Electron Spectroscopy. II, X-Ray Photoexcitation David
M. Hercules, Department of Chemisfry, University of Georgia, Athens, Ga. 30601
A
is commercial instruments on the market to review the field of electron at the time of this writing. Whereas spectroscopy for the period 1970-71, a few years ago ESCA chemical shifts because it is the first review on this topic had been observed only for a few elein the Analytical Chemistry Fundaments, a t the present time chemical mental Reviews, it seems appropriate to shifts have been measured for a t least 48. go somewhat beyond those limits. I The job of reviewing the field of have tried to cover all literature on ESCA has been made somewhat easier ESCA abstracted in Chemical Abstrach because of the publication of two books through Dec 16, 1971, as well as articles by Siegbahn and his associates, one appearing in readily available journals. covering the ESCA technique in general I have attempted to review all articles (136) and the other covering ESCA abstracted beginning with volume 69 applied to free molecules (137). A (1968) of Chemical Abstracts, as well as third book on photionization and photosome earlier articles. The choice of electron spectroscopy has been pubarticles prior to 1968 will reflect my lished (98). This reviews recent depersonal bias about what types of arvelopments in the field of photon impact ticles would be most useful and interionization and reviews direct ionization, esting to the readers of ANALYTICAL superexcitation, and preionization. I n CHEMISTRY. addition, a number of general articles This review will attempt to cover only dealing with various aspects of ESCA those studies where soft X-rays were have appeared, mostly written in used for photoexcitation. This techEnglish. Siegbahn and his associates nique has been referred to in the literahave published at least five (45, 117, ture as X-ray photoelectron spectros130, 138, 13’9, 143’); a number of other copy (X-ray PES), photoelectron specEnglish language general articles have troscopy of inner shell electrons (PEappeared (16,33,36,66,78-74,113, 135). SIS), and as electron spectroscopy for General articles on ESCA have apchemical analysis (ESCA). The term peared in Swedish (QZ, Q4), French ESCA will be adopted in the present (161), Italian (185, l Z 6 ) , ,Japanese review. ( l i ,60, 69, 70, 99,146, 166), Norwegian It is always somewhat difficult re(lZ3),and German (155;. viewing a rapidly developing field such The first international conference on as ESCA. This is particularly true electron spectroscopy was held a t the when the developments occur both in Asilomar Conference Grounds in Pacific applications and fundamentals of the Grove, Calif., on Sept. 7-10, 1971. The papers presented a t this conference technique simultaneously. Such has have been published in a preprinted been the case for ESCA in the past and program (77) and are scheduled to be will probably continue to be so in the future. This interaction between appliprinted by a commercial publisher early cation and fundamentals is healthy for in 1972. However, because these papers are not currently available to most the development of an experimental technique. It stimulates interest, it readers of this journal, they will be develops utility, and it helps to bring reviewed in a subsequent Fundamental about a fusion between theoretical and Review. applied science. I have not tried to Theory. One aspect of attempting bias this review either in the direction of t o understand a new spectroscopic application or theory but have tried to technique is to correlate the data with make it cover both aspects. This those obtained by other techniques. stems from the belief that in the present Because the origin of core electron chemical shifts is caused primarily stage of development, persons wishing by the effects of chemical bonds in the to use the ESCA technique will need to valence shell, a direct link between have available both types of information. NMR and electron spectroscopy has The growth of the field of electron been predicted (14). Such a correlation spectroscopy has been quite dramatic. has been obtained for an homologous Research papers dealing with ESCA as a series of phosphonium salts (247) but attempts to apply this to a broad scale technique or using ESCA as a technique to investigate chemical phenomena have correlation have not been successful. been appearing only for about three A relationship has been reported beyears. Despite this, there are a t least five tween C(1s) electron binding energies LTHOUGH THE PRESENT ARTICLE
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ANALYTICAL CHEMISTRY, VOL. 44, NO. 5, APRIL 1972
and lac NMR chemical shifts for the halomethanes. The binding energy was also related to the nuclear quadrupole coupling constants of the chloromethanes (17). One would also anticipate a correlation between ESCA chemical shifts and those observed in Mossbauer spectroscopy. This has been observed for both tin compounds (11) and iron compounds (19). Mossbauer and ESCA spectra have been compared for compounds of the type, K,FeF3 where the iron exists in mixed (31) valence states. Another problem of fundamental importance to electron spectroscopy is escape depths of the electrons. Various estimates have been reported ranging from ca. 5 A fr2m dense matrices to as much as 100 A from lighter material (136). In the case of gold, an escape depth of 22 A has been measured for 1.2 keV electrons (6). Another important problem for ESCA is the nature of the Madelung potential in a solid. The question arises as to whether the surface Madelung potential can be represented by the bulk Madelung potential or whether some correction is necessary. By using a rigid lattice model in relation to a semi-infinite lattice, it was calculated that the Madelung potential was not significantly affected a t the surface (144). ,4 linear relationship between C(1s) and N(ls) core binding energies and atomic charges is linear if the interatomic Madelung potential is included (156).
A variety of studies has been reported attempting to correlate ESC.4 chemical shifts with various types of electron density calculations. ESCA chemical shifts were calculated using self-consistent field MO theory for the first row elements in the periodic table (13). A linear correlation with net charge was obtained and agreement with experiment was found. Similarly, a relationship between core binding energies and charges for carbon was observed if the molecular potential was taken into account. Binding energy shifts for C(1s) electrons in different molecular environments were correlated with the average molecular potential a t the carbon nucleus for a variety of carboncontaining compounds. Except for CO good agreement was found (135’). Xitrogen Is electron binding energies were correlated with CNDO charges (71). -4linear relationship was observed between calculated charge and observed
binding energy although points seemed
to fall on two lines, one characteristic of anions and the other characteristic of neutral molecules. Correlation between calculated charges on nitrogen and N(1s) binding energies was accomplished for a larger number of compounds (63). N (1s) binding energies and charges determined by ab initio SCF-MO calculations were reported for a small number of nitrogen containing compounds (163). Other ab initio calculations have been done for sulfur containing molecules (47) and agreement between theory and experiment has been compared. Several types of different charge calculations were performed for phosphorus compounds (124) and compared with experimentally measured binding energies. A detailed study of the chemical effects on core electron binding energies in both iodine and europium was carried out in an attempt to calculate chemical shifts by inclusion of the Madelung potential (37). Although the calculated values for chemical shifts exceeded the observed values, these calculations indicate the magnitude and importance of the Madelung correction. A different approach to the calculation of binding energy shifts has been the use of thermodynamic data (81). This involves the fundamental approximation that atomic cores having the same charge are chemically equivalent and these are thermodynamically interchangeable. A method has also been reported (8.2) for the estimation of dissociation energies and its application to the correlation of core binding energies obtained from ESCA data. Other calculations of interest to electron spectroscopists have been reported. For example, ionization potential calculations on transition metal complexes (157) and ab initio LCAO-SCF-hlO calculations to determine the average quantum mechanical potential a t an atomic nucleus. One example of the breakdown in linearity between calculated charges and observed binding energies has been reported (6). Simple ab initio SCF calculations have been compared with photoelectron spectra for a series of simple hydrocarbons (84). The binding energies for 1s electrons of C, h’, 0, and F, have been calculated in different molecular environments by ab initio LCAOSCF-MO wave functions (80). The contribution of the Is binding energy to the potential is insensitive to the environment. Generally, shifts in inner and outer levels are similar. The whole problem of the persistence of atomic orbitals in transition metal complexes and other compounds has been reviewed (79). The potential applicability of ESCA to problems of this type was discussed. Another matter of importance to ESCA is whether Koopman’s theorem
holds for the process of ionization by removing a core electron. Measurements of binding energy shifts of 1s electrons from carbon, oxygen, and nitrogen were compared with calculations based on Koopman’s theorem and shown to agree within 1 eV (30). The C(1s) chemical shift for CO relative to methane agreed closely with that calculated by Koopman’s theorem; however, the O(1s) chemical shift for CO relative to water was closer to the value calculated for orbital relaxation (67). Calculations of inner shell ionization energies from Koopman’s theorem have been shown to relate to a weighted average energy for singly and multiply excited states in the photoelectron spectrum (104). The K electron binding energies have been obtained for both the sudden and adiabatic approximations for H20, NH,, and CHd using several SCF-LCAO-MO wave functions with different Slater-type orbital basis sets (49). The general validity of the linear relationship obtained between binding energies and calculated charges is analyzed. A generalized energy expression has been used with Slater’s rules to study Koopman’s theorem for core electrons and its dependence on nuclear charge and the number of valence electrons of an atom (146). An electronic relasation energy is defined and is proportional to the number of electrons in the esternal shell but independent of nuclear charge. Arguments have been given to show that the core hole should be thought of as localized even when there are several equivalent sites over which it may be delocalized. Energy splitting of the core electron levels in paramagnetic molecules has been observed in several instances. The spectra of the paramagnetic nitrogen osides and oxygen show core splitting whereas diamagnetic gases do not (59). Splittings of approximately 6 eV have been reported for the 3s levels in MnF2, MnO, and FeF3 (40). Splittings have been explained by multiplet states formed by coupling of a hole with an unfilled valence subshell (4.2). Splittings of 3s electron states are exhibited by inorganic compounds containing manganese and iron, as well as iron, cobalt, and nickel metals. The 3p binding energies of many of these materials also show multiplet effects but the interpretation is less straight forward. The 21, binding energies in MnF2 are broadened by a t least 1.3 eV which is consistent with multiplet splitting (49). Satellite lines have been reported in the ESCA spectra of alkali halides (158), and these are shown to arise from configuration interaction. Splitting observed in the ESCA spectra of rare earth ions with more than half filled 4f shells has been reported to arise from the exchange energy difference between
the final 4f states (160). ESCA has also been used to study the density of states in the valence bands of Fe, Co, Ni, Cu, Ru, Rh, Pd, Ag, Os, Ir, Pt, Au, Zn, S, CdCI,, and HgO (41). Some have been treated by use of a rigid band model. Another interesting phenomenon is the angular distribution of ESCA electrons observed in spectra from a single crystal (141). For the case of sodium chloride, some variations of electron intensity with the tilt angle of the crystal can be correlated with the various crystal planes of the NaCl lattice (14.2). The angular distributions for 1.25-keV photoelectrons ejected from Au showed a strong modulation due to diffraction effects. Those ejected from C(1s) and O(1s) impurity levels were relatively flat (44). Satellite lines are also observed in ESCA spectra as a result of shake-off and shake-up phenomena. Studies related to this problem are just beginning to emerge (90). Satellite peaks observed in the ESCA spectra of metal carbonyls a t binding energies 5-6 eV higher than normal have been reported to arise from transitions to ionic states which correlate with components of the ground state (12). Thus, one can describe the ground states of these molecules using a weak bond formalism. Satellites on the 21, electron lines of transition metal ions have been reported to arise from simultaneous outer shell excitation. The coupling mechanism and selection rules have been discussed (230). Energies of the 3d -+ 4s transition have been obtained from a number of difluorides and oxides. Instrumentation. Two papers have appeared describing improvements in magnetic instrumentation for ESCA. A modification of the original Siegbahn instrument for the iron-free doublefocusing magnetic spectrometer has been reported (112). The coil design allows for greater access to source and detector locations. Continuous control of external fields during the recording of spectra is possible. I n this same publication, the authors describe an X-ray tube suitable for providing high intensity, soft X-rays with good stability. A computer design study of an iron free double-focusing magnetic spectrometer has been carried out (39). For single detector operation, the design is more efficient than any existing spectrometer and the focal plane characteristics permit a hundred-fold increase in the rate of data accumulation when using a multichannel detector. An electron spectrometer using a cylindrical mirror type of electrostatic analyzer has been described (200). The analyzer uses second order focusing, space focusing, and retardation by a lens system prior to the analyzer. A counting technique is used for the detector. The characteristics of this spectrometer have been tested
ANALYTICAL CHEMISTRY, VOL. 44, NO. 5, A P R I L 1972
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on the gold Na, N, doublet; values comparable to magnetic instrumentation have been obtained (101). Several patents have appeared for ESCA instrumentation. An electrostatic spectrometer is described which has high resolving power and transmission and a large solid detecting angle. The sample is held on a cylindrical electrode and a potential is applied between the sample and the entrance slit of the analyzer to provide scanning (61). An addendum to the patent describes a spectrometer containing a conduce ing sample housing operating a t a potential independent of the spectrometer slit electrode (6.8). The sensitivity of an electron spectrometer has been increased by using an electron-optical device to focus the electrons, with magnification and with or without retardation prior to entering the electrostatic analyzer (61). Several papers have been reported which relate to components for electron spectrometers or present aids for data handling. A soft X-ray tube using an aluminum anode has been constructed for use with cylindrical electron spectrometers (169). A method has been described for designing parallel plate electron multipliers (110). This is possible by depositing semi-conductor layers onto glass plates by sputtering and placing a series of plates parallel. These multipliers are reported to have a high gain (110) and to give output pulses with a very narrow pulse height distribution. The multipliers also have small dimensions. One very important problem in ESCA is deconvolution of recorded spectra. A paper examined the effect of noise on the application of a deconvolution technique using Fourier transforms (34). The truncation criteria for the removal of noise from the transforms can be bettered by an iterative smoothing procedure. More closely related to ESCA, deconvolution techniques in Auger spectroscopy have been used to study band structure (108). I n this application, the effects of instrument broading and inelastic interactions were significantly reduced. Chemical Shifts. Although the earlier chemical shift work has been summarized (136), it is well to review it a t this time for the sake of completeness. The binding energies of core levels of copper and its oxides was studied early (119); a shift of 4.4 eV observed between Cu and CuO. Chemical shifts for sulfur compounds were the first published data to establish the potential utility of ESCA as an analytical technique (63,64, 118). Chemical shifts for Na2SO4, Na&Oa, NanSOa, Sic, and a variety of organosulfur compounds were included. A series of chlorine compounds ranging in oxidation states from - 1 to f 7 showed a range of chemical shifts of ca. 10 eV (46) between 108R
NaCl and NaClOd. ESCA chemical shifts for an extensive series of nitrogen compounds, both organic and inorganic, were recorded (116, 116). Thase data were correlated with atomic charges calculated by simple electronegativity considerations. Chemical shift data for carbon compounds were reported (S), establishing the difference between aromatic and carboxylic acid carbons for sodium benzoate and terphthalic acid. Chemical shifts were also reported for iodine and europium (43). More recent work on chemical shifts for the light elements will now be summarized. The K-level shifts for Be, BeO, and BeFz have been studied by ESCA (66). Boron 1s electron binding energies were measured for 25 compounds showing a range of shifts of 8.4 eV (64). A plot of B(1s) binding energies ua. extended Huckel-MO calculations showed a reasonably linear correlation. A number of compounds were run containing boron atoms in different environments, but only a single boron line was observed. Approximately 15 carbon compounds (64) were also measured and correlated with CNDO charge calculations. Nitrogen 1s electron binding energies have been measured for 57 nitrogen compounds (63). Correlations of these data with different types of MO calculations were attempted. Electron binding energy shifts for N(1s) electrons in gaseous compounds have also been reported (83). Binding energy data for Si(2p) electrons have been reported for 16 compounds. A range of approximately 8 eV was observed between Si and NazSiFs (114). Correlation between the measured binding energies and charges calculated by the SCF method were reported. ESCA spectra of the O(1s) levels of olivines were observed to contain a single component whereas those of pyroxenes contained two components with an intensity ratio 2:1 and an energy separation of ca. 1 eV (166). This has been interpreted as a result of binding energy differences between bridging and nonbridging oxygen atoms in the silicate chain of the pyroxene structure. Phosphorus 2p electron binding energies were measured for 53 phosphorus containing compounds, showing a range of approximately 8 eV. These included inorganic phosphorus compounds as well as some organophosphorus compounds and cyclic structures such as the phosphonitrilic trimer. The P(2p) electrons for 17 phosphorus compounds were measured (107) along with binding energies of O(ls), S(2p), and Se(3p). This study involved the comparison of a series of compounds having the structure RsPM where M is an electron pair, oxygen, sulfur, or selenium. When M goes from oxygen to sulfur to selenium there is essentially no change in the P(2p) binding energy; this is attributed to
ANALYTICAL CHEMISTRY, VOL. 44, NO. 5, APRIL 1972
charge equalization through *-bond feedback. DilTerences in the binding energies of P and C1 core electrons in PCL and POCl have been measured and interpreted in terms of the nature of the PO bond (68). Chemical shifts for S(2p) electron binding energies for 11 compounds have been reported (90) and correlated with atomic charges calculated by the EHMO method. Chemical shifts for the Br(3d) and As(3d) electrons have been reported for several compounds (76). For a given change in oxidation number, the shift for Br is intermediate between those reported for chlorine and iodine. Chemical shifts observed for As were correlated with variations in the effective charges on arsenic. Chemical shift data were reported for both iodine and europium (37). The shifts for iodine per unit change in oxidation number were smaller than those observed previously for chlorine. A shift of 10 eV was reported between Eu +2 and f 3 which is the largest shift for a given change in oxidation state reported to date. Other workers (109) have measured chemical shifts between Eu and EuzOa for several electrons. They found differences of 7.8, 7.5, and 6.4 eV for the 4p 3/2, 4d 5/2, and 4f electrons, respectively. To date there has been no resolution of the differences in Eu binding energies measured by these authors. The 4s, 4p, 3d, and valence band electron binding energies have been reported for a number of rare earth atoms (111). Measurements have also been performed on promethium, measuring the L, M, and N electron binding energies. The 4p and 4d levels were measured using AlKa radiation and the 39, 3p, 3d, and 4s and 4p levels were measured using the CuKa line (102). The energies of all L, M, N, and 0 levels in platinum have been measured (86). Electron binding energies for the Se(3p) and Te(3d) electrons have been measured for a number of simple selenium and tellurium conipounds (149). Generally, the chemical shifts were smaller than those observed for comparable sulfur compounds. The results were correlated by HFS semi-empirical molecular orbital charge calculations. The Mo(3d) electron binding energies were measured as a function of oxidation state for 20 molybdenum compounds. A linear correlation between electron binding energies and oxidation state was observed (148).
Chemical shift data have been reported for some of the transition metals and platinum metals as well as studies of bonding in transition metal complexes. The 3p electron binding energies for 18 chromium compounds have been reported (64, 66). A range of 5.5 eV was observed between Cr and KZCrz07. The Fe(3p) electron binding energies for 11 iron compounds were
measured and were found to correlate with EHMO atomic charge calculations (90). ESCA studies have also been carried out on VC, NbC, and TaC and the results are compared with heats of formation of these compounds (128). ESCA spectra were recorded for a series of iron- and sulfur-containing compounds in which the formal oxidation state of iron varied between 0 and 6 and sulfur varied between -2 and +6. Ranges of chemical shifts were 5.5 eV for iron and 7.5 eV for sulfur. Changes of ca. 1 eV per unit change in formal oxidation state was observed for covalent iron compounds containing the same ligands while the effect in ionic compounds was twice as large (89). Several workers have been concerned with the nature of the iron sites in Prussian Blue. Two discrete maxima corresponding to Fe + 3 and Fe(CN)8'were observed (91). In addition chemical shifts for other bi, and tri-valent compounds are given. Subsequent work (169) has indicated that Prussian Blue the low spin iron is covalent +2 iron and the high spin iron is ionic f3. These workers also found two narrow lines agreeing with those of KJ?e(CN)s and F e +3 (169). The ESCA spectra of coordination compounds of triphenylphosphine complexes have been studied (16). The P(2p) binding energies in coordination compounds fall at lower values than the binding energies of the P(2p) electrons in uncoordinated (C&)&.' Studies involved chemical shifts of the P(2p) electrons in complexes of Ni, Pd, and Cd. Opposing features of sigma bonding and s-bonding resulting in cancellations of electron changes account for the constancy of electron density on the phosphorus (16). Core binding energies for Cr(CO)s, Fe(C0)6, and Ni(C0)4 and M ( K - C ~ H ~where )~ M = Cr, Fe, CO, and Ni; CsH5 = cyclopentadienyl have been reported and discussed qualitatively in terms of charge distribution (24). ESCA studies have been reported on high spin Ni (11) compounds (80). Discussions of the Madelung potential and the relationship between optical electronegativities and ionization potential is included. Molecular core binding energies and multiplet splittings in Cr(C0)S and chromium hexafluoroacetylacetonate have been recorded. The ligand (IS) chemical shifts, and multiplet splittings for Cr(3s) electrons are discussed. Platinum 4f electron binding energies were obtained for complexes of platinum in which the metal is in a low formal oxidation state (19). Binding energy changes were interpreted in terms of the degree of oxidation of the metal by assuming that within the series of complexes the binding energies depended only on the electronic charge transferred from the metal to the ligand. Correlations were attempted between the
charge transferred to the ligand and the anticipated geometry of the molecule. ESCA studies of platinum and palladium complexes have allowed distinction between terminal and bridging chlorine atoms in these compounds (86). Band Structures of Solids. I n addition to its utility for providing information on core electron binding energies, ESCA is also a useful technique for studying electrons in the valence bands of solids and for determining the densities of states in metals. Both photoelectron and Auger spectra have been obtained with X-ray excitation for copper, nickel, and iron. The photoelectron lines have been used as internal standards to measure the energies of the prominent L, M, N , 0 Auger lines (164). Densities of states in core electron levels for Fe, Co, Ni, Cu, and Pt were studied (58) and the data shown to be in good agreement with other experimental techniques and with theory. Additional data have been reported on the densities of occupied states in the valence bands of Fe, Co, Ni, Cu, Ru, Rh, Pd, Ag, Os, Ir, Pt, and Au (4). Another study has reported density of states measurements of Fe, Co, Ni, Cu, Ru, Rh, Pd, Ag, Os, Ir, Pt, Au, ZnS, CdC12, and HgO. The d bands of these solids have systematic behavior with changes in atomic number and have been shown to agree qualitatively with theory. The d bands of Ag, Ir, Pt, Au, and HgO have a two-component shape. The position and shape of the energy bands of Fe, Co, Ni, Cu, Ru, Rh, Pd, Ag, Os, Ir, Pt, and Au were studied (6). The Fermi levels of the metals with unfilled d bands are found in the high energy flanks of the valence band spectra. These observations are in general agreement with the overlap between d and sp bands in the transition metals. Splitting5 observed for some of the bands seemed too large to be attributable to spin-orbit interactions. I n addition to the transition metals reported above, density of state measurements have been performed on rare earth metals as well. ESCA measurements on films of Yb show a large spinorbit split of the 4f levels. Measurements also were reported for core levels in Yb (62). Additional measurements have been performed on Nd, Sm, Dy, and E r (68). The band is basically due to excitation of 4f electrons and the spectra showed a complex structure which is a result of multiplet splitting. Valence band studies of materials other than metals are also possible. The valence bands of LiF, Be0, BN, and graphite were studied (67). The differences between the inner levels and the valence bands were compared with X-ray energies. ESCA data obtained on Cu20, CuCl, CuO, CuS, and CuSO4 showed satellites having various degrees of complexity adjacent to the Cu(2p) '
lines (180). The satellites are explained by a multiple excitation process in which emission of a 2p photoelectron is paralleled by a valence band-conduction band excitation. ESCA data have also been obtained on a single crystal of GaSe, showing peaks corresponding to the valence band levels. The results agree with calculations (1). The valence band width has been determined for NaCl, and correlated with configuration interaction (161). The valence band electrons for different forms of elemental carbon have been studied (160). The band widths were larger than expected theoret,ically, and on the basis of previous ESCA measurements. The major part of the density of states curve is largely unaltered in going from the crystal to the disordered solid. Also, it was found that the valence band of coronene is very similar to graphite. The C(1s) binding energies for diamond and graphite were found to be similar, and chemisorption of oxygen at prismatic faces of the crystals of graphite was detectable. The peak positions of X-ray photoelectron spectra of the valence regions of LiClOd, and Li2SOI have been reported (127). Orbital assignments for these lines are given. A change in cation had no measurable effect on the line positions and only a small influence on their widths. Organic Compounds. Because of its versatility, a natural application of ESCA is to structure investigations of organic compounds. A number of studies have emerged relating chemical shifts to some particular aspect of organic structure. These will be summarized here by element or by structure type. C (1s) binding energies have been measured for simple hydrocarbon types (66, 162). Ionization energies for both valence shell and core electrons have been reported. For methane, two lines in the valence shell have been correlated with molecular orbitals (66). Core binding energies as well as valence shell orbital energies have been compared with energies obtained by the use of Koopman's theorem, calculated values being somewhat larger than the observed values. C(ls) binding energies decrease with hydrogenation and when a hydrogen atom is replaced by an alkyl group (162). C(ls), O(ls), and valence shell electrons have also been measured for carbon monoxide. The results indicate a positive charge on the carbon and a negative charge on the oxygen in agreement with the relative electronegativities. Ionization potential measurements agree with those observed by other techniques (164). C(ls) ionization potentials for halomethanes have been measured relative to methane and chemical shifts determined. The carbon and halogen binding energies increase linearly with the
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number of halogens. A linear relationship is observed with the sum of differences between the electronegativities of the ligands and that of hydrogen (163). Chemical shift measurements in the fluoronated methanes have been analyzed in terms of simple concepts of bond dipoles and effective atomic charges (131). Shifts in the C(1s) energy level for multiple fluorination are sums of the shifts produced by a single fluorination. Binding energies of the C(1s) electrons are linear functions of calculated charge. Carbon 1 s electron binding energies have been measured for an extensive series of organic compounds, this representing the most extensive series of measurements to date. The series includes saturated hydrocarbons, carbonyls, and some aromatic compounds (48). The use of carbon shift data in chemical structure is considered by calculation of charge parameters obtained from electronegativity considerations as well as analysis of the data in terms of group shifts from which group electronegativities are derived. Data for solid and gaseous compounds are compared and solid-state shift effects are shown to be small for non-ionic compounds. The observed shifts are compared with several types of free molecule MO calculations (48). Core binding energies have also been determined for compounds of the type CaHs X where X is H, F, C1, CHI, CF,, and CCb (22). The data observed were correlated with CNDO charges. ESCA has been applied to the measurement of carbonium ions by using frozen SbFsSO2 solutions for carbons in the tertbutyl cation (121). Data are also reported for the trityl cation and the tropylium cation. An extensive ESCA study of the N(1s) binding energies of quaternary nitrogen compounds has been reported (78). Tetraalkyl ammonium and mono, bi, and tricyclic aromatic quaternary nitrogen systems have been studied. Binding energies decreased as delocalization of the nitrogen electrons increased. A serious effect of counter ion on the binding energy was observed and ring substituent effects were investigated in the pyridinium series. It is evident that crystal effects are important in ionic organic compounds. Approximately a 5-eV range of binding energies was observed for quaternary nitrogen compounds (78). Probably the most extensive work on the ESCA of organic compounds has been done for sulfur compounds. An extensive series of these compounds has been studied (96) ana a correlation established between the S(2p) electron energies and chemical structure. The influence of structure on binding energies has been discussed as well as correlation of measured binding energies with 110R
charge calculated on the basis of simple electronegativity concepts. The S(2p) chemical shifts for a series of substituted nitrobenzenes has been reported (98) and the results have been compared with shifts observed in other sulfur compounds. The shifts are interpreted in terms of resonance and inductive effects. ESCA measurements for sulfur substituted nitrobenzenes have been correlated with infrared spectra and polarographic half wave potentials (97). The results indicate that the binding energies of the sulfur atom linking the sulfur groups to the aromatic ring are largely affected by the inductive effects of the substituents. The influence of the positive character of sulfur on the S(2p) binding energies in sulfides and sulfones has been reported (96). The effect of the charge on sulfur increases the C(1s) electron binding energy of the adjacent carbon atom approximately one-tenth the value of the shift in the S(2p) level. .4n extensive series of organo-selenium compounds has also been studied by ESCA (103). The electron binding energies for both the 3d and 3p levels were determined, and correlations between binding energy and calculated charge on the selenium given. In addition to measurements on specific atoms in organic functional groups core binding energy measurements have been performed on a number of elements in ring systems. The binding energies for C, S, N, and 0 in thiophene, pyrrole, furan, isoxazole, pyrazole, imidazole, and thiazole have been measured (25). The core binding energies were correlated with atomic charge distributions obtained from molecular orbital calculations. Likewise, core binding energies for C and N(1s) levels in adenine cytosine and thymine have been reported (7, 9). The core binding energies for C(ls), S(2p), S(2s), and S(2p) electrons were measured for thiophene and compared with ab initio calculations (21). Core binding energy measurements for thiathiophenes have permitted distinction between symmetrical and unsymmetrical structures (27). Observed shifts were interpreted by CNDO calculations. An ESCA study of mesoionic compounds has been reported (122). The N(1s) binding energies of 3-methyl-2-phenyl-l,3,4-thiodiazoro-Sthione and its 2(p-chlorophenyl) derivative differ by 2.3 eV while their methiodides differ by 1.2 eV. Similarly, binding energy differences of the S(2p) electrons in the thiodiazoles differ by 2.5 eV whereas in the methiodides they are equivalent. 4-Methyl-2phenyl-1,3,4-thiodiazole-Sthioneshows an N(1s) electron difference of 0.9 eV and an S(2p) electron difference of 2.1 eV. These measurements are indicative that mesionic 1,3,4-thiodiazole thiones exist predominantly in the betaine form (122). The ESCA spectra of DPPH
ANALYTICAL CHEMISTRY, VOL. 44, NO. 5, APRIL 1972
and diphenylpicrylhydrazine have been recorded. The N(1s) lines are considerably broader in DPPH than its diamagnetic parent. The reason for this broadening is that multiplet splitting of the core levels occurs in the free radical (36). Applications. Although ESCA is a relatively new technique, applications to various areas of science and technology are being reported. The application of ESCA to chemical structure problems has been reviewed (93). Problems reviewed are rotation barriers caused by charge displacement, hydrogen bonding effects on oxygen and fatty acids, and suggestions for the use of ESCA in organometallic chemistry and catalysis. ESCA has been used as evidence for the thiosulfonate structure of cystine S-dioxide (2). It was possible to distinguish between the thiosulfonate us. the disulfoxide structure for these compounds. ESCA has also been used to establish how oxygen and sulfur are linked to carbon in ~~-3,5,7-trimethyl2-oxa-4,6-dithia-l-adamantanol and 4,4’thiobis (y-valerolactone) (60). Perhaps ESCA has its greatest potential utility as a technique for the study of surfaces. Some applications in this area have been reported. A review of preliminary experiments, which points out some possibilities of ESCA in catalyst research has been given (32). Experimental examples using ESCA to study chemical states of molecules at catalyst surfaces, catalyst characterization, activation, aging, and poisoning are included. The potential use of ESCA for studying surface structure and the reactivity of solids has also been suggested (106). ESCA binding energies for Cu(2p) electrons of dispersed surface phases of CuO supported on alumina have been shown to agree with those determined by other X-ray techniques (162). Likewise, ESCA spectra have been obtained for calcined and uncalcined Mo-Al203 systems. After calcining, the 3d levels of the 1.10 doublet were no longer resolved because of an interaction between the Mo and the alumina (106). ESCA has been used to study chemisorbed oxygen on platinum surfaces (86). Platinum oxide surfaces prepared by reaction with molecular oxygen or electrochemical oxidation both showed the presence of oxide formation. Comparison was made with bulk samples of various platinum oxides. ESCA has been used to examine carbon fiber surfaces ( I O ) . Profiles of the C(1s) peak closely resemble those of graphite. ESCA has been used to study doping of semi-conductors (154). The semi-conductor lines were unaffected by changes in the bulk Fermi levels produced by doping. This probably results from pinning of the surface Fermi level by contaminants independent of the bulk Fermi level. Ge, GaAs, and CdSe were studied.
Application of ESCA to compounds of biological interest has also been reported. A brief article has reported investigations of transfer RNA and its nucleotide bases (75). Preliminary experiments have demonstrated that total protein may be estimated by quantitative determination of the nitrogen peak (87). In proteins rich in lysine and arginine, the amide nitrogen may be distinguished from the amine nitrogen. Sulfur content may be observed by observing the S photoelectrons (87). A study has determined the extent to which ESCA could be used to study large molecules such as insulin (18). Particular emphasis was given to the evaluation of chemical shifts arising from sulfur photoelectrons. Other types of investigations have been reported as well. A method has been described for studying the components of frozen aqueous solutions (88). This is particularly valuable because it allows one to study species which cannot be isolated as solids. Uinding energies were determined for KiFe(CN)e solutions as well as NaCl solutions (88). ESCA has also invaded the polywater controversy (SI). The ESCA spectra of polywater have been shown to contain high concentrations of Na, K, SO2-, C1-, Koa-, borate, silicates, and C-0 compounds with trace amounts of other impurities, but very little water. On the basis of this evidence, the esistence of polywater has been doubted. The quantitative analysis of molybdenum oxide mistures by ESCA has been reported (148). Although much Moo3 contamination occurs on the surface of MOO? oxide particles, ESCA has been used successfully to estimate the percentage of MOO, in h~oO3-hfoOa mixtures. For high percentages of MoOz, precision of ca. 2% has been reported. Indications are that PbO-PbOs, CraOy-CrOa, and As~03As206mixtures could also be measured quantitatively. ESCA has also been used to detect arsenic in soil samples indicating it to be a possible tool for application to pollution problems (76). ACKNOWLEDGMENT
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WORKsupported in part by funds provided by the United States Atomic Energy Commission under Contract AT-( 38- 1)-645.
