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(78) J. C. Russ. "EXAM Methods", Edax International Inc., Praire View, 111. (79) C. E. Fiori, R. L. Myklebust, and K. F. J. Heinrich, Anal. Chem., 48, 172 (1976). (80) D. B. Brown, J. V. Gilfrich, and M. C. Peckerar, J. Appl. Phys., 46, 4537 (1975). (81) J. W. Criss. unpublished intermed. thickness. (82) A. W. Witmer and M. L. Verheijke, Proc. 78th Colloq. Spectrosc. lnt., 3, 658 (1975). (83) D. Laguitton, R . Rousseau, and F. Claisse, Anal. Chem., 47, 2174 (1975). (84) J. Sherman, ASTM Spec. Tech. Publ, 157, Jun. 1953. (85) W. H. McMaster et at., Lawrence Radiation Laboratory Report UCRL 50174 (1969). (86) W. Bambynek et al., Rev. Mod. Phys., 44, 716 (1972). (87) A. Lurio and W. Reuter. Appl. Phys. Lett., 27, 704 (1975). (88) R. H. Miilar and J. R. Greening, J. Phys. 8, 7, 2332 (1974);7, 2345 (1974). (89) E. Storm and H. Israel, Nucl. Data Tables A7 (1970). (90) Contact J. W. Criss. Code 6480, Naval Research Laboratory, Washington, D. C. 20375. (91) S. D. Rasberry and K. F. J. Heinrich, Anal. Chem., 48, 81 (1974). (92) K. F. J. Heinrich and S. D. Rasberry. Adv. X-Ray Anal., 17, 309 (1974). (93) J. W. Colby, Adv. X-Ray Anal., 11, 287 (1968). (94) K . F. J. Heinrich and H. Yakowitz, Anal. Chem., 47, 2408 (1975). (95) A. R. Hawthorne and R. P. Gardner, Anal. Chem., 47, 2220 (1975). (96) J. Kashiwakura and Y. Gohski, Spectrochim. Acta, Part8, 30, 471 (1975).
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1975. (104) "Handbook, of Spectroscopy", (includes x-ray tables) CRC Press, Cleveland, Ohio, 1974. (105) J. L. Heilbron, "H. G. J. Moseley, The Life and Letters of an English Physicist", University of California Press, Berkeley, Calif., 1974: see also Ref. 8.
Electron Spectroscopy: Ultraviolet Excitation A.
D. Baker* and M. A. Brisk
Department of Chemistry, Queens College of the City University of N e w York, 65-30 Kissena Boulevard, Flushing, N. Y. 11367
D. C. Liotta Department of Chemistry, The Ohio State University, Columbus, Ohio 432 10
In this review, we cover ultraviolet photoelectron spectroscopy and closely related areas for the period late 1973 to late 1975. These two years have seen another tremendous increase in the use of the technique, and it is now finding expanding applications in fields remote from the original areas of investigation. Organic and inorganic chem-
ists, physicists, metallurgists, and surface scientists are among those who have applied ultraviolet photoelectron spectroscopy (UPS) to particular problems in their disciplines. As a result, the field is even more diffuse now than it was two years ago, when the previous review of this topic appeared in ANALYTICAL CHEMISTRY( I ) . Consequently, ANALYTICAL CHEMISTRY, VOL. 48, NO. 5, APRIL 1976
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we have had to be extremely selective in our coverage, and we apologize in advance for omitting specific references to the works of numerous individuals who have helped advance “the state of the art”. Our aim has been to focus on the developments and applications which seem to have excited the most interest and attention. General review articles, chapters, and books continue t o appear regularly. Chapters on electron spectroscopy are starting to appear in college level texts ( 2 ) .Eland has authored a very useful book dealing specifically with UPS, written mainly from the standpoint of the physical chemist or chemical physicist (3). A most valuable book authored by Robin, containing a critical survey of UPS and related uv excitation data was published in 1974 (4). A major conference on electron spectroscopy took place a t Namur, Belgium, in 1974. The proceedings constitute Volume 5 of the Journal of Electron Spectroscopy and Related Phenomena ( 5 ) . Contained in this volume is a review of UPS by Frost (6).A general review of all types of electron spectroscopy has been written by Brion (7). Other important reviews of UPS have been given by Price (8) and DeKock and Lloyd (9). A simplified guide to the interpretation of uv photoelectron spectra has appeared (10).As in the last ANALYTICAL CHEMISTRY review, we draw attention to the excellent Chemical Society (London) series of specialist reports on UPS ( 1 1 ) . A new series of comprehensive reviews of different aspects of electron spectroscopy is to appear in the next year (12). Turning our attention now to specific areas of development in UPS, we have found it convenient to subdivide this review into the following areas: Instrumentation and Experimental Developments, Intensity Measurements and Angular Distributions, Inorganic Chemistry, Adsorption Studies, Organic Chemistry, and Other Aspects.
INSTRUMENTATION A N D EXPERIMENTAL DEVELOPMENTS A high intensity vuv lamp suitable for UPS has been described by Poole et al. (13). I t consists of a glow dc discharge (100 mA) maintained between water cooled aluminum electrodes via an uncooled quartz capillary. A helium 30.4-nm source is described by Maier et al. (14). I t operates a t the low pressure of Torr, a charged particle oscillator arrangement maintaining sufficient intensity of plasma to sustain the discharge. The effects of gas pressure on photoelectron kinetic energy analysis have been discussed (15).Various discussions of energy analyzers have appeared, both from a practical and fundamental viewpoint. The ultimate factors dictating the resolution achievable at a given transmission have been analyzed (16). Read et al. have discussed the optimization of electrostatic energy selection systems ( 17). The collecting efficiency of a cylindrical mirror analyzer with pre-retarding lens has been determined over the range 0-30 eV (18). The practicalities and fundamental problems associated with the construction of a Hadamard Transform photoelectron spectrometer have been discussed (19, 20). A high temperature spectrometer is described by Allen et al. (21). Berkowitz and colleagues have also done much work with a high temperature photoelectron spectrometer. Berkowitz has a review of this work in press (22). We will reference studies on particular systems studied by the Berkowitz group in the “Inorganic Chemistry” section of this review. A simple-to-construct, good resolution photoelectron spectrometer utilizing magnetic focusing has been described (23). Photoelectron-photoion coincidence experiments have continued and a paper pointing out the limitations and optimal conditions pertinent to this technique has appeared (24). Among specific studies of compounds by photoelectron-photoion coincidence spectroscopy are those on SO2 (25), CS2 (26), N20 (27), COS (27), CHd (28), CH2C12 (29), CH2Br2 (301,CH212 (30),CzFe (31, 32), and CsHe (33). Another type of coincidence electron spectroscopy experiment is receiving increasing attention, viz., electron-electron coincidence spectroscopy. In this experiment, an electron beam is used to ionize a sample, and the ejected electrons are measured in coincidence with forward-scattered electrons of specific energies. Brion et al. have shown that it is 282R
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possible to simulate the conditions of UPS and XPS by the use of fast impacting electrons, then carrying out such coincidence measurements. Because such a coincidence technique allows “photon” sources of any desired energy to be selected (by choosing a specific energy loss for the scattered vs. impacting electrons), any ionization process of the molecule can be studied a t various excitation energies (impractical with conventional vuv and x-ray sources). Relative ionization probabilities for the formation of various ion states have been determined for CO (34,35),NH3 (36), and HzO (37). Additionally, such coincidence experiments have allowed the angular distribution of ejected electrons from Ne (38), Ar (39),and Kr (39) to be derived as a function of electron energy. Fixed frequency laser photoelectron spectroscopy of negative ions has been used to determine the electron affinities of various atomic and molecular species (40-43). In an elegant experiment, Turner and Bloch have studied the relative lifetimes of molecular ions produced in photoelectron spectroscopy (44). They use a modified photoelectron spectrometer to determine the radiative lifetimes of some electronically excited diatomic and triatomic molecules, relying on the measurement of the time delay between electron ejection and photon emission. The lifetimes they measured varied from 59 ns (for one of the N2+ excited states) up to 260 ns for an N20+ excited state.
PHOTOELECTRON ANGULAR DISTRIBUTIONS A N D PHOTOIONIZATION CROSS SECTIONS A substantial effort is now being made by several theoretical chemists and physicists to develop mathematical methods for predicting photoionization cross sections and angular distributions. Several approaches are currently under investigation and a lively debate seems likely concerning the relative merits of the different approaches. Methods based on a plane-wave approximation for the outgoing electron have been popular. For example, Schweig and Thiel (47) developed a theory which describes the initial state of the ionized molecule by LCAO MO’s with a valence basis set of Slater atomic orbitals, and the final state by a plane-wave approximation. The transition moment for the ionization process is then proportional to the overlap between the ionized MO and the phase wave of the ejected photoelectron. In work on angular distributions, calculations center on determinations of 6 , the anisotropy parameter, which characterizes the outgoing electron wave. fi appears in the equation expressing intensity as a function of angle:
where u represents the cross section. For example, ionization from an s orbital gives rise to an outgoing p photoelectron wave, and ionization from a p orbital results in outgoing s and d photoelectron waves. Ionization from molecular orbitals can be expected to result in a mixture of s, p, d , etc., waves, characterized by a particular 6 value. In principle, then, measurements of angular distributions can aid in the interpretation of a photoelectron spectrum, but this presupposes that the angular distributions can be measured accurately, and that accurate calculations of expected angular distributions can be carried out. The experimental work of Carlson et al. (45,46) on angular distribution measurement stands as a prime reference source for data used for judging calculations. There is clearly a need for more experimental work in this area. Experimental determinations of the variations of photoionization cross sections (0)with wavelength can be carried out using the electron-electron coincidence technique described in the preceding section of this review, or by using a synchrotron to produce a variable wavelength source, or by carrying out studies with several different types of resonance lamps (e.g., He I, He 11, Ne I, Ne 11, Lyman F, etc) and different x-ray lamps. All these approaches are finding some application. The plane-wave approach used by Schweig and Thiel appears to reproduce experimental band intensities in He I and He I1 photoelectron spectra fairly well, a t least in a rel-
A. D. Baker is currently Associate Professor of Chemistry at Queens College of the City University of New York. He was born in England, and received his BSc. (Special Honors Chemistry, 1965) from Imperial College, University of London. From 1965 to 1968. he worked as a graduate student of D. W. Turner, first in London, and later at the University of Oxford, obtaining the Ph.D. degree in 1968. His Ph.D. work involved studies on the original retarding field type of photoelectron soectrometer and on the first high resolu-r tion focusing type ultraviolet photoelectron spectrometer. Between 1968 and 1971, he held an appointment as Research Fellow at the Swansea campus of the University of Wales, working with D. Betteridge on analytical aspects of photoelectron spectroscopy. In 1971, Dr. Baker moved to the United States, first to be Assistant Professor, and later (1975) Associate Professor at Queens College. He has co-authored two books on photoelectron spectroscopy, and more than 20 review articles and research papers. He is a member of the Editorial Board of the Journal of Electron Spectroscopy and Related Phenomena, and is co-editor with C. R. Brundle of the forthcoming series of volumes, "Electron SpectroscopyTheory, Techniques, and Applications" to be published by Academic Press. In addition to electron spectroscopy, his research interests lie in other types of instrumentation for chemists, and in aspects of organic nitrogen chemistry. He has published several papers on the chemistry of nitrones and related compounds, and teaches organic chemistry courses at Queens College. Additionally, he has interests in the history of science and in chemical education at all levels.
ative sense ( 4 7 ) , and such variations can be useful in assignment of bands to particular orbitals (8,48). Several groups have, in fact, relied upon a plane-wave approximation for the unbound electrons (49-51 ) to calculate both cross sections and angular distributions. Rabalais et al. for example (50, 51) have used ab initio wavefunctions of the Gaussian lobe type coupled with an orthogonalized plane-wave for the final state to calculate photoionization related properties. Manson and Shyu (52) and Ritchie (53)have struck a note of caution, however, concerning the plane-wave approximation, and their objections will have to be answered. Their work seems to indicate that nonCoulomb phase-shift terms are crucial to the correct determination of 6 , and since plane-wave (or Coulomb wave) continua omit this, it apparently precludes the possibility that absolute calculated values of angular distributions made by the plane-wave approximation could be valid. More insight into this problem can be anticipated in the following two-year period. Chapman (54) has described a computational method for the calculation of photoionization cross sections and angular distributions within the general framework of a singlecenter expansion method discussed by earlier workers (referenced by Chapman). His method employs a partial wave expansion for the continuum wavefunction to generate a set of coupled radial equations which are solved exactly using a noniterative integral equation technique, and in addition, single-center expansions are used for the initial and final state wavefunctions. It appears that the method has promise in handling moderately complex molecules. Also of interest is a series of papers by Ritchie (53,55-57) in which angular distributions are calculated. In Ritchie's work the radial coupled equations are decoupled by use of the distorted-wave Born approximation, and the resulting equations are solved by the use of the Coulomb-Born approximation. Other approaches to photoionization calculations include an expansion of the unbound electron wavefunction in a two-center ellipsoidal co-ordinate system ( 5 8 ) ,and the consideration of the electron-molecule interaction with an approximate potential whose form permits the use of multiple-scattering techniques (59). INORGANIC COMPOUNDS
Several trends appeared in the literature involving the application of UPS to inorganic systems over the period covered by this review. For example, both He I and He I1 spectra were commonly obtained in photoelectron (PE) studies, coupled with results of MO calculations. One can frequently assign bands by a comparison of relative peak
Marion A. Brlsk received a B.A. and M.A. from Queens College in 1970 and 1972, respectively. She received a Ph.D. in 1975 from the City University of New York in physical chemistry under the supervision of A. D. Baker, working with both the UPS and ESCA techniques.
Dennis Liotta received his undergraduate training at Queens College of C.U.N.Y. from which he received his B.A. degree in 1970. He attended graduate school at the City University of New York and received his Ph.D. degree in 1974. Currently, he is doing postdoctoral research at The Ohio State University.
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intensities in these spectra since band intensities relate to orbital compositions. Molecular orbital calculations using both ab initio and approximate methods are also of value in the interpretation of spectra. By assuming the validity of Koopmans' Theorem (KT), calculated eigenvalues can usually be correlated with experimentally determined ionization potentials. In general, this approach has enjoyed widespread success in photoelectron spectroscopy until recently when K T is shown to break down. It appears to be deficient when applied to localized molecular orbitals such as predominantly metal d orbitals in transition metal complexes. This means that the technique cannot always reveal the relative energies of molecular orbitals in the ground state molecule. Consideration of spin-orbit coupling effects has also been shown to be a powerful tool in spectra elucidation. MO's which contain significant heavy atom character very often exhibit spin-orbit splittings upon ionization. Such observed splittings are usually smaller than those corresponding to the free ion as a consequence of delocalization over more than one center. The magnitude of the splitting, therefore, is also indicative of bonding properties of the molecule. Other trends to be noted include utilization of data from other experimental techniques, correlation of UPS data with the chemical properties of systems under consideration, and a reassignment of already published spectra as a result of new information. Simple Compounds. In this section, results of photoelectron studies on molecules containing relatively few atoms are presented. Brehm and Hofler (60) and Hotop et al. (61) examined the P E spectrum of Ba in a cross-beamed experiment using He I radiation. A high ionization cross section for production of Ba2+was observed relative to Ba+ in the ground state. In addition, Baf ions were detected in several excited states including 8p, 5g, 6f, and 10d electron configurations. Walker et al. (62) observed the rotational broadening accompanying some fine structure in the P E spectra of H F and DF, and us,ed these data to calculate an accurate spinorbit coupling constant. Ber mark et al. (63) reported shifts in the P E spectra of H2&0 and H2180 caused by the isotopic substitution of 0. Tanaka and Tanaka (64) reported photoelectron studies on 0 2 using a continuum light source and a monochromator with emphasis on the autoionization process and associated Franck-Condon factors. Dromey et al. (65) presented a time-averaged and deconvoluted P E spectrum of 0 2 ; the spin-orbit splitting of the ground state 0 2 + ion is shown to be 25 meV as compared with the spectroscopic value of 24.2 meV. Samson ANALYTICAL CHEMISTRY, VOL. 48, NO. 5, APRIL 1976
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and Petrosky (66) attributed the appearance of some structure in the spectrum of atomic 0 as being caused by the presence of the 0 I resonance line in the exciting radiation. This line arises from the 0 2 impurity in the He lamp. Gardner and Samson (67) re-examined the He I1 P E spectra of CO, Nz, 0 2 and COz employing a grazing incidence monochromator. I t was shown that bands appearing between 30-40 eV correspond to dissociative final states. The same authors recorded the high resolution P E spectra of N2 and CO using the 736- and 744-A Ne lines (152). Vibrational structure was observed corresponding to transitions which probably involve an autoionizing intermediate electronic state. Frost et al. (68) and Brundle (69) studied the P E spectrum of 03;band assignments were not straightforward and further studies were suggested (69). The He I P E spectrum of the transient species S a 0 was interpreted by the application of CND0/2 and ab initio MO calculated results, as well as comparisons to spectra of some isoelectronic analogues: NSF, NSCl, and SOz. Frost e t al. (70) indicated that these calculations require the inclusion of the S 3d orbitals for both the SO2 and S 2 0 molecules. The same authors (71) employed measured and calculated Franck-Condon transition probabilities to establish the geometries of the OCSe, SCSe, and CSez molecular ions. Cradock and Duncan (72) noted weak bands in the P E spectra of SCSe and CSe2 which they assigned to a shake-up transition accompanying direct ionization. The positions of these peaks were shown to be related to transition energies observed in the electronic spectra of the compounds. Frost et al. (73) proposed that the first band in the spectrum of HCN is a composite of two peaks representing ionization from both x and u MO’s; the x electrons being of lower IP. Their analysis of the spectrum of HCP facilitated this band assignment. The calculations of So and Richards ( 7 4 ) supported the interpretation of Frost et al. I t was also shown that the first two low-lying electronic states of HCN and N2 are in reversed order, probably because of the large differential correlational energy associated with the two states in Nz. Fridh and Asbrink (75)conducted P E studies on HCN and DCN and also supported the assignments of Frost et al., although there is some discrepancy regarding the vibrational analysis. A P E study on the unstable molecule HBS was performed by two groups: Fehlner and Turner (76) and Krato et al. (77). Both groups showed the orbital orderings in HBS and HCP to be the same. In addition, the observed fine structure indicates that the ionic states involved are linear. Comparison of experimental IP’s and eigenvalues resulting from H F calculations on S(CN)2 indicates a breakdown of KT; the N lone-pair orbital appears out of the predicted order. Rasmus et al. (78) noted that K T becomes less valid when one is considering ionization from localized orbitals. As already mentioned, such effects are seen in Nz, and can be accounted for on the basis of correlational energy considerations. Several high temperature P E investigations of metal halides in the vapor state have been reported. Goodman et al. ( 7 9 ) , Berkowitz et al. (80), and Potts et al. (81) obtained P E spectra of the alkali metal halides. There are apparent discrepancies in the interpretation of some of the spectra; the presence of dimers in the vapor phase in particular complicates the task of making band assignments (80). Poole et al. employed UPS to establish the valence band ordering in some solid alkali metal halides (82, 83). Cocksey et al. (84),Bogges et al. ( 8 5 ) ,Cusachs et al. (86),and Berkowitz (87) reported spectra of some Group IIb metal halides. Cocksey et al. used observed shifts of the metal d electrons to conclude that Zn is more electronegative than Hg in disagreement with the Pauling Scale. Berkowitz et al. however, examined the He I and He I1 spectra, and concluded that the electronegativity trends of the Pauling Scale in this matter are in agreement with their data. Evans e t al. (88), Lappert et al. (89), Dehmer et al. ( g o ) , and Wittel and Manne (91) recorded spectra of some Group IIIb and IVb gaseous halides. The variable temperature studies conducted by Lappert et al. showed a correlation between relative peak intensities in the spectra of Al, Ga, and In halides and known equilibrium constants re284R
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garding dimer formation in the vapor state. The work of Dehmer et al. on Ga and In trihalides suggests a revision of the earlier interpretation of the BBr3 and BIB P E spectra. The P E data of Burroughs et al. (92)suggests participation of the inner 5d electrons of Hg in covalent bonding of Hg(CN)Z i.e. d x > px* back donation in the Hg-CN bond. Baker et al. (93) correlated the dihedral angle in MezSz with the observed splitting (0.27 eV) between the first two bands in the P E spectrum of the compound. These two bands are predominantly of S 3p character. Colton and Rabalais (94) employed results of CNDO/2 calculations as well as He I and He I1 spectra to establish the orbital ordering in SC12, S2C12, SZBrz, and MeZS2. The observed splitting involving the S 3p orbitals in Me2S2 agrees with that reported by Baker e t al. This splitting was reproduced when a dihedral angle of 85’ was assumed in the calculations, also in agreement with the value determined by Baker et al. Osafune and Kimura (95) observed a splitting (1.0 eV) between the primarily 0 lone-pair MO’s of H202. CND0/2 calculations correlated this experimental splitting with a 1 1 1 O dihedral angle. Davies (96)compared results of ab initio calculations on H202 and HzSz with experimentally determined IP’s. Solouki et al. (97)also compared ab initio results on SClz with P E data. Lloyd and Roberts (98) conducted a variable temperature study of CF4, SiF4, and GeF4 with the objective of establishing the origin of some fine structure; analysis of this fine structure had prompted Jonas et al. (99) to propose a significantly different interpretation from the original work on these compounds (100).The results of Lloyd and Roberts however, supported the original assignment by identifying observed fine structure in the SiF4 spectrum as being due to the presence of “hot bands.” Dekock (101) investigated bonding in the XeF4, XeOF4, and IF4 molecules. His assignment of the XeOF4 P E spectrum disagrees from that presented by Brundle and Jones (102). Lloyd et al. (103) determined the electronic structure of H N 0 3 by examining both the He I and He I1 spectra, comparison of experimental IP’s with those theoretically determined (ab initio SCF MO calculations), and by comparison of spectra with those of some isoelectronic species. Mines and Thomas (104) detected evidence for Jahn-Teller distortion in three bands belonging to the P E spectrum of SO3; it was shown that these peaks relate to degenerate electronic states of SO3+. Connor et al. (105)recorded P E sDectra of some oxvanion comDounds of S O P . c104-, C103’, and Pod3;. Frost et al. (106) designed a P E spectrometer to handle reactive and short-lived species including some nitrosyl and nitryl halides. Spectra appeared to be successfully interpreted by considering the effect of X (halogen) on the known electronic levels of NO and NO2 in XNO and XN02. I t is interesting to note that the stabilization of the a1 orbital of the NO2 group (strongly bonding over NOz) is reflected in the nitrating ability of the molecules investigated. Inorganic Phosphorus Compounds. The P E spectra of some trivalent P compounds have revealed some interesting aspects of bonding in these molecules. The P lone pair orbital usually corresponds to the highest energy level and, in addition, tends to be well separated from the rest of the spectrum; hence readily identifiable. The I P and broadness of this band in general is affected by hyperconjugative effects, (p-d)x-bonding (charge density transferred to virtual P d orbitals), geometry (hybridization of lone-pair MO) and of course inductive effects. I t is interesting to note that the lack of fine structure and general broadness of this band in most P E spectra indicate that these P electrons are significantly delocalized and, therefore, not really lone-pair electrons a t all. Lappert et al. (107) examined the P E spectra of some halogen substituted phosphines: RnPX3-, (R = Me or Bu, X = H, C1, or F, n = 1-3), (MezN),PC13-, ( n = 1-3), and (R2N)PF2 (R = Me or Et). I t is shown that the P lone-pair I P provides a satisfactory measure of relative basicities or electron donor capabilities within a related series of phosphines. Cowley et al. (108) also reported the spectra of some halogen containing phosphines: (CF3)3-nPHn and (CF3)3-nPCln ( n = 0, 1, 2). The IP of the P lone-pair electrons appears to follow expected inductive trends; (CF3)3P ~I
> (CF&PX > (CF3)PXz > PX3 for X = H or C1. However, i t should be noted that successive replacement of CF3 by C1 leads to a progressively smaller change in the P lone-pair IP. Such behavior can be explained by conjugative effects or even changes in molecular geometry upon C1 substitution. For example, the P lone-pair I P is greater in PH3 than in PC13 (109); the X P X angle in PH3 and PCl3 is 93.3 and 100.3’, respectively, indicating greater percentage s character in the phosphine P lone-pair orbital. In fact, changes in hybridization of the lone-pair orbitals of X in Me3X (X = N, P, As, Sb) compounds was reported by Elbel et al. (110) to account for the essentially constant lone-pair IP’s in this series. The CXC bond angle is known to decrease as X is changed from N to S b indicating an accompanying increase in s character of the lone-pair orbitals. This trend counterbalances the decreasing energies of the atomic p orbitals in the series N < P < As < Sb. Debies and Rabalais (111) implicated the existence of (p-d)a bonding in some phenylphosphines, -arsines, and stibines. It was shown that the lone-pair orbitals are destabilized relative to the N analogues. Schmidt et al. (112) detected P-C U-ahyperconjugation in several a systems: vinyl-, phenyl-, benzyl-, and allylphosphines. These interactions were determined by measuring the shifts of the ethylenic 7c and P-C u IP’s after accounting for inductive effects. CND0/2 calculations indicate that it is this hyperconjugation interaction which determines the conformation of the vinyl-, allyl-, and benzylphosphines. Cowley et al. (113) measured lone-pair IP’s of N and P in MezNP(CF&, MezNP(Cl)CF3, and Me2NPC12. The N lone-pair I P is essentially constant in the series. This is consistent with the gauche geometry of these molecules, i.e. an angle of 90’ between N and P orbitals so that interaction cannot occur. Both the trans and gauche rotamers of some diphosphines were studied by Cowley et al. (114). The P E of Me4Pz exhibited three peaks due to the P lonepairs. The first and third peak ( A = 1.66 eV) are due to the n+ and n- combinations of the P lone-pairs in the trans rotamers, while the second peak corresponds to the P lonepair in the gauche rotamer. Similar assignments were made for the P E spectrum of Me4Asz (A = 1.59 eV). The IP’s of P lone-pairs are also of interest in some polyphosphines. Cowley et al. (115) showed that (p-d)r bonding is probably not significant in those compounds. Goodman (116) presented the P E spectrum of PF5. The energy level ordering predicted from approximate MO calculations disagrees with the experimentally determined ordering. Nicholson and Rademacher (117) studied the P E spectra of S b I11 halides. Their results indicated that (p-d)?r back-donation is probably not important in these compounds. Transition Metal Complexes. Transition metal complexes have become popular subjects for UPS investigations. Consequently, application of the technique has revealed a great deal of information concerning the bonding in these molecules. Qualitative arguments, based on ligand field theory, can be used to interpret spectra, particularly when symmetrical molecules are involved. In the spectra of most transition metal complexes, the bands in the low ionization potential region correspond to the molecular orbitals which are mainly of metal d character. These bands are of particular interest since they tend to be well separated from the rest of the spectrum. In addition, their ionization energies and bandwidths reflect important aspects of metal-ligand bonding. It should be noted however, that these molecular orbitals tend to succumb to the weaknesses inherit in KT, and caution must be exercised in correlating the energy levels as determined by UPS with those of the ground state molecule. The transition metal carbonyls and sandwich complexes have received considerable attention. In fact, the hexacarbonyls and some substituted carbonyls served as models for testing the ability of K T to reproduce spectra. Higginson et al. (118) reported the He I and He I1 PE spectra of the Cr, Mo, and W hexacarbonyls. Published MO SCF ab initio calculations as well as relative band intensity data permitted a satisfactory interpretation of these spectra. Four groups of bands were predicted and were experimentally observed. The first band involves the 2tzg MO shell which is predominantly of metal d character. As the Cr, Mo, W series is transgressed, the first ionization
potential decreases while the intensity of the band increases (“heavy atom effect”). Mono-substituted complexes of Cr and W carbonyls (M(CO)hL, L = amine, PH3, isonitrile) were investigated by Higginson e t al. (119). In these complexes, the symmetry seen by the metal is reduced to C4”; the metal tzg orbitals should now transform as e and b2 representations. The spectra of the amine substituted complexes exhibit the p r e dicted two peaks in the metal d band region, while the spectra of the phosphine and isonitrile complexes show a t most an asymmetric band shape. Curve analysis yields an intensity ratio between the two peaks (lower IPhigher IP) of 2:l. In addition, the first peak in the W(C0)5NH3 spectrum is spin-orbit split. Therefore, the e type MO was assigned to the lower IP band followed by the bz peak. It is interesting to note that the calculated ordering of the e and b:! levels is reversed from the experimentally determined order. This result is not surprising since it is known that the relaxation energy accompanying ionization is dependent on the metal character of the orbital from which ionization occurs. Calculations show the e MO to possess greater metallic character than the b2 MO, thereby implying a greater associated relaxation energy with the e MO and a possible reversal of the predicted ordering of energy levels. When K T is shown to fail in reproducing spectra, A SCF calculations can be performed. These calculations are achieved by taking the difference between the ground state energy and energy of the ionic states. Such calculations have not only been successful in reproducing spectra, but also produce values for the relaxation energies associated with the ionizations under question. Results of A SCF calculations on Ni(C0)4, Fe(C0)2( N o h , and Co(C0)3NO were reported by Hillier et al. (120). Thus, the spectra of the nitrosyls could be reproduced since K T was shown to be inadequate in these cases. A larger relaxation energy was calculated for ionization from the 5t2 (4.7 eV) and 2e (5.7 eV) MO’s of Ni (CO)4 which are mainly of Ni 3d character. These values reflect the dependence of relaxation energies on metallic character; the 2e MO has an associated 90% metallic character while the 5 t 2 MO consists of 47% metallic character. The nitrosyl complexes exhibit an even more striking dependence of relaxation energy on the percent metal orbital composition of the MO. Photoelectron spectra of Mn(C0)5X complexes, where X = C1, Br, I, H, or CH3, have been the source of considerable disagreement regarding band assignments. The original order of XAe) > X,,(a,) > e(dxz, d > bz(d,,) has been amended by Caesar et al. (121) to x‘, > b2 > e > Xu. Their new assignment is based on trends observed in the spectra of substituted Mn carbonyl complexes and their Re analogues. However, M. B. Hall (122), by calculating spin-orbit splittin s in the Re complexes, arrived a t band assignments which cfiffered from those of other workers. For example, the first band in the spectrum of the C1 substituted complex was shown to be mainly of Re character. In addition, the ionization potential associated with the e(d,,, dyz)MO is reported to be lower than the I P corresponding to the bp(d,,) MO. This latter assignment is analogous to that observed in the substituted Cr and W carbonyls. Guest et al. (123) have shown that K T is again deficient in interpreting the P E spectra of Mn(C0)5H, Mn(C0)5CH3, and Fe(C0)dHz. In these cases, K T places the M-X u bond a t lower IP’s than the metal 3d MO’s. A SCF calculations also failed to produce the appropriate ordering. However, CNDO calculations did reproduce the experimentally determined order in all three cases. Connor et al. (124) reported the He I and He I1 spectra of iron tricarbonyl butadiene. K T again produced an incorrect band order, and A SCF calculations were reported. I t was shown that there is a net increase of electron density on the CO ligands due to a back-bonding being greater than u donation, as in other carbonyls. In addition, the butadiene ligand resembles the butadiene anion because of extensive back-bonding through a 7c virtual M.O. Baerends et al. (125) used results of discrete variational X a calculations to satisfactorily interpret the He I spectra of Fe(C0)b and C2H4-Fe(C0)4. I t is interesting to note that the energy levels of free ethylene are almost unaffected upon bonding ANALYTICAL CHEMISTRY, VOL. 48, NO. 5, APRIL 1976
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to Fe in agreement with calculated energies. The negative charge on the CzH4 moiety was explained as resulting from electron acceptance by both the a* and u* virtual orbitals of ethylene. UPS studies on sandwich complexes have provided interesting information about the bonding in these molecules. In general, band assignments were made by examining spectra of structurally similar complexes in which either the metal or ligand is changed in a series. Then, observation of trends and application of simple MO schemes permitted band assignments. Evans et al. (126) reported the He I spectra of some open shell metallocenes; M(C5H5)2, M = V, Mn, Co, Ni, and their 1,l’-dimethyl derivatives. The spectra of these molecules were shown to bear a striking resemblance to ferrocene. Additional studies by Evans et al. (127) on some bisa-arene Mo and V compounds, and on some mixed sandwich complexes containing the C7H7 and C ~ H B ligands, shed light on the most important sources of bonding in these molecules. It was shown that the ring size determines the ligand MO which is most stabilized upon complexation. When the same MO scheme was applied to molecules of the formula (C5H5)M(C7H7) where M = Zr, Nb, Mo, trends were noted which are similar to those observed in the spectra of the 3d transition metal analogues (127). Groenenboom et al. (128) explained observed differences by con‘sideration of the more diffuse nature of the 4d orbitals and {changes in the percent metal character of some MO’s. It appears from the aforementioned work that bonding of the cyclopentadienyl ligands is due mainly to interaction of the MO’s of a symmetry, while the cycloheptatrienyl and cyclooctatetraene ligands participate mainly in 6 bonding. Ab initio calculations and He I and He I1 spectra of c r ( C s H 6 ) ~and (C6Hs)Cr(C0)3 were presented by Guest et al. (129). They indicated that benzene carries a net negative charge while CO accepts more electron density than in the hexacarbonyl complex. KT is shown to satisfactorily interpret bands arising from ionization of predominantly ligand electrons, but is unsuccessful in reproducing bands corresponding to mainly metal MO’s. Whitesides et al. (130) interpreted the P E spectra of some cyclic pentadienylmanganese tricarbonyl complexes (“ring whizzers”) by establishing the electronic structure of the Mn(C0)3 and ligand fragments separately, and then permitting the two to interact. The principal bonding interaction involves the el” level of C5H5- with the Mn d,, orbital. Other studies have been conducted on cyclopentadienyl transition metal complexes containing ligands such as H, Me, olefins (1311, C1 (1321, and NO (133). In general, bonding models were proposed which agree with experimental and theoretical results when orbital relaxation effects were considered. He I P E spectra of some tetrahedral trifluorophosphine and dialkylamide transition and Group IVb metal compounds have been examined. Qualitative ligand field theory appeared to suffice in the interpretation of the spectra. Bassett et al. verified the d’O electronic configuration of M(PF3)4 compounds where M = Ni, Pd, and Pt (134). As in the spectrum of Ni(C0)4, the d level is split into t 2 and e components as a result of the local tetrahedral symmetry. These bands increase in intensity as the metal is changed from Ni to Pt, in the series. This “heavy atom effect” was also observed for Group VIa hexacarbonyls. The observed shifts of the u, ligand, and metal d bands indicate that the bonding in these molecules consists of 0 donation as well as a back-bonding. These bonding interactions appear to be strongest for the Pt complex. P E data also revealed information about the geometry of the following complexes: M(NR& and M’(NR’2)4 where M = C, Si, Ge, Sn, Ti, Zr, Hf, or V; R = Me; M’ = Ti, Zr, or Hf; and R’ = Et. This was primarily accomplished by observing the extent of interaction between the N lone-pair electrons. Gibbons et al. (135) ascertained that the carbon atoms are probably fixed, i.e., no free rotation about the M-N bond. Other four coordinate metal compounds investigated by UPS include MR4 and M’R4 where M = Ti, Zr, Hf; M’ = M, Ge, or Sn; and R = Me3CCH2; R’ = Me3SiCH2 (136). In these molecules, the HOMO appears between 8-9 eV and corresponds to the M-C bonding MO. Although this level is insensitive to the nature of the transition metal, its I P de286R
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creases with increasing size and decreasing electronegativity of the Group IVb central atom. W(NMe2)6 (135) and W(Me)6 (137) are among the few octahedral complexes other than the hexacarbonyls that have been investigated by UPS. He I and He I1 spectra of W(Me)6 and TdMes indicate that there is a significant contribution by the 5d metal orbitals to bonding. Several UPS investigations of some metal oxy compounds, viz., Cr02C12 (138, 139), Ru04, Os04, VOC13, and Mo02C12 (139, 140) have been reported. It is shown that the original interpretation of the tetroxide spectra must be amended as a consequence of spurious peaks due to impurities. The electronic structure of CrOzClz was determined by treating the molecule as a distorted Cr042- tetrahedron of CzU symmetry (138). The MO structure retains the general sequence of that in a tetroxide of Td symmetry. The existence of a quadruple bond in the dimers [Mo(RC02)2]2 (R = H, Me, CMe3) has been supported by UPS studies (141). Three bands appear in the spectrum of the formate complex (D4h) pertaining to the b2,(6), e,(a), and alg(u) metal bonding MO’s at 7.7, 9.5, and a shoulder a t 11.0 eV, respectively. Khandelwal and Roebber (142) obtained P E spectra of some biologically important molecules: tetraphenylporphine and its complexes with dipositive ions of Mg, Mn, Fe, Cu, and Zn. All of the spectra are very similar and a detailed peak assignment is difficult because the bands tend to be broad and featureless. Organometallic Compounds. The work described in this section involves P E studies of structurally similar sets of molecules, in which Group IVb metals (Si, Ge, Sn, Pb) are substituted for carbon. Substitution, in general, results in noticeable spectral changes, especially in the low ionization potential region; peaks are shifted and sometimes broadened. These differences have been attributed to geometry changes accompanying substitution, as well as (pd ) r bonding, hyperconjugation (n-a, a - ~ ,u-u mixing), and inductive effects. The latter two effects tend to destabilize lone-pair and A orbitals. On the other hand, (p-d)r bonding leads to a delocalization of charge, and therefore, has a stabilizing effect. It should be noted that these interactions are generally discussed in terms of semi-localized molecular orbitals. Distefano et al. (143) presented the spectra of S[X(CH3)3]2,and CH3-S-X(CH3)3 molecules where X = C, Si, Ge, Sn, Pb. The band corresponding to the S lone-pair orbital is of primary interest in these spectra; this localized MO can interact by symmetry with XC3 group orbitals (hyperconjugation) as well as with the empty d orbitals of X ((p-d)a bonding). In addition, its associated I P is also affected by inductive changes accompanying substitution. The observed spectral trends suggest that the most important interaction in the Si substituted compounds is between the S n a and Si 3d orbitals ((p-d)r bonding). This is a reasonable result since the same principal quantum number is involved. Hyperconjugation appears to predominate in the heaviest compounds. However, a strong inductive effect ( + I ) is correlated with the Pb(Me)3 group. Bock et al. (144) attempted to account for observed differences in the energy levels of (CH3)2O, CH30SiH3 and (SiH3)20. CND0/2 and E H T calculations coupled with K T were used to isolate the various factors which tend to alter IP’s upon substitution. It was also shown in this work that substitution of Si for C causes delocalization of the 0 lonepair electrons primarily through (p-d)r bonding. P E data and CNDO calculations on (CH3)2S, CH3SSiH3, and (SiH3)2 by the same group (145) also indicate a strong (pd ) a interaction accompanying Si substitution. The I P of the S lone-pair electrons (HOMO) in these spectra shifts to higher values, and the associated bands are broadened, reflecting the delocalization afforded by the (p-d)a interaction. Calculations on the ethers show a large dependence of energy levels on geometry. However, the S analogues appear to be relatively insensitive to the geometry change introduced. The work of Ramsey et al. (146) suggests a strong interactiop between the 0 lone-Dair orbital and the M-C u bond in Me3MC(O)Me (M = Si, Ge). This interaction probably explains the shift of the 280-nm uv transition of aliphatic ketones to longer wavelengths in the electronic spectra of R3MCR and (R3M)zC(O); M = Si, Ge compounds. A par-
ameterized hyperconjugation model was used by Ensslin et al. (147) to explain the observed a splittings in the PE spectra of H~XCECH and H3XCrCXH3; X = C, Si. In these molecules, the hyperconjugation interaction involves the a and M-C a orbitals. A a, M-C mixing is also thought to exist in some allyl (CH2=CH-CH2MR3) and benzyl (CsHb-CH2MR3; M = C, Si, Ge, Sn) (148) compounds. Starzewski et al. (149) also noted a decrease in I.P. of the a MO with Si substitution in some ylidic systems. They, however, attributed this trend not to a a energy lowering, but to a decrease of the MO coefficient of the ylidic carbon atom. Hosomi and Traylor (150) have demonstrated interactions between adjacent C-Sn bonds in some tin organometallics. A 2.6-eV interaction is indicated between adjacent C-Sn bonds in Me3SnCH2CHz-SnMe3. This interaction is even larger than that observed in allyltrimethyltin (2.2 eV), where a-a mixing is shown to exist (151).A u-a interaction is also apparent (1.6 eV) between the C-Sn a and the cyclopropane orbitals in cyclopropyl-carbinyltrimethyltin (152). Bischof et al. (153) investigated the relative importance of hyperconjugation, (p-d)a bonding, and inductive effects in (CH3)sMPh and (CH3)3MCH2Ph where M = Group IVb metals and P h = phenyl. First-order perturbation theory was employed to establish the contribution of the aforementioned effects on the electronic structure of the molecules. The PE spectrum of SiFsMn(C0)s exhibits a band a t 10.4 eV which is assigned by Cradock et al. (154) to the Si-Mn u bond. This band could not be located in the PE spectrum of the SiH3 pentacarbonyl derivative; it was suggested that the a band was obscured by stronger bands a t =9 eV due to the Mn 3d levels (155). It is interesting to note that the Mn 3d levels show no splitting although the local symmetry involved is C d U . Ensslin et al. (156) reported the P E spectra of some po2, 3, 4, 5 ) . P E spectra of the n = 4 lysilanes; Si,H2 and 5 species c&$$b: correlated to CNDO calculated eigenvalues only if several rotamers were considered. The disilane and trisilane spectra, however, do not present such conformational problems because of their known D3d and CzVsymmetry. I t was also noted that d orbitals were included in those CNDO calculations which reproduced the disilane P E spectrum. Cradock et al. (157) obtained the He I P E spectra of boratran and some silatrans. Spectra were interpreted by consideration of u donation from N to the empty p orbital of B (or d orbitals of Si), and of interactions of 0 lone-pair levels. It was concluded that the observed proximity of N to B (or Si) is a consequence of a bonding interaction involving the N lone-pair level and the metal.
ADSORPTION STUDIES Many UPS studies using standard He I and/or He I1 radiation have recently been performed under uhv conditions in order to investigate adsorption of gases on metal surfaces. Such studies are of particular importance to the understanding of catalytic reactions which are known to occur a t these surfaces. It should be noted that other electron spectroscopic techniques have been applied to similar surface problems, and were reviewed by Brundle (158, 159). The same author described the various commercial spectrometers which have been employed for surface studies (160). XPS in particular is undoubtedly a valuable technique in this area of research. The kind of information that one hopes to acquire when conducting UPS studies on particular adsorption systems includes: a) nature of the adsorbed species; b) strength of adsorbate-substrate bond, i.e., degree of interaction; and c) mode of adsorbate-substrate bond, i.e., which, if any, adsorbate MO’s interact with the substrate. Answers to c) will also relate to the position of the adsorbate molecules on the surface. In order to abstract the desired information from UPS, it is usually necessary to compare the spectra of the surface with those of the free molecules, so that IP’s can be compared and bands can be assigned. Such a task can be formidable and may require considerable theoretical and experimental data as was the case in the adsorption of CO on various metal surfaces. The two main problems appear to involve reference levels and relaxation energies (161, 162).
The IP’s of valence energy levels of the gaseous molecule are referenced to the vacuum level, while the Fermi level serves as the reference level for the adsorbed state. Comparison can only be made after the addition of an appropriate work function to the IP’s of the adsorbed molecule. The use of the work function for the metal is only an approximation, since the presence of adsorbed species may change this value (163). In addition, the possible effect of differential relaxation energies must be considered. It is known, for example, that the relaxation energy associated with ionization for electrons in the solid state is larger than that associated with the free molecule (164). The relationship between gas phase IP’s and those of the adsorbed molecule can be summarized by the following equation (159): (B.E.,
- B.E.,d,);
=4
+ AEjR f U j B
where 4 is the appropriate work function for the adsorbed state, A E j R is the difference in relaxation energies associated with the molecular orbital j and aEjB relates to the genuine chemical shift, if any, for orbital j . Demuth and Eastman (166) suggested a method to deal with these serious problems. They compared IP’s of the condensed phase molecule with those of the chemisorbed species, and assumed that those orbitals which are nonbonding with respect to the substrate are shifted only as a result of a change in associated relaxation energies. Consequently, a relaxation energy value could be tabulated and applied to the bonding orbitals as well. Thus, the authors assume that all of the adsorbate IP’s have an equivalent associated relaxation energy, a point that has not been proved. However, their results were used to calculate a chemisorption bond energy for ethylene which was in good agreement with experimental data. A significant number of adsorption systems have been studied bv UPS: 0 2 , c d , NO, N20, Con, H20, H2S, C2H4, C6H6, C2H2, CH30H on Ni (159, 161, 166-168, 179, 181, 186: 187); CO, C02, 0 2 on Ti, H2, N2, CO, 02, CpH4 on W (167, 169-172); CH30H and H2CO on W (173); 0 2 on Sr and Ba ( I 74,175); CO, COz, 0 2 , and H20 on Mo and Au (176, 177, 189, 190); CO2 and 0 2 on Pt (178-180, 184); CO on P d (181); CO, C&, and C2H2 on c u (159, 182, 185, 186); co, C6H6, C2H4, C2H2, and H2S on Fe (159, 183,185,186) and CO on Ru (184.188). In many of these’studies information concerning the nature of the adsorbed species could be derived without a comparison of absolute binding energies between the free and adsorbed molecule. For very weak interactions associated with condensation or physisorption, the solid-state spectra exhibit the same general features of the free molecule spectra except that bands are broadened (159). However, if adsorption causes dissociation of the molecule, the similarity disappears. The interpretation of UPS spectra becomes more complex when the adsorbate molecule is strongly chemisorbed in the molecular state. For the sake of brevity, results of only a few studies will be presented here in order to exemplify the ability of the technique to reveal valuable information about adsorption systems. The UPS of molecularly adsorbed CO has been reported for a large number of transition metal substrates including Ni (161, 168, 181), W (161, 170), Fe (183), Cu (182), Mo (161, 176, 1771, Ru (1841, Pd (181), Pt (178, 180, 184) and Au (177). In general, two bands, belonging to molecular CO levels have been observed in the He I spectra of these systems. The original work (192) assigned these two bands to the gas phase 5u and la MO’s. However, a great deal of work followed and the balance of evidence strongly suggests that the band at lower IP relative to the Fermi level is primarily 1~with some admixture of the 50. level, while the second band corresponds to the gas phase 4 u MO (159, 165, 180, 181, 183, 186-188). Such a conclusion implies a significant shift of the 50. electrons so that the CO molecule is thought to be adsorbed via interaction of the 5u level with the substrate orbitals. Atkinson, Brundle, and Roberts (176) showed that although CO is chemisorbed in its molecular state on polycrystalline Mo at 77 K, it is present in a dissociated state a t 300 K. The UPS of CO on Mo at 300 K exhibits features which were correlated to atomic C and 0 atomic levels. Kishi and Roberts (183) reANALYTICAL CHEMISTRY, VOL. 48, NO. 5, APRIL 1976
* 287R
ported the presence of dissociated CO on polycrystalline Fe at 295 K and 350 K. In addition, it was shown that pre-adsorption of H2S stopped dissociation of subsequent CO adsorption a t 295 K. This result is of interest since sulfur doping is used in industrial catalytic processes to promote yields and specificity. The work of Bonze1 and Fischer (184) on the NO/Pt, Ru systems is also related to a known catalytic reaction, viz., reduction of NO on these metals. Their study confirms dissociation of NO on Ru but not on Pt a t low temperatures. Adsorption studies involving C&, C2H4, and C2H2 (162, 166) a t various temperatures imply the interaction of the higher gas phase a level with the substrate in cases of chemisorption. In the case of the C2H4/Ni system, the UPS spectrum a t 300 K loses all resemblance to free C2H4 and assumes an appearance identical to that of the C2H2/Ni system a t 300 K, im lying a dehydrogenation reaction. Analysis of the C 2 H 2 h i spectrum reveals the removal of the P orbital degeneracy since only one a orbital is shifted upon adsorption (166). Most UPS studies involved the valence levels of the adsorbate; changes in the metal bands due to adsorption are usually subtle and difficult to explain since one cannot easily separate contributions from the surface and bulk. How.‘ever, a genuine attenuation in the intensity of photoemission from states close to the Fermi level (d band) in Ni were observed upon adsorption (191). These observations were made by analysis of difference curves (subtraction of clean surface spectrum from adsorbate spectrum). Page et al. related these intensity changes to the strength of metal adsorbate bonding ( I 68). An area which may prove to be amenable to UPS involves the use of the technique to follow surface reactions. Such studies are few (159, 168) but are encouraging. For instance, UPS was used to follow the successive displacement of CO by 0 2 on Ni (168). I t should also be mentioned that UPS‘has been used to determine the electronic structure of clean surfaces of various solids including pure metals (193), semiconductors (194-196), alloys (197),metal chalcogenides (198, 199) and metal oxides (200). ORGANIC COMPOUNDS Recent years have seen an ever-increasing number of photoelectron spectroscopic studies involving a wide variety of organic compounds. Since in the past virtually all classes of “simple” organic compounds have been examined, current studies usually involve: 1) systems designed to show specific types of orbital interactions; 2) systems which show unique or anomolous reactivity patterns; and 3) systems which possess unique stereochemical and/or electronic sub-groups. While space does not permit an indepth discussion of all the recent work involving organic compounds, a representative selection of recent publications in the area will be discussed. Polynuclear Aromatic Hydrocarbons. Boschi, Clar, and Schmidt have obtained the high resolution photoelectron spectra of 65 polynuclear aromatic hydrocarbons, a t temperatures between 20 and 450 OC (201).Positive assignments of all bands in the 6-11 eV region were accomplished with the aid of semiempirical molecular orbital (MO) calculations as well as careful vibrational analysis of the spectra. In sterically overcrowded systems, excitation of lowfrequency twisting and out-of-plane bending modes result in a blurring of the vibrational structure. These authors contend that in the absence of reliable x-ray data, this observation allows a distinction to be made between planar and nonplanar polynuclear aromatic hydrocarbons. D e w a r Benzene a n d Hexamethyl( D e w a r benzene). Schroder et al. (202) have recently reassigned the first two bands in the photoelectron spectrum of hexamethyl(Dewar benzene) as al(x) followed by bp(7r). Previously Geotz et al. (203) proposed the opposite assignment. The actual ordering was ot interest since Dewar benzene (1, n = 0) terminates the series of bridged bicyclic dienes (I, n = 1, 2, 3 , 4 ) shown below. The photoelectron spectra of these compounds show that as n increases from 1 to 4, “through-space’’ interactions become increasingly less important relative to “through-bond’’ interactions (204). 288R
ANALYTICAL CHEMISTRY, VOL. 48, NO. 5, APRIL 1976
n
In an effort to end this apparent controversy, Heilbronner, Goldstein, et al. have reported the photoelectron spectra of 1 ( n = 0) (205),as well as its dihydro (2) and tetrahydro (3) derivatives.
2
3
In agreement with a variety of MO calculations, the a l ( a ) and b 2 ( ~orbitals ) of 1 ( n = 0 ) were shown to be accidentally degenerate, Le., “through-space” and “through-bond” interactions cancel in contrast with the pattern set by its higher homologs. Cyclobutadiene Derivatives. Schweig, Krebs, et al. have reported the photoelectron spectrum of cyclobutadiene derivative 4 (206).
4
The ionization potentials a t 6.89 and 8.20 eV (Intensity 1.2:2) were assigned to “ 2 and n(s), respectively. Since all bands below the first do not appear as doublets or quartets, the authors concluded that the molecules exist as a rectangular, ground state singlet. The rectangular geometry was later confirmed by x-ray crystallography (207). Analysis of the spectrum also indicates that the sulfur atoms do not show any significant interaction with the a orbitais. In a related study, the photoelectron spectrum of tri(tert-buty1)cyclobutadiene was reported (208). The lowest ionization potential was shown to be 6.83 eV in good agreement with the previous study. Furthermore, it was again concluded that this system must exist as a rectangular ground state singlet. All assignments were in good agreement with MO calculations performed on this system. Cyclic a n d Acyclic Acetylenes. The photoelectron spectra of a series of halodiacetylenes (6a,b,c), dihalodiacetylenes (7a,b,c), and halomethyldiacetylenes (8a,b,c) have been studied by Heilbronner et al. (209). X-C=C-C=C-H 6
X-C=C-C=C-X 7 X-C=C-C=C-CH3 8
a = C1, b = Br, c = I In this work, the authors demonstrate that simple Linear Combination of Bonding Orbitals (LCBO) models, which use a basis consisting of only bonding orbitals can be parametrized to exactly reproduce ionization potentials. However, these same data yield erroneous charge distributions and/or spin-orbital couplings for the radical cation, M+. Only when P*CCorbitals are added to the basis set is a selfconsistent rationalization obtained.Ensslin, Bock, and Becker (210) have examined the photoelectron spectra of a number of mono- and di-substituted, silyl- and methylacetylenes. The observed a-splittings were rationalized in terms of a parametrized hyperconjugation model. Interestingly, it was not necessary to explicitly include 3d Si orbitals in the basis set. The implications of these results in terms of pa-da back-bonding is ponderous.
The photoelectron spectra of cyclic acetylenes 9-14 have recently been examined. (211,212).
11
10
9
12
13
14
The relative sequence and the position of the bands were rationalized in terms of “through-space” and “throughbond” interactions between basis A orbitals and u orbitals of appropriate symmetry. It was found that a cis-bend of the acetylene moiety by 0 < 20° leads to a split in the energy of the in-plane and out-of-plane basis A orbitals of less . deviations in 0 lead to observthan 0.2 eV ( 2 1 1 ~ )Larger able splittings (e+., 13, 14) (211b). This ,finding suggests that the increase in the ay MO energy in strained cycloalkynes is the source of the enhanced reactivity of the triple bond in these systems. Through-Bond vs. Through-Space Interactions. The concepts of “through-space” and “through-bond’’ interactions, which were originally introduced by Hoffmann, (212), have been applied in a large number of reports involving the interpretation of photoelectron spectra of organic compounds. “Through-space” interactions occur when two or more basis orbitals of the same symmetry and reasonably similar energy are oriented in a suitable fashion to produce orbital overlap. The orbital sequence which results is the so-called “natural sequence”, x+below X- (the in-phase basis orbital combination below the out-of-phase combination). “Through-bond’’ interactions occur when basis orbitals are properly aligned with high energy u-orbitals of the same symmetry; this can produce either the “natural” or the “inverted” (X- below X+) sequence. Although most of the systems studied show a competition between “through-space” and “through-bond” interactions, systems capable of “spiroconjugation” (213, 214) afford almost pure “through-space’’ interactions between the highest occupied a orbitals of the spirane a systems. The photoelectron spectra of spiranes 15, 16, and 17 have recently been examined (215,216).
21
22
The authors also make use of their orbital assignments as the basis for rationalizing the photochemistry of 21 (220). In contrast to the large “through-bond” interaction observed in the photoelectron spectrum of tricycl0[4.2.0.0~~~]octa-3,7-diene 23 (221), a separation of only 0.3 eV is observed for the first two A orbitals of tricyclo[4.2.1.02,5]nona-3,7-diene, 24 (222).
24
The bridging carbon in 24 decreases “through-bond” interactions by not only changing the stereochemistry of the two interacting T systems, but also by lowering the orbital energy of the u bond MO’s which relay the interaction. The photoelectron spectrum of 25 (223) provides verification of the orbital assignment reported for barralene (1, n = 21, (2041,
25
since the only symmetry allowed interaction which can occur is between the lower energy T orbital of the conjugated diene and the out-of-phase olefin orbital combination. Finally, the photoelectron spectrum of 1,3-diazaadamantan-6-one, 26, has been interpreted in terms of “throughbond” interactions between the nitrogen lone pairs and the carbonyl T system (224).
00 15
16
17
The energy difference between the interacting a orbitals was found to be approximately 1.2, 0.06, and 0.3 eV for 15, 16,and 17,respectively. An interesting example involving the competition between “through-space” and “through-bond” interactions has been reported by Heilbronner and Maier (217). These authors examined the photoelectron spectra of [2,2]paracyclophane, 18, 4-amino 2,2 paracyclophane, 19, and 1,1,2,2,9,9,10,10-octafluoro 2,2 paracyclophane, 20.
\ \
NH
18
the original orbital assignments in 18 (218). Furthermore, the fluorine substitution in 20 effectively eliminates the “through-bond” interactions observed in 18 by substantially lowering the orbital energy of the u orbitals, thus minimizing the magnitude of the interaction. Substitution of a &amino group in 19 causes “orbital switching”, i.e., the A orbitals choose to interact with the amino group, causing the A symmetry to be rotated by 60°, relative to 18, and thus substantially altering the nature of the a interactions. Although “through-bond” interactions generally occur through three u bonds, Martin and Schwesinger report two cases, 21 and 22, in which “through-bond” interactions are observed over four u bonds (219).
19
26
1,2-Non-Bonding Orbital Interactions. Although the interaction between non-bonding orbitals located on adjacent atoms is, in reality, just another example of a “through-space” interaction, there has been a sufficient amount of work done in the area to merit a separate discussion.
r
r 20
The effects of the fluorine substitution in 20 help confirm
x=o. s ANALYTICAL CHEMISTRY, VOL. 48, NO. 5, APRIL 1976
289R
Batich and Adam found that the differences in energy between the in-phase and out-of-phase non-bonding basis orbitals of alkyl peroxides correlated well with cosine 8, where 0 is the dihedral angle formed by the two electron pairs. (225). Similar results were obtained for dialkyl disulfides by Baker, Brisk, and Gellender (226). This relationship between the dihedral angle of the electron pairs and the orbital splitting observed by photoelectron spectroscopy has been utilized as a means of determining ring conformations of rings containing two adjacent heteroatoms. For example, analysis of the photoelectron spectrum of 27 indicates that a t least in the vapor phase this compound exists in the half-chair conformation only (227).
s-s 27
Buschek and Nelson have studied the photoelectron spectra of a large number of cyclic hydrazine compounds (228). These data, supplemented by calculations, enabled the authors to construct a curve which correlates the difference in lone pair ionization potentials as a function of the lone pair-lone pair dihedral angle. Based on this correlation, the conformation of a variety of cyclic hydrazines was determined from photoelectron data exclusively. In previous reports, these authors determined the ratio of conformers 28a vs. 28b and 28c by a similar type of analysis (229, 230).
Me 28a
0
0
0
0
0
0
33
32
34
Their results reconfirm Turner’s original assignment (234) and invalidate subsequent proposals (235, 236). The
origins of the assignment difficulties is traced back to the validity of Koopmans’ theorem for the “oxygen lone pair” ionizations. Heilbronner et al. have reported the photoelectron spectra of seventeen tropones (237). This work confirms the previously assigned orbital sequence in tropone itself (238, 239). All attempts to separate inductive and conjugative effects were shown to depend heavily on the models which were chosen. Finally, the authors concluded that photoelectron spectroscopy yielded no information about the “aromatic character” of tropone. In a classic paper, Schweig and coworkers recorded the photoelectron spectra of acetylacetone, 35, 3-methylacetylacetone, 36, and 3,3-dimethylacetylacetone, 37, using a variable temperature target chamber (240).
35
37
36
From the changes in the spectra with temperature, the equilibrium constants for the keto F? enol tautomerism were obtained. The reaction enthalpies also derived from this technique showed good agreement with vapor phase values previously obtained by other methods (241). Azo Compounds. Photoelectron data for a large number of cis and trans azo compounds has recently been reported by Houk, Chang, and Engel (242). Correlations were found between the orbital ionization potentials and quantities such as polar substituent, D, the dihedral angle of the nonbonding electron pairs, as well as the energy of the n K* transition. Furthermore, a rationale which relates ground state electronic structure of azo compounds to the propensity to lose nitrogen upon photolysis was proposed. Schweig, Trost, et al. (243) obtained the photoelectron spectra of cyclic azoalkane 38,39, and 40.
-
2 8b
28c
Carbonyl Compounds. Photoelectron band assignments in a large number of simple mono- and a-dicarbonyl compounds have recently been reviewed by McGlynn et al. and therefore will not be discussed here (231). Schweig et al. reported the photoelectron spectra of cyclopropenone, 29, as well as di-tert-butylcyclopropanone, 30, and di-tert-butylcyclopropenone,31 (232).
29
30
31
Analysis of these spectra indicates that the double bond is stabilized through both inductive interaction with the carbonyl group as well as conjugative interaction with the K*CO orbital. Based on the data, the authors suggest that the cyclopropenone system bears some resemblance to the aromatic cyclopropenyl cation. The photoelectron spectra of p-benzoquinone, 32, 1,4naphthoquinone, 33, and g,lO-anthraquinone, 34, have been determined by Schweig and coworkers (233). The true sequence of the lone pair orbitals was determined through application of a correlation technique to these compounds. 280R
ANALYTICAL CHEMISTRY, VOL. 48, NO. 5, APRIL 1976
39
40
These authors were able to correlate the destabilization of the e, cyclopropane orbital in 39 and 40 with their facility to lose nitrogen. Amines. The photoelectron spectra of a number of cyclic amines have been examined by Yoshikawa, Hashimoto, and Morishima (244). Although the ionization potentials of non-bonding orbitals did not correlate with solution basicities, a good correlation of ionization potentials with vapor phase basicities was obtained. Morishima et al. have also studied the interactions of nitrogen lone pairs with endocyclic double bonds, e.g., 41 and 42 (245).
i-) Ti I H 41
0 N
H 42
Whereas the nitrogen lone pair in 41 is destabilized relative to its saturated counterpart in a fairly typical fashion, the lone pair in 42 has the same ionization potential as its saturated counterpart. Since symmetry restrictions are removed in 42, the nitrogen lone pair can interact with both the TCC and T*CC orbitals, apparently causing a net cancellation of effects. Aue and coworkers (246) have reported the photoelectron spectrum of manxine, 43.
43
The first band (IP = 7.05 eV) exhibited a very different band shape than all other amines examined. The authors interpreted this in terms of a planar radical cation which results from ionization of an electron from a pure p orbital. Miscellaneous. A large number of other studies involving organic molecules have been reported in the literature which can not be discussed here (247). Many of these reports provide interesting insight into a variety of subjects and are thus recommended reading for all (Table I). OTHER ASPECTS There has been considerable effort directed a t obtaining the photoelectron spectra of transient species. Jonathan and co-workers have been particularly active in this area (272-276). They have followed up an earlier study by reporting fully on the photoelectron spectrum of singlet oxygen, produced in 10% concentration by a microwave discharge. T o enhance the spectrum of the singlet oxygen, they used a phase-sensitive method of detection, based on the modulation of the microwave discharge used to excite the oxygen. They concluded that such a method of obtaining the spectrum is advantageous compared to the standard method in cases where a poor partial pressure of the transient is a problem. For concentrations of transients in excess of 5, the standard technique becomes superior. The first ionization potential of singlet oxygen is 11.09 eV, some 1 eV lower than that of triplet oxygen. Other studies by Jonathan and co-workers focused on SO, 0 3 , CF2, and Sz (273-275). Additionally Golob and coworkers obtained the photoelectron spectrum of the methyl radical by a microwave discharge in an argon/dimethylmercury or argodazomethane mixture. The ionization potential is 9.8 eV, and associated with the first band in the spectrum is a clear vibrational fine structure (276). Other radicals and transient species to be studied include CS, NF2, HBS, S 2 0 , HzCS and FzCS. All are discussed in Frost's review (6). An important aspect of the photoelectron spectra of radicals is the multiplet or exchange splitting that results from the open shell occupancies characteristic of
LITERATURE CITED
(1)D. Betteridge and M. A. Williams, Anal. Chem., 46, 12513 (1974). (2)G. W. Ewing, "Instrumental Methods of
Table I Subject p -Quinodimethane
Bridge annulenes C2s orbitals Nucleic-acid bases Phosphorous ylides Pyridine N-oxides Alkyl aryl ethers, alkyl aryl sulfides Homoallylic methyl ethers Tetrazenes Dithianes Hyperconjugation Substituted naphthalenes Substituted benzenes Triisopropylidenecyclopropane Thiocarboxylate compounds Acetanilide Sulfones Carboranes N-Nitrosamines these species. Such splittings have been observed in the core-level XPS spectra of radicals such as di-tert-butyl nitroxide (277). Anticipating future electron experiments on radicals by UPS, Yarkony and Shaeffer have carried out calculations on the trimethylene radical (275). Complementary data for UPS studies can be obtained from another type of electron spectroscopy that is receiving increasing attention-electron impact energy loss spectroscopy. By measuring the loss in energy that an impacting electron suffers on being inelastically scattered from an atom or molecule, it is possible to deduce the excitation energies of the species. Among studies reported are those on allene (2791, aldehydes, ketones, alcohols, and unsaturated compounds (280-283). Another varient of electron spectroscopy with electrons is also being developed, and appears to be of considerable importance. This new method is electron transmission spectroscopy, and has been developed largely by Schulz and co-workers at Yale (284287). One variant of the experimental method devised by Schulz involves passing an electron beam of progressively increasing energy through vapor of a substance. At certain values of the impacting electron energy, electrons can be captured by the gas molecules, thereby forming metastable negative ions. These negative ions dissociate after a short period of time, re-emitting the initially captured electron with its initial kinetic energy. Since the re-emitted electrons may come off over a wide angular spread, a change in the transmitted "straight through" current is detected, thus showing the resonance energies a t which electrons can be captured. Other developments of interest to those working in UPS include the use of a vacuum-uv photoionization source as a selective detector for liquid chromatography (288) and, very recently, the development of instrumentation for obtaining the photoelectron spectra of liquid samples (289293).
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Electron Spectroscopy: X-ray and Electron Excitation David M. Hercules Department of Chemistry, University of Georgia, Athens,
Ga.30602
The present review is for the fields of x-ray photoelectron spectroscopy (ESCA) and Auger Electron Spectroscopy (AES) for the period of 1974-75; it is the third review of these topics in Analytical Chemistry ( I , 2 ) . This review will cover literature abstracted in Chemical Abstracts starting with the December 23, 1973, issue through December 25, 1975, issue no. 25. The reviews of ESCA and AES will be in separate sections for convenience of reference. The editors of Analytical Chemistry have suggested that this be a critical review, rather than all-inclusive. However, ESCA is a young and rapidly growing field, 545 articles were abstracted in the last review and 630 in this one. The period of the present review has seen ESCA become established as a field; there are many fundamental studies being reported and applications of ESCA to a variety of problems have appeared. In such a rapidly growing area, it is difficult t o be selective because of the diversity of interests on the part of readers. Therefore, I will try to cover most of the published literature on ESCA and selected literature on Auger spectroscopy. This review will not include theses, government reports, patents, or articles published in obscure foreign language journals. During the present review period, the second international conference on electron spectroscopy was held a t Namur, Belgium. The proceedings of this conference have been reported as a book (3) and as volume 5 of the Journal of Electron Spectroscopy and Related Phenomena. The first Gordon Research Conference on Electron Spectroscopy was held in 1974 and another is planned for 1976. The International Union of Pure and Applied Chemistry Committee on Molecular Structure and Spectroscopy has published recommendations for nomenclature and spectral presentation ( 4 ) . During the period covered by this review several books have appeared, either devoted to electron spectroscopy or to related phenomena. Ertl and Kuppers have published a monograph entitled “Low Energy Electrons and Surface Chemistry” ( 5 ) ; Jorgensen has published a book “Photoelectron Spectra Induced by X-rays of Over 600 Non-Metallic Compounds Containing 77 Elements” (6); Nefedov has written a book in Russian (7) and Kane and Larrabee 294R
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have chapters in their book “Characterization of Solid Surfaces” dealing both with ESCA and Auger spectroscopy (8). 1 should like to make some comment about the format of the review. The last two reviews have been written only with major subheadings and in a normal journal prose style. My experience, and that of my students, has been that this format often makes it difficult to find a specific article, or articles on a specific topic. Therefore, in the present review I have tried to use a more extensive outline, with a larger group of subheadings. I would appreciate feedback on this format. One barometer that ESCA is maturing as a field is the large number of review articles that have appeared in the past two years. In combing the literature, I counted no less than 105 review articles either on ESCA, some aspect of ESCA, or the application of ESCA to some area. In tabulating these reviews, I have selected only those published in English and only those either of general interest or of specific interest to analytical chemists. The more general review articles will be tabulated here, but review articles will also be treated in separate subsections where appropriate. Several plenary lectures from the Namur Conference make good review articles of various aspects of ESCA. Seigbahn’s article on outlook (9) provides a broad perspective of the field. Other articles deal with correlations with chemical and physical data ( I O ) , correlations between NMR and ESCA (111, multiplet structure (12), adsorption and surface reactions ( 1 3 ) , applications to industrial chemistry ( 1 4 ) ,theoretical models (Is),new developments in instrumentation (16),and applications to chemical analysis ( 17). Other interesting reviews have appeared on chemical aspects of ESCA (18); the Society of Analytical Chemistry has two short reviews in the Analyst (London), (19,ZO), relaxation effects are discussed @ I ) , and an excellent review has appeared on the applications of electron spectroscopy to analytical chemistry (22).
THEORY In this section I will cover only those articles where the intent of the article is some contribution to theory. This
