3 Photoelectron Spectroscopy and Related Phenomena of Gaseous Species By J. P. MAlER Ph ysikalisch -chemiscbes lnstitut University of Basel Klingelbergstrasse 80 4056 Basel Switzerland Previously there has been a tendency to separate the ultraviolet- (u.v.)and X-ray- excited processes; however in this Report it seems opportune to encompass the whole energy region and to make the division by reference to progress and study of the species in the gas phase. The field of X-ray p.e. spectroscopy and the implications in organic chemistry wzre covered in Annual Reports by Clark up to 1972 and for an up-to-date panorama the reader is referred to a conference publication reviewing the state of the art up to 1974.’ This Report highlights what in the author’s opinion have been the fruitful advances in p.e.spectroscopic and related techniques in the past two years with emphasis placed on the applications and information available predomi- nantly for the valence region of organic species. An extensive compilation’ exists for the U.V. p.e. spectroscopic studies of heterocyclic molecules up to 1973 and aspects of inorganic and organometallic molecules are thoroughly reviewed and discussed annually in the Specialist Periodical Re~ort,~ and are not reported here. Application of the technique to non-metallic species has also been sur~eyed.~ Two more books devoted to p.e. spectroscopy have appeared’ and these with those of Turner et al. and Siegbahn et al. provide the necessary introductions whilst the Faraday Discussionsare representative of the develop- ments up to 1972.6 J.Electron Spectroscopy 1974 5. E. Heilbronner J. P. Maier and E. Haselbach in ‘Physical Methods in Heterocyclic Chemistry’ ed. A. R. Katritzky Academic Press New York 1974 Vol. 5. A. Hamnett and A. F. Orchard in ‘Electronic Structure and Magnetism of Inorganic Compounds’ ed. P. Day (Specialist Periodical Reports) The Chemical Society London 1974 Vol. 3 p. 218. H. Bock and B. G. Ramsey Angew. Chem. Internat. Edn. 1973 12 734. ’ (a)A. D. Baker and D. Betteridge ‘Photoelectron Spectroscopy’ International Series of Monographs in Analytical Chemistry 1972 Vol. 53; (b)J. H. D. Eland ‘Photoelectron Spectroscopy’ Butterworths London 1974. Faraday Discuss. Chem. SOC.,1972 No. 54. 75 J.P.Maier 1 Photoelectron Spectroscopic Studies of Organic Molecules The technique has continued to be used extensively to obtain ionization energies (IE) and to use these as a basis for the description of the electronic structure of the molecule in terms of MO models.By this means the bands in the p.e. spectra are related to electron ejection from orbitals. The additional detail discernible Franck-Condon profiles excitation of vibrational modes in the ion spin-orbit coupling vibronic features and the band intensity dependence on the angular asymmetry parameter and photoionization cross-section provide substantial indications of the respectability of the model. The past two years have witnessed advances in sophistication in interpretive methods and more appreciation of their limitations.These aspects are exemplified in an article by Heilbr~nner.~ There has been considerable success in the use of the technique to investigate specific problems and to relate the data with other experiments. In this area the He-Ict (21.22eV) photon source has by and large been utilized in order to benefit from the resolution attainable. Photoelectron-Electronic Spectroscopy Correlations.-The IEs and the assign- ment of the bands have been used to draw inferences regarding the interpretation of the electronic absorption spectra of molecules or their cations. In the latter the correspondence between the separation of the bands in the p.e. spectra and the electronic bands of the cation is illustrated by the absorption and emission spectra of the cations in the vapour phase of diatomic and triatomic molecules.* For organic molecules the correspondence is illustrated by the absorption spectra of their radical cations trapped in matrices.The benefits of the comparison (and complemented by open-shell MO studies) are thoroughly discussed in a recent review.’ The gas-phase absorption spectra of the cations are ‘simulated’ to an extent by the technique introduced by Dunbar of photodissociation spectroscopy. lo The ion currents of fragments obtained by photon irradiation of the parent ions are monitored in an ion cyclotron spectrometer. In Figure 1 is shown such a spectrum where the loss of hydrogen yields the fragment ion part of the work involving substituted benzene cations.lo Differences from the p.e. spectra are ex- pected as the method relies on dissociation products the thermodynamic dissociation pathway has to be accessible and the excited states of the parent cation generated are dependent on optical selection rules. The important indica- tion of these experiments is that the ions examined by the two techniques are structurally similar. Electronic transitions of ions are also observed in the X-ray emission spectra. Some of sufficient resolution to identify the valence states E. Heilbronner in ‘The World of Quantum Chemistry’ ed. R. Daudel and B. Puliman D. Reidel Publishing Co. Dordrecht 1974. G. Herzberg Quart. Rev. 1971 25 201. P. Carsky and R. Zahradnik Topics Current Chem. 1973 43 1. lo P. P. Dymerski E.Fu and R. C. Dunbar J. Amer. Chem. Soc. 1974 96,4109 and references therein. Photoelectron Spectroscopy and Related Phenomena of Gaseous Species Energy feV1 Figure 1 Photoelectron and photodissociation spectra of p-Juorotoluene (Reproduced by permission from J. Amer. Chem. Soc. 1974,96,4109) have been reported by Siegbahn and co-workers,'' for diatomic and triatomic ions and for ions of C,H, C2H40 CHF, and CF in the gas phase. Following the generation of the core vacancy by electron impact relaxation transitions corresponding to core from valence states are observed. This approach can help in the assignment of the valence bands due to the selection rules governing the transitions. For example in CF4+ only transitions from the t,-orbitals are strongly allowed when the Cls electron is ionized and the Cls X-ray spectrum consists of one line.The similarities in vibrational structures observed on p.e. and Rydberg bands converging to the same ionic state have been discussed for fluorobenzenes.12 The consideration of the quantum defects and selection rules for the Rydberg transitions with the type of MO with which the p.e. band is associated allows the interpretation of both sets of data to be advanced. Similarly the assigned p.e. spectra of fluoromethanes and cyclopropenone allow the prominent features of electron-impact spectra of these molecules to be identified.I3 An approach has been made to correlate the separation of the cationic states (z,x) from the p.e. spectra with the electronic absorption spectrum of the molecular states where the two excited states of the molecule correspond in the orbital picture to excitation of the electrons to an unoccupied orbital and which L.0. Werme B. Grennberg J. Nordgren C. Nordling and K. Siegbahn UUIP-815 Uppsala University Institute of Physics Report March 1973. R. Gilbert P. Sauvageau and C. Sandorfy Chem. Phys. Letters 1972 17 465; R. Gilbert and C. Sandorfy ibid. 1974 27 457. l3 W. R. Harshbarger N. A. Kuebler and M. B. Robin J. Chem. Phys. 1974 60 345; W. R. Harshbarger M. B. Robin and E. N. Lassettre J. Electron Spectroscopy 1973 1 319. 78 Maier 8e-la2 8e-lb I I p-AE(1,2) _cI I A-i 0.75 h .z 0.50 5 a .C. -3 Ii f-+ 23 "C Y a 0.25 0.0 20OOO 30 0oO 4oOOo 50OOO (b) an-' Figure 2 (a) Photoelectron spectrum of spiro[4,4]nonatetraene;(b) electronic absorption spectrum of spiro[4,4]nonatetraene (Reproduced by permission from J.Amer. Chem. Soc. 1974,96,7662) Photoelectron Spectroscopy and Related Phenomena of Gaseous Species 79 when photoionized leave the ion in the respective (8or 2)state.14 In an in-dependent-electron model such as Huckel theory the energy separation of the cationic states would equal that of the excited states when Koopman's theorem is used for the IEs. When electron interactions are considered provided that configuration interaction is not important then by evaluating the coulomb and exchange integrals from SCF MO calculations the excitation energies may be estimated This has been done recently for pyrazine and its 2,6-dimethyI deriva- tive.' A striking example is that of spiro[4,4]nonatetraene1 where the separa- tion between the first two bands in the p.e.spectrum (Figure 2a) corresponding to electron ejection from the la2 (7t)and lbl (z)MOs respectively is equal to the separation of the first two bands in the U.V. spectrum (Figure 2b). The latter are associated with transitions of E symmetry represented by 8e (z*) +la2 (7t) and 8e (z*)t-lbl (7t). In this instance the symmetry and spiroconjugation result in approximate cancellation of the two-electron integrals. A study involving the p.e.-electronic spectra correlation for benzene with the aid of PPP-SCF-CI calculations resulted in assignment of the 12.3eV IE band to the sca,,,M0.16 In a series of related compounds it has been assumed that electron interactions are comparable and the variations of the orbital energies derived from the p.e.spectra have been used to suggest the location of the olefinic z-+ cr* band of Me2C=CMe2.l7 Conformational and Hydrogen-bonding Phenomena.-P.e. studies of conforma-tional phenomena have illuminated both the potential and the limitations of the method. Requisite are well defined changes of the IEs of the pertinent bands which are dependent on the conformation adopted. Through the study of structurally related series the variations may be used to establish empirical correlations. For example in the case of conjugated molecules twisting around the essentially single bonds is readily perceptible from the IE of the z-bands whereas around double bonds the resolution limits preclude definite deductions.The former situation is encountered in cyclo-octatetraene and its hydrogenated derivatives,' * biphenyls and phenylethylenes,' alkyl aryl ethers sulphides and amines,20 and hydrazine-type systems.' This enables an estimation of the E. Haselbach and A. Schmelzer Helv. Chim. Acta 1972 55 1745; E. Haselbach Z. Lanyiova and M. Rossi ibid. 1973 56 2889. Is C. Batich E. Heilbronner E. Rommel M. F. Semmelhack and J. S. Foos J. Amer. Chem. SOC. 1974,96 7662. '' F. Marschner and H. Goetz Tetrahedron 1973 29 3105. W. Fuss and H. Bock J Chem. Phys. 1974,61 1613. C. Batich P. Bischof and E. Heilbronner J. Electron Spectroscopy 1973 1 333.l9 J. P. Maier and D. W. Turner J.C.S. Faraday ZZ,1973,69 196; Faraday Discuss. Chem. SOC.,1972 No. 54 p. 149; T. Kobayashi T. Yokota and S. Nagakura J. Electron Spectroscopy 1973 3 449. 2o J. P. Maier and D. W. Turner J.C.S. Faraday II 1973 69 521; S. A. Cowling and R. A. W. Johnstone J. Electron Spectroscopy 1973 2 161;J. P. Maier Helv. Chim. Acta 1974,57,994; P. S. Dewar E. Ernstbrunner J. R. Gilmore M. Godfrey and J. M. Mellor Tetrahedron 1974 30,2455; A. D. Baker M. Brisk and M. Gellender J. Electron Spectroscopy 1974 3 227. 21 S. F. Nelsen and J. M. Bushek J. Amer. Chem. SOC. 1973 95,2011 2013; ibid. 1974 96 2392 7930; P. Rademacher Angew. Chem. Znternat. Edn. 1973 12,408. 3.P.Maim I "'""'" Figure 3 Photoelectron spectrum of 1,2-dimethylhexahydropyridazine (Redrawn by permission from J.Amer. Chem. Soc.,1973,95,2013) dihedral angle between the coupling ?t-systems to be derived. The study of deformed double bonds illustrates the latter case.22 When there is a significant proportion of more than one conformation in flexible systems these may be detected provided that the criteria specified above are satisfied. The work on hydrazines is a nice instance where the nitrogen lone pairs provide such a probe.,' In Figure 3 is shown such a p.e. spectrum the analysis of which reveals that at least two conformers are present. By contrast in some substituted hydrazines where several conformers dominate the differences are not clear in the p.e. spectra. In a study of geometrical isomers of cis-and trans-Zsubstituted cyclopentyl and cyclohexyl bromides small differences in the first IEs were found and some change in the shape of bands.23 Open-chain conformations for (a-naphthyl)(CH,),(a-naphthyl) (n = 4 or 6) at -500 K were deduced by comparison with the p.e.spectrum of a-n-butyl-na~hthalene.,~ This supports the mass spectral studies for n = 4-16 which indicate that these species exist in the open-chain form with rapid intramolecular 22 C. Batich 0. Ermer E. Heilbronner and J. R. Wiseman Angew. Chem. Internat. Edn. 1973 12 312. 23 R. Botter F. Menes Y. Gounelle J. M. Pechine and D. Solgadi Internut. J. Mass Spectrometry Ion Phys. 1973 12 188. 24 J. Berkowitz J. L. Dehmer K. Shimada and M. Szwarc J. Electron Spectroscopy 1974 2 21 1 ; ibid.3 164. Photoelectron Spectroscopy and Related Phenomena of Gaseous Species 8 1 AA 114°C MeAA 075°C Me2AA ~ 8 10 8 10 I.P./eV -Figure 4 Photoelectron spectra of acetylacetone (AA) 3-methylacetylacetone (MeAA) and 3,3-dimethylacetylacetone(Me,AA) (Reproduced by permission from Chem. Phys. Letters 1974,26,229) collisions to form a ring form after ionization. The temperature dependence of the p.e. spectra has also been examined for inter- and intra-molecularly hydrogen- bonded systems. In the first approach the dimer formation of carboxylic acids was encouraged by lowering the temperature to 200-245K and comparing these p.e. spectra with those at -300 K.*' However the complexity of overlap of the bands allows only a few comparisons.The p.e. spectra of intramolecularly hydrogen-bonded amino-alcohols at -300 and 500 K suggest that the decrease in IE of the nitrogen lone pair at the higher temperature may be taken as an indication of the hydrogen-bonding stabilization.26 In Figure 4 is shown the variation of relative intensity of the lowest IE bands of acetylacetone and its '' R. K. Thomas Proc. Roy SOC.,1972 A331 249. 26 S. Leavell J. Steichen and J. L. Franklin J. Chem. Phys. 1973 59 4343. 82 J. P.Maier 3-methyl and 3,3-dimethyl derivatives as a function of temperat~re.~~ This is interpreted as a consequence of the keto-enol tautomerism and from the band intensities equilibrium constants and reaction enthalpies were inferred.Series Correlations and Substituent Effects.-A successful way to interpret the p.e. spectra of organic molecules has been through the study of series of molecules where the trends in the IEs establish a recognizable at tern.^ When the IEs of the n-bands correlated with the el,(s)and a2”nbands of benzene of the hetero- cyclic derivatives of benzene C5H5X (X = N P As or Sb) are plotted against the first IEofthefree heteroatoms,linear regressionsareobtained.28Thegradients of0.36 and 0.16 are in accord with the values 4 and 4 predicted from perturbation consideration of the benzene orbitals. The changes in the IEs have then been regarded in terms of simple models such as the LCBO (linear combination of bond orbitals) type. If the molecules contain a heavy atom particularly iodine or bromine the spin-orbit coupling discernible yields a sensitive probe of the electronic environment and hence of the adequacy of the model.In the p.e. spectroscopic study of hal~genodiacetylenes,~~ X-C-C-C=C-H X-CSC-C-C-X and X-C-C-C-C-Me (X = Cl Br or I) the spin-orbit splittings reveal that even though parametrization may be success-ful for IEs of the bands the predicted spin-orbit splittings are erroneous. The p.e. spectra are of considerable simplicity. Figure 5a shows a spectrum and Figure 5b the model used. The consistency of the interpretation is witnessed by the spin-orbit coupling vibrational excitation Franck-Condon profiles and correla- tion with data on other halogenoacetylenes. The p.e. spectra of iodoethylenes are rather more complex and were interpreted with the aid of modified extended Huckel calculations with spin orbitals as basis.30 Semi-empirical MO calculations have continued to be used in obtaining cor- roboration of the assignments.However such assignments may be unreliable. For instance the study of the IEs of cycloalkenes by ab initio SCF MO calculation3’ demonstrated that for small rings especially the results obtained from the SE MO theories are rather poor. In other work the IEs of the n-bands of the a~enes~~ were considered in relation to models of differing approximations from Huckel to SCF calculations. In an impressive compilation of PIE data of 65 eata-and peri-condensed aromatic hydrocarbon^^^ the n-IEs below 11 eV are identified with the aid of SE MO calculations.For molecules larger than naphthalene the a-IE onset is placed at 10.6 f0.2 eV. In the sterically overcrowded systems the authors argue that the Franck-Condon profiles of the n-bands suffice to 27 A. Schweig H. Vermeer and U. Weidner Chem. Phys. Letters 1974 26 229. ’* C. Batich E. Heilbronner V. Hornung A. J. Ashe D. T. Clark U. T. Cobley D. Kilcast and I. Scanian J. Amer. Chem. Sac. 1973 95 928. 29 E. Heilbronner V. Hornung J. P. Maier and E. Kloster-Jensen J. Amer. Chem. Sac. 1974 96,4252. ’O K. Wittel H. Bock and R. Manne Tetrahedron 1974 30,651. ’l D. C. Clary A. A. Lewis D. Morland J. N. Murrell and E. Heilbronner J.C.S. Faraday II 1974 70 1889. 32 P. A. Clark F. Brogli and E. Heilbronner Helv. Chim. Acta 1972 55 1415.33 R. Boschi E. Clar and W. Schmidt J. Chem. Phys. 1974,60,4406. Photoelectron Spectroscopy and Related Phenomena of Gaseous Species 1 r. Iv) 9 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 I.PJeV t %-If -Be# Figure 5 (a)Photoelectron spectrum of di-iododiacetylene;(b)orbital correlation diagram for di-iododiacetylene (Reproduced by permission from J. Amer. Chem. SOC.,1974,96,4252) distinguish deviations from coplanarity. The good correlation generally attained in such series with the calculated orbital energies suggests that the 'defects' of the Koopmans' theorem are similarly related. In one approach the results of ab initio calculations for azabenzenes were corrected empirically for reorganiza- tion and correlation by dividing the NOS into related categories.34 A para-metrization of the iterative Extended Huckel method also gave good agreement 34 J.Almlof B. Roos U. Wahlgren and H. Johansen J. Electron Specfroscopy 1973 2 51. 84 J. P.Maier with the p.e. spectra of azabenzenes and azanaphthalenes by considering groups of similar orbitals in separate regressions and thus ‘compensating’ for the frozen orbital notion.35 The n-IEs of alkyl-f~lvenes~~ provide an example where the relaxation process on ionization is strongly dependent on the nature of the state of the cation generated. Substituent E’ects. The effect of alkyl groups on the n-IE in 63 alkenes as well as in alkynes and carbonyl compounds has been disc~ssed.~’ The influence of alkyl substituents on the Jahn-Teller splitting of the first band has also been considered in allene derivative^.^^ In the study of 4-substituted quinuclidines the dependence of the nitrogen lone pair IEs was viewed in terms of local potential at the nitrogen due to the dipole of the sub~tituent.~~ These IEs were also found to be linearly related to o* substituent parameters.Also in substituted pyridines the first n-IEs give linear correlation with o+ constants and the nitrogen lone pair IEs with the pK values of the m01ecules.~~ In para-substituted pyridine N-oxides the effect of substituents has been investigated for the ground state and two excited states corresponding to ionization of electrons from correlated MOs discernible throughout the series.41 The IEs of these three processes gave good linear correlations with dipole moments pK values and cr and o+ substituent constants.In the above-mentioned empirical correlations the sig- nificance of the changes of IEs in regard to the electronic mechanism changes are not clear. The effects of halogen (F C1 or Br) substitution on benzene and pyridine x-IEs have also been discussed in terms of a model comprising short- and long-range inductive terms as well as the rnesomeric term.42 Through-bond and Through-space Notions. Questions of through-bond/space interactions have continued to attract the interest of p.e. spectroscopy and here the necessity of data on related molecules is especially evident. The analysis of the n-IE p.e. data of butadiene hexatriene and their methylated derivatives shows the influence of the methyl groups and affords the postulation of through-space interaction between the n-MO and the non-bonded pseudo n-MOs of the methyl group in the 2-or 3-po~itions.~~ Interactions between nitrogen lone pairs have been investigated in 2,3-diazabicyclo[2,2,n]alk-2-enes(n =14) 35 J.Spanget-Larsen J. Electron Spectroscopy 1973 2 33; ibid. 1974 3 369. 36 F. Brogli P. A. Clark E. Heilbronner and M. Neuenschwarider Angew. Chem. Internat. Edn. 1973 12 422. 37 P. Masclet 0. Grosjean G. Mouvier and J. Dubois J. Electron Spectroscopy 1973 2 225; P. Carlier R. Hernandez P. Masclet and G. Mouvier ibid. 1974 5 1103; G Hentrich E. Gunkel and M. Klessinger J. Mol. Struct. 1974 31 231.38 F. Brogli J. K. Crandall E. Heilbronner E. Kloster-Jensen and S. A. Sojka J. Electron Spectroscopy 1973 2 455. 39 G. Bieri and E. Heilbronner Helv. Chim. Acra 1974 57 546. 40 B. G. Ramsey and F. A. Walker J. Amer. Chem. SOC. 1974 96 3315. 41 J. P. Maier and J.-F. Muller J.C.S. Faraday IZ 1974 70 1991. 42 D. G. Streets and G. P. Ceasar Mol. Phys. 1973 26; 1037; J. N. Murrell and R. J. Suffolk J. Electron Spectroscopy 1973 1 471. 43 M. Beez G. Bieri H. Bock and E. Heilbronner Helc. Chim. Acta 1973 56 1028. Photoelectron Spectroscopy and Related Phenomena of Gaseous Species (1) and 3,4-diaza-analogues of tricyclo[4,2,1 ,02*5]nonadiene (2) and its 7,8-dihydro-deri~atives.~~ n-Orbital interactions were also discussed from the p.e.spectra of anti-and syn-tricyclo[4,2,0,02*5]octadienes(3) homoconjugative interactions of polyunsaturated [4,4,2]propellanes and bridged bicyclic dienes (4; n = l--4).4’ In bridged cyclohexanes (5; X = 0,S or C=CH2) the interac- tions of the substituent X with the cz ‘ribbon orbitals’ of the framework were con- sidered from the p.e. spectral IEs.~~ In 1,5-cyclo-octadiyne (6)the through-space/ bond interactions have been shown to be of comparable magnitude as far as the n-IEs are concerned and the effects of cis-bending of the acetylene moiety on the n-IEs have also been disc~ssed.~’ Homoconjugative interactions between exo- cycIic and endocyclic double bonds have also been considered by p.e. spectro- scopy as has spiroconjugation.’’,48 The assignments of the p.e.spectra of quadricyclanes have been used to discuss their cycloaddition reactions.49 Fluorosubstitution has been used to change the balance of through-bond/space interactions by preferentially stabilizing the o-MOs relative to the n-MOs. This has been demonstrated in [2,2]paracyclophanes diazabenzenes and diazanaphthalenes.’ 2 Transient Species and Free Radicals The stable nitroxide radicals di-t-butyl nitroxide and 2,2,6,6-tetramethylpiperi-dine N-oxyl have been studied in the gas phase by p.e. spectroscopy with U.V. 44 J. C. Bunzli D. C. Frost and L. Weiler J. Amer. Chem. SOC.,1973 45 7881 ;F. Brogli W. Eberbach E. Haselbach E. Heilbronner V. Hornung and D. M. Lemal Helv. Chim. Acta 1973 56 1933. 45 R. Gleiter E. Heilbronner and H.D. Martin Chem. Ber. 1973 106 28; R. Gleiter E. Heilbronner L. A. Paquette G. L. Thompson and R. E. Wingard Tetrahedron 1973 29 565; M. J. Goldstein S.Natowsky E. Heilbronner and V. Hornung Helv. Chim. Acta 1973 56 294. 46 R. Hoffmann P. D. Mollere and E. Heilbronner J. Amer. Chem. SOC.,1973 95 4860; R. J. Boyd J. C. Biinzli J. P. Snyder and M. L. Heyman ibid. p. 6478. 47 G. Bieri E. Heilbronner E. Kloster-Jensen A. Schmelzer and J. Wirz Helv. Chim. Acta 1974 S7 1265; H. Schmidt A. Schweig and A. Krebs Tetrahedron Letters 1974 1471. 48 P. Asmus and M. Klessinger Tetrahedron 1974 30 2477; A. Schweig U. Weidner R. K. Hill and D. A. Cullison J. Amer. Chem. Soc. 1973 95 5426 and references therein. 49 H. D. Martin C. Heller E. Haselbach and Z. Lanyiova Helv.Chim. Acta 1974 57 465; E. Haselbach and H. D. Martin ibid. p. 472. 50 E. Heilbronner and J. P. Maier Helv. Chim. Acta 1974 57 151; D. M. W. van den Ham D. van der Meer and D. Feil J. Electron Spectroscopy 1974,3,479; R. J. SuRolk ibid. p. 53; D. M. W. van den Ham and D. van der Meer ibid. 1973 2 247. 86 J. P.Maier excitation and the former species also with X-ray excitation in the 1s binding energy regi~n.~ From the multiplet splitting of the core levels due to exchange interactions between the unpaired core and valence electrons the spin density distribution is inferred. The unpaired electron is mainly in the n* MO of the NO group and is delocalized and the Nls and 01s binding energies reflect the net flow of charge from alkyl to NO.The valence p.e. spectrum indicates an IE of 7.31eV for the photoejection of the odd electron from the '7t* MO resulting in a singlet ground ionic state. The subsequent bands IE = 8.94 and 9.27 eV correspond to ionization of the nonbonding electrons of oxygen and the exchange splitting between the unpaired electrons in the ion results in triplet and singlet states whose intensity ratio in the p.e. spectrum 3 :1 is as expected on the basis of spin multiplicities. These two studies illustrate the complementary nature of the experiments to provide the complete data. Transient species so far studied have been reasonably long-lived. Most of the species have been generated in sufficient amounts from the precursor(s) to domi- nate the spectrum or else the bands have been separated or well defined from those of the precursor(s) and/or side-products.Pyrolysis or microwave discharge methods can also be used and fast-flow systems enable steady-state concentrations to be attained. The p.e. spectra of CS,O2('Ag) SO (%-) P2 HBS NF2,HCP S20 H2CS F2CS,and others have been obtained.6*s2 The first band of the methyl radical (from Me2Hg as precursor; lifetime <2 ms) has been observed.53 The IE of 9.837 eV is in good agreement with the known spectroscopic value and the dominance of the 0-0 transition is indicative of similar geometry for the ground ionic state to that of CH ,which is expected to be planar. The latter is in accord with the planarity of NH,' which is iso-electronic with CH,.. A single vibrational excitation of 2720 & 30cm-' is ascribed to the vl stretching mode.The identification of the species involved is one problem encountered. One means of characterizing the reactive species is by mass analysis. This has been demonstrated by Turner and co-workers where in situ analysis is achieved by utilizing the same analyser for electron detection and for mass identification from the time-of-flight of the ions.54 Even in routine measurements this gives reassurance that the sample is not only pure but of the correct structure. Recently phase-sensitive detection with a modulated micro- wave discharge has been used successfully to separate the p.e. spectra of the Oz (lAg) (10% in concentration) and 0 (3P)transients from that of O2(,&-) and SO (,I=-) from SOz." These authors also report the p.e.spectrum of the 51 I. Morishima K. Yoshikawa T. Yonezawa and H. Matsumoto Chem. Phys. Letters 1972 16 336; D. W. Davis and D. A. Shirley J. Chem. Phys. 1972,56,669. 52 References given by D. C. Frost in ref. I. L. Golob N.Jonathan A. Morris M. Okuda and K. J. Ross J. Electron Spectroscopy 1972 1 506. 54 D. L. Ames J. P. Maier F. Watt and D. W. Turner Faraduy Discuss. Chem. SOC. 1972 No.54 p. 277. '' N. Jonathan A. Morris M. Okuda K. J. Ross and D. J. Smith J.C.S. Faraduy II 1974,70 1810; J. M. Dyke L. Golob N. Jonathan A. Morris M. Okuda and D. J. Smith ibid. p. 1818; J. M. Dyke L. Golob N. Jonathan A. Morris and M. Okuda ibid. p. 1828. Photoelectron Spectroscopy and Related Phenomena of Gaseous Species 87 CF2 radical produced by microwave discharge through C2F4-He.The extensive vibrational excitation on the first band suggests considerable bond-angle change on ionization. 3 Coincidence Experiments Although coincidence measurements have been widely used by physicists the application of these powerful techniques to probe the electronic structure and nature of the ionic states of organic molecules is as yet limited. Photoelectron- photoion coincidence measurements aim at mass and kinetic energy analysis of the ions obtained corresponding to selected delayed photoelectrons and uia the latter at determining the initial internal energy of the ion. Alternatively the p.e. spectrum for a selected mass ion is determined. Thus when dissociation products are detected the ionic state which was at first produced is identified.The ionic states are correlated with ejection of electrons from MOs and thus one may try to link the reaction of ions of known initial energy to the description of the electronic configuration. For the triatomic molecules studied COS N20,C02 CS2 SO2,and HzO it was found that all ionic states that lie above the thermodynamic dissociation limits fragn~ent.~ Quasi-equilibrium theory or direct dissociation models did not suffice for rationalization of the branching ratios and the kinetic energy of the ions. The fragmentation pathways can be postulated from the energetics data and Wigner-Witner correlation rules. In Figure 6 are shown the mass ion spectra of benzene taken in coincidence with the electronic states of the benzene which are represented in the Figure via the MOs from which the electron is removed.The ground (1elg-')and first excited (1e2g-') ionic states which lie at IEs of 9.25 and 11.49eV respectively are not shown. The Franck-Condon region of the hypersurfaces reached in the ground and first two excited states is below dissociative pathways while for higher states the fragments shown are detected on removal of electrons as shown. The studies on CF4+confirm that the ground ionic state is fully dissociated with the CF3 + fragment possessing large kinetic energy releases ( -1eV),5* whereas for C2F6+ the results suggest that C2F6+ ions in their first excited state fragment directly to C2F5' rather than via the ground ionic state which yields only CF3+.59 The authors relate the difference to the ionization of electrons from the strongly C-C a-bonding MO and n-MOs of the fluorines for the first two ionization processes respectively.The fragmentations of CH4+ CD4+ C2H6+ and C2D,+ have also been studied but instead of the fixed photon source (He 584 A) a continuous light source with a monochromator was used and only electrons of sf J. H. D. Eland Ado. in Mass Spectrometry 1974 6 917; references given in ref 5(6). ST J. H. D. Eland Internal. J. Mass Spectrometry Zon Phys. 1974 13 457. B. Brehm R. Frey A. Kiistler and J. H. D. Eland Internat. J. Muss Spectrometry Ion Phys. 1974 13 251. '' I. G. Simm C. J. Danby and J. H. D. Eland Internat. J. Mass Spectrometry Zon Phys.1974 14 285. J. P. Maier m/e 39 52 63 78 f 5 2.5 0 5 0 A m/e 39 52 63 78 Figure 6 Coincidence mass spectra of benzene (Reproduced by permission from Internat. J. Mass Spectrometry ion Phys. 1974,13,457) zero kinetic energy were detected.60 This obviates compfications arising in cross-section and autoionization. Another approach has been to use fast electrons (3.5 keV) to simulate photon impact and to detect the scattered and ejected electrons in coincidence. This provides a means of varying the impact wavelength and in this way partial ionization cross-sections have been determined for NH ,CO and H20which are in agreement with the available photon impact data.61 Application of this approach to larger molecules can yield the cross-sections as well as the asymmetry parameter fl.6o R. Stockbauer J. Chem. Phys. 1973,58 3800. M. J. van der Wiel and C. E. Brion J. EIectron Spectroscopy 1973 1 309 439 443; G. R. Branton and C. E. Brion ibid. 1974 3 123 129. Photoelectron Spectroscopy and Related Phenomena of Gaseous Species 89 4 Angular Distribution of Photoelectrons from Organic Molecules Measurements of the angular distribution of ejected photoelectrons from some organic molecules have been carried out in order to determine how this can be related to their electronic structure. The asymmetry parameter /I (-1 to + 2) describes the results. For the molecules of interest here the He-Icc (21.22eV) resonance line has been used throughout with the exception of benzene where the Ne-I line was also used as well as the synchrotron radiation at these wave- lengths (as a source of polarized radiation).These data illustrate the intensity differences as a consequence of polarization of the radiation.62 The energy dependence of /I was found to follow the pattern suggested by calculations for CT-and n-orbitals of ethylene. Carlson and co-workers have determined /I for the p.e. bands of halogen-substituted methanes CH,,X4-, (n = 0-3 X = F or C1) and MeX (X = Br or I) and tetramethyl compounds of Group IV Me,X (X = C Si Ge Sn or Pd) and attempts were made to correlate the data with 1.6 1.2 0.8 0.4 0.0 .0.4 I I I I I I I I I 1 1 19 18 17 16 15 14 13 12 11 10 9 8 Binding energy (eV) Figure 7 He-I photoelectron spectrum of butadiene and the angular parameter /3 (Reproduced by permission from J.Electron Spectroscopy 1974,3,59) 62 J. A. Kinsinger and J. W. Taylor Internat. J. Mass Spectrometry Ion Phys. 197213 10. 445. J. P.Maier the associated MO parameter^.^^ Such studies on pyridine and the three diaza- benzenes also revealed trends.54 In Figure 7 is shown the B variation in the p.e. spectrum of butadiene. In the homologous series of unsaturated linear hydro- carbons it is noted that IT-and a-orbitals may be so di~tinguished.~~ The difficulty arises in interpreting the data via theoretical models and recently semi-empirical SCF calculations have been used to provide the link.65 5 X-Ray-excited P.E. Spectra and Complementary Uses of He-I and He-I1 Excitation The development in performance of X-ray-excited p.e.spectrometers by Siegbahn and co-workers66 has allowed the study of the valence bands of gaseous species with sufficient resolution to permit band-intensity corn par is on^.^^ In Figure 8 -... eV 293 290 35 3b 2k 20 1'5 10 Binding energy Figure 8 Photoelectron spectrum of benzene excited by monochromatized A1 Ka radiation (Reproduced by permission from J. Electron Spectroscopy 1974,5985) 63 T. A. Carlson and R. M. White Faraday Discuss. Chem. SOC. 1972 No. 54 p. 1972; A. E. Jonas G. K. Schweitzer F.A. Grimm and T. A. Carlson J.Electron Spectroscopy 1972 1 29. 64 R. M. White T. A. Carlson and D. P. Spears J. Electron Spectroscopy 1974 3 59. 6s B.Ritchie J. Chem. Phys. 1974 60 898; ibid. 1974 61 3279 3291; S. Iwata and S. Nagakura Mol. Phys. 1974 27 425; J. N. Rabalais and T. P. Debies ref. 1 p. 847. 66 B. Wannberg U. Gelius and K. Siegbahn J. Phys. (Ej 1974 7 159. 67 K. Siegbahn ref. 1 p. 3; U. Gelius ibid. p. 895. Photoelectron Spectroscopy and Related Phenomena of Gaseous Species t I TRITHIAPENTALENE s GASEOUS PHASE s2.3 ‘2 2Pq2 .2V\- AE, =I .4eV -eV 166 164 162 160 0 I I I 75 100 125 Channel number Figure 9 The S 2p photoelectron spectrum of irithiapentalene excited by monochromatized X-ray irradiation (Reproduced by permission from J. Electron Spectroscopy 1974,5,985) are shown the monochromatized A1 Ka-excited valence bands of benzene and here the intensities of the bands show clearly that the 16.9 eV band is that asso- ciated with the 3al MO and the preceding band with the 2bl MO owing to its increased intensity.In contrast to u.v.-excited p.e. spectra in the X-ray spectra s-type bands are stronger than the p-type. All the deeper valence bands can be excited and the intensity reversal is an asset in the location of bands that are difficult to detect with He-I1 radiation especially for IE >25 eV (e.g. the 2a, benzene band in Figure 8). Intensity models have been proposed and related to gross atomic populations from MO calculation^.^^ The higher resolution has revealed interesting data on the core level bands. The spectrum of trithiapentalene (Figure 9) shows clearly how the S 2p bands overlap and the assignment is shown.The old spectrum is shown in the inset and the authors note that this was ‘deconvoluted’ by three groups differently and then interpreted. These latest data suggest that there is a flat minimum around the S-S equilibrium position. The Clsband of CH4+is shown in Figure 10,and the contour shows vibrational excitation associated with the relaxation in the hole state leading to change of geometry; the equilibrium at 0.05 A shorter C-H distance is calculated Other molecules whose valence structures have been studied with these improvements include C,H, C2C14 and SF, as C302 well as some diatomic and triatomic molecules.67 The gas-phase X-ray-excited J. P.Maier EXP. c1s -LEAST SQUARES FIT 0.43 eV 291.5 291.0 290.5 Binding energy/eV Figure 10 The C 1s photoelectron spectrum of methane (Reproducedby permission from Chern.Phys.Letters 1974,28 1) p.e- spectrum of the pyridine-iodine monochloride charge-transfer complex has been reported68 and the technique has also been applied to He-IIa (40.80eV) radiation has been used as a source of excitation with two objectives firstly energy extension of the accessible ionic states and secondly for changes in the photoionization cross-section. The p.e. spectra of inner valence shells of saturated and unsaturated hydrocarbons have been pre~ented.~’ In Figure 11 the division of the inner valence region is seen and the s-type MO pattern is evident. The band positions which are less complex than of the outer valence shell have been found to fit a simple Huckel model.Shake-up bands often seen in X-ray-excited spectra have also been observed with He-I1 radiation. These forbidden transitions involving simultaneous electron excitation accom- panying the ionization gain their intensity through configuration interaction and have been discussed for diatomic and triatomic molecules in relation to their detection in the He-11-excited p.e. spectra.70 In the He-I-excited p.e. spectrum of butatriene an additional band of comparable intensity to the other bands is 68 A. Mostad S. Svensson R. Nilsson E. Basilier U. Gelius C. Nordling and K. Siegbahn Chem. Phys. Letters 1973 23 157. 69 A. W. Potts and D. G. Streets J.C.S. Faraday II 1974 70 875; ibid. 70 1505. ’O A. W.Potts and T. A. Williams f.Electron Spectroscopy 1974 3 3. Photoelectron Spectroscopy and Related Phenomena of Gaseous Species p-type bands s-type bands 15 20 25 30 ionization energy/eV Figure 11 The He-IIa photoelectron spectra of cycloalkanes (Reproduced from J.C.S. Faraday II 1974,70,875) also attributed to such a two-electron process and the increased transition probability has been di~cussed.~ The change of photoionization cross-section has been used as a means of identification of MOs in which a high contribution of an A0 is indicated. This has been used predominantly in instances where the changes in relative intensity of the bands are drastic on passing from He-I to He-I1 radiation. This is the case with the 3p A0 in comparison with the 2p.The former represented by chlorine- sulphur- and phosphorus-containing molecules decrease appreciably in cross- section towards the shorter wavelengths. The differential cross-sections have been used in connection with the assignment of the p.e. spectra of halogeno- ethylene~~~ and of oxygen- ~hlorine-,~’and fluorine-containing corn pound^.^^ There has also been some work in this direction by correlation with calculation^.^^*^^ 71 F. Brogli E. Heilbronner E. Kloster-Jensen A. Schmelzer A. S. Manocha J. A. Pople and L. Radom Chem. Phys. 1974,4 107. 72 A. Katrib T. P. Debies R. J. Colton T. H. Lee and J. W. Rabalais Chem. Phys. Letters 1973 22 196. ” A. Schweig and W. Thiel J. Electron Spectroscopy 1974 3 27 and references therein.74 F. 0.Ellison J. Chem. Phys. 1974,61 507; J. W. Rabalais T. P. Debies J. L. Berkosky J.-T. J. Huang and F. 0.Ellison ibid. p. 516.