
spectrum. It is generally necessary to study a series of related compounds and correlate changes in the IP’s of recognizable bands with changes in the substitution pattern of the molecule. The I P of an orbital is a reflection of its chemical environment and as such is sensitive to changes in the substitution pattern of the molecule. By monitoring the change in I P of a particular band with various substituents, it is often possible to determine the nature and origin of that band. One effect of substitution may be to alter the charge distribution in the molecule as a result of the electronegativity or inductive effect of the substituent. The replacement of an atom by one which is more electronegative will result in an increase in the IP’s of orbitals associated with adjacent atoms, and vice versa. Correlations between IP’s and (a) the electronegativity of substituents (104, 182, 271) and (b) the partial charge of the atom on which the orbital is localized (36) have been demonstrated. These correlations are useful in that they help to predict the I P of an “identifiable” orbital in the spectrum of a more complex molecule. A particular electronegativity effect which can be very useful in spectral interpretation is known as the perfluoro effect. The substitution of a fluorine for hydrogen in a planar molecule has a much larger stabilizing effect on the u than on the A M.O.’s. Brundle et al. have shown (68,69, 240) that for planar molecules u M.O.’s are stabilized by 2.5-4 eV on fluorine substitution, whereas the stabilization can be an order of magnitude smaller for r M.O.’s. Thus, by simple comparison of the P.E. spectra of a perhydro/perfluoro pair, it should be possible to identify A and u ionizations up to about 15 eV, where the spectrum is complicated by F lone-pair ionizations. The perfluoro effect is applicable to both nonaromatic (69) and aromatic (68) planar systems although the A shift is usually greater for the latter class of compounds. However, in the case of nonplanar systems, such as those containing the CF3 group, there is no u-A distinction and the IP’s of all the M.O.’s exhibit a large upward shift on fluorination. Nevertheless, the perfluoro effect is particularly useful in interpreting the spectra of compounds in which 7r M.O.’s and heteroatom lone-pair orbitals are close in energy, for example those of quinoline and isoquinoline (282). Price and coworkers (228) have reported the He(1) and He(I1) spectra of all the fluorobenzenes except one, and by correlation of spectral peak shifts with progressive fluorine substitution, have made an assignment of the spectrum of benzene. Orbital Interactions. Substituent shifts are also observed if the substituent possesses an orbital of comparable energy and symmetry so that it can interact through space with the orbital under consideration. This interaction results in the formation of a new pair of molecular orbitals with energies different from each interacting orbital. The energy of the symmetric combination will be lower than that of the antisymmetric combination. In the photoelectron spectrum, the interaction is observed as a shifting of one peak to a higher and one to lower I P than expected in the absence of interaction. When there is a non-interacting orbital which is initially degenerate. with one of the interacting orbitals, it acts as a marker and enables the extent of splitting to be measured. The size of the splitting depends upon the degree of overlap and relative energies of the interacting orbitals, but it is normally 0.1-0.6 eV. The shift arising from orbital interaction will be in addition to any shift arising from inductive or other effects. Substituent effects can often be interpreted by such interactions and thus the PE spectra reflect substitution patterns. The splitting may also depend on the conformation of the molecule. It is hardly surprising that the effect of different substituents on the orbital energies of benzene has been studied by P.E.S. to a considerable extent (147, 217, 228). Rabalais et al. (104, 235) have shown that the IP’s of substituted benzenes are greatly dependent on the electronegativity of the substituents and the amount of conjugation between the orbitals of the phenyl and substituent groups of the composite molecule. The substitution of a methy-
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lene group between the interacting groups destroys the interaction (235). However, discrepancies in the assignment of peaks are common whether elaborate calculations or simplified methods are used in interpretation. For example, there is still disagreement (167, 185) about the ordering of the highest filled M.O.’s in pyridine. The effect of methylthio (44, 45), and silyl (214) substituents on A systems has been investigated by Bock and his group. They suggest that p d back-bonding can be neglected for sulfur-* interactions, although this is not the case for silicon. A series of papers has been published by Schweig et al. in which the P.E. spectroscopic evidence for hyperconjugation is discussed. They attempt to estimate the importance not only of C-H hyperconjugation (251), but also of C-halogen (252), C-sulfur (247, 248), C-silicon (290, 291), C-germanium and C-tin (260) hyperconjugative interactions. Spiroconjugation (257, 258, 289) is a further complication. Heilbronner and coworkers (55) have referred to the effects of hyperconjugation in the interpretation of the spectra of some benzocycloalkenes. It is important to emphasize that the combined influence of inductive, conjugative, hyperconjugative, and other effects should be taken into account when interpreting a P.E. spectrum. Bock and Wittel (46) have attempted to estimate the relative importance of these electronic effects in their P.E. study of the bonding in trans-dihaloethylenes. Angle of Twist. A recent innovation has been the use of P.E.S. to study steric inhibition of resonance. Maier and Turner have carried out an extensive study of steric inhibition in biphenyls (205), phenyl ethylenes (207), and in anilines, phenols, and related compounds (208). In the case of the biphenyls, full resonance interaction between the r electrons of the phenyl rings occurs when the rings are co-planar. If co-planarity is prevented by steric factors, the resonance interaction is reduced according to the degree of steric inhibition. This, in turn, is reflected by changes in the arrangement of the occupied energy levels, for a series of related molecules. Maier and Turner report an empirical relationship between the separation of bands 1 (rs) and 4 ( A S ) in the spectra of biphenyls, and the dihedral angle ( 8 ) between the planes of the two rings. 8 is determined from electron diffraction studies. A plot of Id-Il(AE) us. cos 8 gives a reasonable linear correlation, which they consider enables the conformation of substituted biphenyls to be determined as accurately as with electron diffraction. The theoretical background to the argument is based on interaction between the initially localized r-M.O.’s of the benzene ring, which are used as basis functions. This sim‘ple treatment neglects the effects of u-T interaction, electron correlation, and interaction with anti-bonding orbitals. The authors regard these as second-order perturbations, for the orbitals under consideration. They emphasize the importance of studying all the related combinations of a series of molecules so that electronic and steric effects can be separated by examination of all the occupied energy levels. Similar arguments are used in the study of phenylethylenes, anilines, and phenols. However, because of the lack of electron diffraction data, the assumed linear regression between LIE and cos 0 is defined with only two sets of coordinates, at 8 = 0” and 0 = 90”. It is interesting that the results indicate an approximately linear correlation between the dihedral angle and the “size” of the hindering substituent-e.g., as represented by Van der Waals’ radius. Cowling and Johnstone have also applied P.E.S. to determine the amount of resonance interaction in a series of substituted anilines (91). They use two different methods to estimate the angle of twist of the N-aryl bond. They first assume a linear relationship between the corrected N “lone-pair” ionization potential and cos 8 (or cosz e ) . They give the theoretical reasons for using cosz 8 and suggest that, in the anilines, neither a simple cos 8 nor a cosz 8 relationship is satisfactory, but that a complex relationship using powers of cos 0 should be used. In the second method, the corrected energy difference +
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ANALYTICAL CHEMISTRY, VOL. 46,
between the 7r3 and rz orbitals, Ax, is plotted against cos 8 (or cos2 e). The results of the two methods agree reasonably well, but, in discussion (72), Maier and Turner have favored using the energy difference between the bands, as this avoids uncertainties arising from inductive effects. Heilbronner and his group (22) have investigated the relationship between angle of twist and the position of the P.E. *-bands for compounds comprised of two conjugated n-systems, each individually planar-e.g., dienes, biphenyl. For these compounds, the splitting between the A bands in the P.E. spectrum is found to vary approximately linearly with cos 8. However, the authors show that an extension of this method for characterizing twisted or deformed double bonds, is applicable only within very narrow limits. Using a simple HMO model, they show that if the two 2p atomic orbitals of a two-centre *-bond are twisted by an angle 8, while keeping the internuclear distance constant, the orbital energy of the planar system ( e = 0”) is shifted by
AE
N
3.6 (1
- cos 0) eV
This crude approximation was used with some success to differentiate between the different conformations of trans-cyclooctenes. However, they point out that where the angle of twist, and thus the peak shift, is small, this approach proves of little or no value. In their study of hydrazines, Nelsen and Buschek have noted a relationship between the splitting of the N lone pair peaks and the lone-pair-lone-pair dihedral angle for tetraalkylhydrazines (220). Through Bond Interactions. The above arguments assume a direct interaction of localized orbitals through space. It is also possible for orbitals of appropriate symmetry, but with no possibility of overlap, to interact through adjacent bonds. The magnitude of the interaction is ca. 1.5 eV. The effect of both “through space” and “through bond” interactions on apparently degenerate orbitals is to remove the degeneracy. However, when both types of interaction are possible, they often tend to cancel each other. Heilbronner has thoroughly investigated the phenomena by UPS. With the aid of carefully selected compounds and backed by calculations, he has deduced their relative importance. The interactions of the highest occupied r orbitals in nonbornadiene and related compounds is largely through space (58, 168). The corresponding r interaction in tricyclo[4.2.0.0.z~J]octadieneis mainly through bond (148). Both are important in azabenzenes and azanaphthalenes (59) and propellanes (150), and they nearly cancel in bicyclo[3.2.2.]nona-6,8-diene(153). These papers, which are of immense interest, show the value of the localized orbital concept for the interpretation of PE spectra. They also reveal the dangers of the simplified methods, which often seem to work despite themselves. Through bond and/or through space interactions have also been invoked to interpret the spectra of cis- and trans-dicyanoethylenes (42) dioxanes, dithianes and related compounds (272), triquinacene (71), diamandoid molecules (253) [2.2.2][1.3 -51 cyclop hane- 1,9,17- triene (48), butadienes (25) and radical cations of hydrocarbons (165). The non-interaction of orbitals in bridged cyclophanes has also been discussed (173). Vibrational Fine Structure, Autoionization, and Molecular Fluorescence. The vibrational fine structure on a peak can help in its assignment. It has been generally assumed that shifts in vibrational frequencies will be dependent on whether the electron is ejected from a bonding or antibonding orbital and upon changes in the shape of the molecule as it becomes an ion. Thus PF3f is thought to be planar and PH3+ pyramidal (206). Rabalais et al. (236) have challenged this view for relatively large molecules (e.g., bromothiophenes) by arguing that the molecular ion must be very similar to the parent molecule so the vibrational frequencies of the molecule can be applied unchanged to the ion. Thus, if the various frequencies of the molecule can be assigned, they can be used to identify PE peaks; e.g., if a frequency of 16000 cm-l is found on a peak in a PE spectrum of a carbonyl, the peak is associated with a localized C - 0 orbital. It is not always possible to obtain well resolved fine structure for large molecules,
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and further complications arise when several vibrational series are superimposed on a peak. Further, there are a number of examples of changes in geometry accompanying ionization of moderately sized molecules, e.g., barrelene (163), and CND0/2 calculations have been made of the changes in vibrational frequency to be expected for a number of hydrocarbons (123). However, application of this principle may assist in spectral assignments. [Derrick et al. have inverted the argument by using the fine structure of PE spectra to assign peaks in Raman and IR spectra of 5-membered heterocycles ( 1 1 5 ) ] . There have been studies of vibrational features. Higginson et al. have investigated the variation of the adiabatic ionization potential of iodine as a function of temperature (170). The width of lone pair peaks has been correlated with the percentage of atomic character of the associated orbital (222). Calculations heve been performed to deduce the vibrational envelope of HzS+ (120), and the fine structure in the spectra of formic acid (287) and pyrazine (228) has been reported. Lindholm’s group have extended their studies on Rydberg series of small molecules with pyrimidine (136), pyrazine (135), pyridizine (137), s-triazine (134), and s-tetrazine (138). The PE results are complemented by electron impact spectra and rigorous theoretical interpretations. Autoionization can result in increased fine structure and variations in intensity (14, 15, 78, 188, 219). Streets, Potts, and Price (268) have proved that a t relatively high sample pressures, photoelectrons interact with the sample to give anomolous peaks in the spectrum. The pressures employed to measure the effect were higher than normal but it is probable that small anomalous peaks may be present in many spectra. They would be detected only by careful comparison of spectra obtained a t various pressures. Intensities of Bands. There is a rule of thumb that peak intensities are proportional to orbital degeneracies, but it is well known that there is no justification for the rule in theory and that it is fallible in practice. Despite its limitations, it provides a very useful guideline and there have been efforts to improve upon it. For a number of alcohols, the area under the oxygen lone pair peak has been correlated (with limited success) with the area under the rest of the spectrum (38).Theoretically sound procedures, which allow the ionization cross-section to be calculated from tabulated parameters, have been proposed (92). The theory of photoionization cross-sections has been discussed by a number of authors (105, 203, 229, 232, 256, 273, 274). There is agreement that two important factors are the energy of the ionizing radiation and the nature of the orbital from which the electron is ejected. This may be expressed simply (105) by
where u,, is the photoionation cross section of the nth ionization process, A is a constant, p n is the momentum of the free electron, and E p h is the energy of the ionizing photon. The Q a n and Qabn terms are the one-centre and two-centre terms based on the individual atomic orbitals or their combinations, respectively. In UPS it is possible by using He(1) (584 A) and He(I1) (304 A) rato vary diation and to calculate the atomic contribution to molecular orbitals for simple molecules. Price et al. have carried out two detailed studies in which the dependence of cross-sectional area on E p h is established (232) and the intensities of all the valence shell peaks in the spectra of some simple hydrocarbons and isoelectronic analogs are accounted for (229). When the orbital is largely atomic in character, Qabncan be neglected and the equation simplified. It then predicts that the intensities of peaks arising from “s” and “p” type orbitals in the PE spectra of a molecule will depend on whether ionizing radiation is 584 or 304 A. Comparison of the He(1) and He(I1) spectra of the same molecule is thus very valuable. For example, Robin et al. were able to identify in the spectrum of the barrelene-type compound P(CHsN-NCH3)3P, the phosphorus lone pair band which is mixed with tfie upper K bands by virtue of the relative differences in intensity between the lone pair and K bands on irradiation with He(1) and He(I1) 1240). This approach has been limited by the tech-
nical difficulty of obtaining He(I1). The theoretical problems are complicated by the practical difficulties of accurately measuring photoionization cross-sections. The photon flux is difficult to measure and, with most electron energy analyzers, the intensity varies with electron energy. The common source of analyzer error is the sweeping of the analyzer plates. A correction factor of (Eph-KE)/KE where KE is the kinetic energy of the photoelectron should be applied. This implies that intensities are low for peaks a t high IP, as observed. A great improvement is effected by spectrometers which sweep the entrance slits instead of the analyzer plates (34, 133) and much improved spectra are obtained. The peak heights are not, however, necessarily proportional to the ionization cross-sections. Careful measurements have been made of a number of small molecules, but the details are beyond the scope of this review (16, 31, 41, 76, 146, 246).
In practice, most assignments based on intensity have been made by comparing the relative areas of peaks which occur in the same region of the spectrum and derive from similar orbitals. Doubtless, this is the reason why the rule of thumb has proved useful, and will continue to be of value when used with the knowledge of its limitations. In addition, there are so many variables in the equation that several of them cancel out so that over a limited range of the spectrum there is a reasonable constancy of the crosssect ion. The intensity is also dependent upon the angle of collection of the photoelectrons. If 9 is the angle between the photon beam and the exit slit of the ionization chamber, and p is an angular parameter, the intensity as a function of 0 is given by (74)
(3)
p is dependent upon the type of orbital from which the photoelectron arises and thus may aid in the interpretation of spectra. The theory is given by Grimm (156) and extensive data of the variations of /3 with energy for many cases are provided by the calculations of Mason (209). In practice, it is found that the angular variations in intensity are real, but comparatively small (say *25%). Further it has not proved easy to correlate them simply with molecular parameters, so, a t present, we feel this work is of little value to analysts. Of the many studies, we select those of Carlson et al. (75) and Kinsinger and Taylor (187) as entries to this area.
COMPOUNDS During the past few years, the number and variety of compounds studied by UPS has increased tremendously. So, for the sake of brevity, we have compiled a table (Table I ) which summarizes most of the compounds reported. Those topics which we feel require special attention are discussed separately below. Transient Species and Free Radicals. The method is, in principle, applicable to free radicals and short-lived species. There are technical problems in obtaining a measurable spectrum from such species, because they are present as minor components in the sample as well as being transient. There are also theoretical problems in interpreting the spectra, because Koopmans’ theorem does not hold for open shell molecules ( ( e . , free radicals), and the species may well have a differelit geometry and charge distribution relative to the parent molecule. These problems have been solved, a t least for some simple radicals, ions, and unstable molecules, and there is likely to be a considerable expansion of this area. Jonathan et al. have continued their pioneering work, reported in the last review (32), and dealt with 0 2 and SO2 (179), the methyl radical (154), and CS (177, 178) formed in a discharge tube from CS2, and complemented their experimental work with CNDO calculations. King et al. (184) have also examined the decomposition products of CSz. Cornford et al. have obtained the spectra of ClOz (87), NF2, (CF3)zNO and S03F (88) and discussed them in terms of SCF MO calculations. Kroto and Suffolk have identified the unstable species HzCS as a pyrolysis prod-
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Table I. S u m m a r y of C o m p o u n d s Examined b y UPS 1972-73 Compounds or class of compounds (Refs.)
Compounds or clam of compounds (Refs.)
Inert gas halides and oxyhalides (61-64, 107) Potassium halides (3) Group I11 monohalides (28) Boron halides (204, 286) Substituted borazines (194) Halogeno- and methyl boranes (183) Thallium halides (26, 27, 106) Group IV hydrides (142, 143, 160, 226) Group 1V halides and pseudohalides (95, 100, 142, 143, 160,
and stannanes (291) 1-sila and 1-germa-3-cyclopentenes (259) Alkanes 1139, 145, 169, 216, 254) Alkyl amines: alcohols; halides; and mercaptans (86; 239;
176)
Methyl, silyl, and germyl derivatives of Groups V and VI (97, 101)
HNCO (193) C3Oz (234) Carbonyl halides and related compounds (79, 196,213,277, 295)
CS (144) Group V hydrides (223,227) Nitric oxide (122, 275) NSF, NSC1, NSF, (89, 90, 112, 118) P1 (65, 132)
Phosphorus halides and related compounds (20, 37, 70, 93,
(162, 166, 210, 253, 293)
99)
Group VI hydrides (227) Sulfur (gas) (52) SzFBisomers 1285) Oxides of sulfur and thionyl compounds (80, 110, 113, 171, 212)
Interhalogens (8, 108, 109) HOF (29) Os04, Ru04 (117) @-Diketonecomplexes (131) Silyl and germyl transition metal carbonyls (96) Ni and Pd bis(n-allyl) complexes (201, 241) T arene complexes (127) Metallocenes (130, 133) (Trimethyl sily1)methyl and neopentyl complexes (128) Group IV tetramethyls (49, 94, 129, 176, 289) Methyl chlorosilanes (155) Strained cyclic organosilicon compounds (102) Trimethyl silyl haloacetylenes (39) Vinyl and allyl silanes, germanes (260, 288)
uct of dimethyldisulfide (195). Morishima et al. have obtained and interpreted the spectra of di-tert-butylnitroxide and 2,2,6,6-tetramethyl-piperidine-N-oxyl radicals (215).
Group IV. Much energy has been devoted to the P.E. study of bonding in simpler compounds of the Group IV elements. It is generally agreed that d orbitals participate to some extent in the bonding of all except carbon (39, 95, 97, 101, 142, 143, 176, 214), although there is some dissent (49). The degree of involvement of d orbitals is greater for silicon than the other Group IV elements. Bock et al.’s paper (214) on the effect of (Y- and p-silyl substituents on a-systems explains the properties of these compounds in terms of inductive effects, hyperconjugation and pn d, interactions. In the case of a-substituted ethylenes, the presence of a methyl or silyl group results in destabilization of the ethylenic a-orbital. This can be explained in terms of the electron releasing, inductive effect of the methyl and silyl groups and hyperconjugation between the a bond and the out-of-plane C-H and Si-H u bonds. Usin; this argument, the a destabilization should be greater in the case of the silyl-substituted ethylenes, but this is not in agreement with the P.E.spectra. The discrepancy is explained by the stabilization of the a orbital in the silyl-substituted compound, which results from p T d, back-bonding. Organometallic and Metal Complexes. Despite recent technical advances in the heating of sample inlet systems, surprisingly little work has been reported on the more involatile metal complexes. Most of the compounds investigated have been sufficiently volatile for their P.E. spectra to be obtained at room temperature.
-
-
130R
119, 181,280; 141)
Acetone (34) Organic sulfides (43) Organic pseudohalides 124) Haloalkenes (46, 81) Dicyano ethylenes 142) Phenyl ethylenes (207) Substituted butadienes (269) 3- and 4-Membered ring compounds (60, 238) Carboxylic acids; and esters (271, 287; 270) Azomethane and hydrazines (164,220) Sulfones (265) Organophosphorus pesticides (37) Fulvenes (54, 140) Benzocycloalkenes (55) Cyclo-octatetetraene and derivatives 121) Cyclic ketones; and ethers (18, 82, 292; 17) Alicyclic ring systems such as norbornane, adamantane, etc. Alicyclic unsaturated compounds such as barralene (48, 53, 153, 163, 174, 192, 237, 240)
Substituted benzenes (9,83,91, 104, 185, 190, 191, 208, 217, 233, 235, 267)
Methylthio substituted aromatics (44, 45) Halopyridines (185, 21 7 ) Methyl and trimethyl silyl-pyridines (167) Azabenzenes 16, 134-138, 149,218) Phospha-, arsa-, and stibabenzene (23, 249) Azines, pyrazines, and azoles (98, 266) Quinoline and isoquinoline (282) Bromothiophenes (236) Tetrathiofulvalene (152) Dithianes (272) Thiathiophthenes (151) Polycyclic hydrocarbons (50) Aromatic polycyclic hydrocarbons (56, 71, 84, 116, 124) Biphenyls (22, 205) Phosphates and azoles (250)
Lloyd and Lynaugh (201) have obtained spectra of the bis(x-allyl) complexes of nickel and palladium. Their interpretation of the spectra indicates that the highest-lying orbitals are metal d rather than ligand in character, and that there is no charge on the central metal atom. This assignment does not agree with the results of an ab initio calculation by Veillard, who comments (241) that the apparent discrepancy is removed by taking into account electronic relaxation upon ionization. It is suggested that in this case, contrary to the assumption made in Ref. (201), Koopmans’ approximation breaks down, so there is and the seno relationship between the sequence of I.P.’s quence of M.O. energies. Nevertheless, Koopmans’ theorem has been assumed in the interpretation of the spectra of metallocenes (127, 130) with apparent success. The spectra of (a-EgHg)&r, ( x C&,CH3)&r, (x-CgHg) (a-CsH5)Mn and (x-CgH6) (asH5)Cr have been studied (127) to obtain information about the electronic structure and to compare a-benzene and the a-cyclopentadienyl radical as ligands. The spectra indicate that the low ionization energy region-Le., below -8 eV-may be assigned to ionization from metal d-orbitals, whereas the higher I.E. region is very similar to that of the parent ligand. A similar study (130) has been carried out by the same group on some closed shell metallocenes including ferrocene, ruthenocene, 1,l-dimethylosmocene and bis(n-cyclopentadienyl) Mg. The aim was to investigate through application of Koopmans’ theorem, trends in the energies of the leading valence M.O.’s as the central metal atom is varied, thereby gaining information on the mode of bonding in these sandwich-type compounds. The details of the
A N A L Y T I C A L C H E M I S T R Y , VOL. 46, N O . 5, A P R I L 1974
spectra are discussed using a simple M.O. model and compared with the expectations of ligand field theory. An extensive UVPES study of a range of tris(p-ketonato) complexes, ML3, where M = Al, Ga, Sc, Ti, V, Cr, Co, Mn, Fe, Ru, yields much information about the interaction of metal and ligand valence orbitals (131). The data for the Co(III), Mn(III), and Fe(II1) complexes suggest that the ligand 7r3 ionizations precede a t least some of the metal 3d ionizations. The spectra of six silyl and germy1 derivatives of Mn, Re, and Co carbonyls have been reported (96). It is suggested that the metal-Si (or Ge) o-bonding level gives rise to a band in the 9- to 10-eV region where it is obscured by the transition metal 'd' electron bands. Novel Applications. Miller et al. have used UPS to elucidate the anodic fragmentation reaction of adamanLITERATURE CITED
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tane ketonic derivatives (211). It is argued that the anodic reaction involves an initial cation radical formation. The conformation of methyl substituted hexahydropyridazines has been established by UPS (221). The findings complement those of NMR studies. Berkowitz et al. have deduced that the radical (a-naphthyl)-(CH*)d-a-naphthyl) is open chain, not cyclic, because the first three IPS are virtually the same as for n-butyl-(a-naphthalene) (30). In the cyclic compound, end to end interaction of the naphthyl groups would be reflected by shifts in the IPS. Interactions of borane and Lewis bases have been observed in the gas phase by Lloyd and Lynaugh (202).
ACKNOWLEDGMENT We are grateful to the T. & E. Williams Trust for a maintenance award to M.A.W.
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I
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(1971).
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Electron Spectroscopy: X-Ray and Electron Excitation David M. Hercules' and James C. Carver Department of Chemistry, University of Georgia, Athens. Ga. 30602
The present article is a review of the field of X-ray excited electron spectroscopy for the period 1972-73; it is the second review of this topic in the Analytical Chemisto Fundamental Reviews. The first review ( I ) covered the literature abstracted beginning with volume 69 of Chemical Abstracts through the December 16, 1971, issue. The present review covers all literature abstracted in Chemical Abstracts starting with the December 23, 1971, issue through the December 10, 1973, issue. The present article also contains a sampling of the literature on applications of Auger Spectroscopy. For the sake of brevity the term ESCA will be used to refer to X-ray excited photoelectron spectroscopy and AES for Auger electron spectroscopy in the present review. .John S i m o n G u g g e n h e i m M e m o r i a l Fellow, 1973-74. T h i s w o r k was supported, in p a r t , by t h e N a t i o n a l Science F o u n d a t i o n u n d e r G r a n t GP-32484.
I t is very exciting to review the literature in a rapidly growing field such as ESCA. This growth cannot be reflected in any better way than to note 166 articles were covered in the 1972 review, while 545 articles are covered in the present one. The two-year interim has seen publication of an informal E S C A Newsletter, as well as the birth of the more formal Journal of Electron Spectroscopy and Related Phenomena. The Proceedings of the First International Conference on Electron Spectroscopy held a t the Asilomar Conference Grounds in Pacific Grove, Calif., on September 7-10, 1971, have appeared in print (2). A similar meeting was the Conference on Inner Shell Ionization Phenomena sponsored by the United States Atomic Energy Commission and the proceedings of this Conference (four volumes) have been published ( 3 ) . Three books dealing with ESCA have appeared during this period. Sevier's book, "Low Energy Electron Spec-
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