Thomas L. James Department of Biophysics and Physical Biochemistry University of Pennsylvania Philadelphia, 19104
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Photoelectron S P M ~ ~ O S ~ O P ~
Photoelectron spectroscopy involves precise measurement of the kinetic energy of photo:lectrons emitted from a sample which is irradiated with X-rays or vacuum ultraviolet photons. The measured kinetic energy can be simply related to the binding energy of the electron in the sample element and is a function of the element irradiated and its 4ectronic environment. The technique involving Xray irradiation has been variously called X-ray photo:lectron spectroscopy (XPS), electron spectroscopy for chemical analysis (ESCA), and induced electron emis3ion spectroscopy (IEE). The variation using ultraviolet irradiation is termed ultraviolet photoelectron 3pectroscopy (UPS) or, simply, photoelectron spectrosCOPY. The primary development of ESCA has been due to 3iegbahn and his group at Uppsala. The technique was initially described in 1946 ( 1 ) and has been continually improved since that time. Many of the developments have been described by Sieghahn and his colleagues in two monographs on the subject (8, 3). In spite of the relatively early origin of ESCA, its value as a general analytical technique has only recently been recognized. Other groups have begun to exploit the capabilities of photoelectron spectroscopy; however, most have used ultraviolet radiation rather than Xrays as the initial energy source. Turner and his co-
workers have been prominent in the development of uv radiation which permits a sufficiently high resolution photoelectron spectroscopy study that the vibrational states of gaseous ions may be distinguished (4, 5). Although the variation of the method using uv radiation is essentially the same, the instmmentation and subjects for study are somewhat different from X-ray photoelectron spectroscopy which will be emphasized here. Principles
Relotionship with Other Spectroscopic Methods Many spectroscopic techniques arise from excitation of the electronic structure of an atom by X-rays, photons, or electrons impinging upon the atom and the subsequent relaxation of the electronic system. Initially we may consider those techniques due to X-ray excitation of a sample as shown in Figure 1. The principles involved with photon excitation are the same; only valence orhitals are affected. If the incident Xray beam is of sufficient energy, the energy may be absorbed by an electron in one of the lower energy levels (e.g., the K level in Fig. 1) with the result that the electron is emitted from the atom. As mentioned previously, the basis for photoelectron spectroscopy is the analysis of the kinetic energy of that emitted electron. However, this same process will lead to other
Figure 1. left, Excitation of on .tom b y X-ray rodiatim followed b y reloxolion. The relationship of the eledronic prorenes m d spastrnruopic techniques is illudrotod. right, Msthemoticd expressions fer the excitation and relaxation processes. The observed parameter isshorn on the left of the equations. EK is kinetic enerpy. EB(K), Es(Lrr), and Ea(111d ore the binding energies of eiechom in energy levels K, 111 and Liu. v is the fraquoncy of the incident X-ray while I' and v a are the measured frequencies after absorption m d fluorescence, rapestirely. Planck'r coastant is h.
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Figure 2. Features in photoelectronspectroscopy, X-ray obwrption spechobcopy, X-ray Rvorasmse spectroscopy, and Auger electron rp&orcopy. Tho pobitiont of the photoeiectmn peaks and X-ray absorption edge are coincident, but the peak positions in the other rpedro ere only relotivdy correlated xnh respect to a e another.
spectroscopic methods. The intensity of the X-ray beam is attenuated by passing through the sample leading to X-ray absorption spectroscopy. By measuring absorption as a function of the wavelength of the Xrays, the absorption spectrum is obtained. Absorption edges occur in the spectrum of those wavelengths where the incident X-ray beam has just a sufficient amount of energy to excite an electron from a particular energy level to infinity. For a metal, infinity is taken to be just above the Fermi level. For an insulator, infinity is taken to be a t the bottom of the conduction hand. The positions of the absorption edges will then reveal the electronic energy structure of an atom. Figure 2 illustrates the Structural features of photoelectron spectroscopy and X-ray absorption spectroscopy. The bottom of Figure 1 shows the two relaxation processes which occur after an electron from a lower energy level has been promoted to infinity. When an electron from a higher energy level drops down to a lower energy level to fill a hole in an ionized atom, the energy may be emitted as an X-ray (X-ray fluorescence) or as an emitted electron with a characteristic Idnetic energy (Auger electron spectroscopy) (6-8). The relationship between atomic number and yield of Auger electrons and of X-ray fluorescence is illustrated in Figure 3 (9). Although the Auger process was neglected for years, this radiationless process is the dominant relaxation mechanism for the light elements. Recogniziig this, Harris (6-8) and others have recently begun to develop Auger electron spectroscopy. Harris has listed these possible subjects for an Auger investigation in solids: diiusion of materials, adsorption and desorption, monolayers, and transfer processes. The alternative relaxation process leads to X-ray fluorescence (9) which is capable of yielding information about the energy level separations. It is apparent that the radiation-producing spectrescopies and electron spectroscopy can inherently give the same information. However, electron qpectroscopy has some advantages. The relative1 narrow electron peak may he more precisely locate than the X-ray absorption edges. This allows a more precise determination of "chemical shifts" in the electron b i n d i i energies due to changes in the electronic environment of the atom. The narrower linewidth also provides an advantage over X-ray fluorescence lines for establishing peak positions. Variation of the excitation method using vacuum
ultraviolet photons as the energy source has analogom spectroswpies. W-optical absorption, fluorescence, and phosphorescence spectroswpies are well known. Ultraviolet photoelectron spectroscopy (UPS) uses a source of lower energy than X-ray photoelectron spee tmscopy, so only the valence electrons are ejected and detected. The biding energies of these electrons are measured and thus give a picture of the molecular orbital (MO) energy levels in the molecule. The linewidth in the photoelectron spectrum is limited by the linewidth of the exciting radiation which increases as the frequency cubed. This means that the excitation souroe in UPS has a much lower natural linewidth than the excitation source in XPS and, consequently, UPS is capable of much better peak resolution than XPS (0.01 eV versus 0.5 eV). This improved resolution results in fine structure often b e i i observed in ~ P S spectra arising from the d8erent vibrational and rotational energy levels. In practice the situation is usually complicated because not all of these peaks are resolved. If the ejected electron originated from a weakly bonding or antibondii orbital, the fine structure can usually be resolved. But, in the case of strongly bonding or antibonding orbitals, the fine structure is lacking and only broad lines are observed. A detailed analysis will distinguish between b o n d i i and antibonding orbitals. Electrons ejected from nonbonding orbitals are c h m acterized by single sharp lines. UPS spectra can be complicated further by the effects of spin-orbit coupling for heavier atoms and by J h - T e l l e r distortion effects in highly symmetric molecules. Analysis of UPS spectra of simple molecules and atoms is no easy task; detailed analysis of spectra from moderately complioated molecules is formidable indeed. Cakulofion of Binding Energies from Elecbon Specfra
In photoelectron spectroscopy the kinetic energy of the electron emitted upon irradiation of a sample is the observable. The following discussion will refer to X-ray excitation, but the results are also applicable to ultraviolet photoelectron spectroscopy. I n the case of X-ray irradiation, the kinetic energy Ex will be determined by the incident X-ray photon energy hv, the recoil energy ER,and the binding energy EB of the electron in the element E, = h.
- Ex - Es
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The recoil energy accounts for the distribution of the
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Figure 3. X-ray Rvorescense yield and Auger electron yield in the K shell as a function of atomic number. Taken from reference (2).
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b t i e energy &ween the emitted de&m and We atarn. By applying the lsw of amwvatiw of momentum, it %as s h w that Ea iflmgligibla in inowes Ea will ire n&e&ed hwe. The bidiag chew is a d y the gar am&^ ef int%mt. With sob& id ismest eonvdwit to reference the b f with reg@ to the. Fend lewd @, 10). W e r 'to F&m 4 for the b U e ~ ~ The wpgy lev& for tBe w p k mat& are bn $he left si&ed the Egum, a d %heenew b w d for the speetrometar m a M d ara rm the right side, T h fillunt
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chlorine in some inorganic compounds (8). Similar correlations have been reported for sulfur (11, id), nitrogen (13-16), iodine (lo), europium (lo), beryllium (81, copper (I@, iron (17), boron (IS), chromium (IS), and carbon (19). Energy levels of the valence electrons are also influenced by the electronic environment. The position of lines in UPS spectra are subject to a chemical sbift caused by diierent substituent groups. Dewar and Worley (80) have obtained the photoelectron spectra of 67 organic compounds, which include several fnnctiom1 groups. The results were compared with semiempirical SCF MO calculations (MINDO) and discussed in terms of different classes of compounds. Models for Interpretation of Chemical Shifts
Theoretical explanations for chemical shifts range from consideration of a simple charged sphere model to quantum mechanical calculations. A qualitative explanation of the origin of the chemical shift in atomic core electron biding energies is provided by the charged sphere model (8, 10). This model shows the effect of change in oxidation state. If the valence orbitals are assumed to be a rigid charged sphere of radius r and q electronic charges are moved from the valence orbital to infinity, the decrease in potential energy of the inner electrons ( i atomic units) is given by
The biding energy will then be increased by the same amount. The free ion shifts may also be calculated using self-consistent field (SCF) theories. Variations of the Hartree-Fock SCF method for determination of chemical shifts have given some interesting qualitative comparisons with experimental results ($?,lo, 81). These free ion calculations, in agreement with experimental results, have indicated that the photoelectron peaks of all core levels will be shifted approximately the same amount when the oxidation state is increased (10). General trends of decreasing shift with increasing atomic number within a group of the periodic chart and increasing sbift from left to right along a row of the periodic chart were also found. The use of these free ion models is not too realistic. The free ion calculations predict a shiit which is larger than the experimental shift. In fact, electrons are not transferred to infinity but to the nearest neighbors of the atom in question. A correction for this may be approximated by use of the Madelung constant a. The modified charged sphere model (eqn. (3)) becomes
where R is the distance from the atom in question to its nearest neighbor. Charged sphere and Hartree-Fock free ion calculations have been incorporated with an energy cycle calculation to give better agreement with experimental results ($,lo, 81). The energy cycle is used to account for crystal field effects and gives a better correction than the maximum Madelung energy contribution assumed in eqn. (4). In general, the average crystal
field contribution will be between 0.5 m d 1.0 times the maximum Madelung energy contribution. Another semiempirical approach is applicable in a few cases (88). This approach allows comparison of binding energy shifts of diierent molecules by comparison of thermodynamic data for certain reactions. Predicted binding energy shifts withii 1 eV of the experimental value are obtained, but the method suffers from the limited amount of thermodynamic data available. One more empirical method for predicting chemicd shifts of core electron binding energies is to use experimental binding energies (93). Good predictions are obtained if contributions to the shift from various chemical groups are added. The contributions due to the different chemical groups are determined experimentally by obtaining spectra of several compounds containing these groups. Splitting o f X-Ray Photoeledron Lines
The p,/, photoelectron l i e s of some heavy atoms have been found to split. Novakov and Hollander (84) found that the 5p./, electron spectral lines of Th metal, U metal, and UOa were split. The 4.fs/, and 4f.1,spectral lines of the same samples did not split. Splitting of the 5~21,line for Au in some gold compounds was found to be linearly related to the quadmpole splittings obtained from the Mossbauer spectra of the compounds (86). Quadrupole splitting is a function of the symmetry of the electric field around the nucleus of the atom in question. These results imply that X-ray photoelectron spectroscopy may also yield information about the symmetry of the electronic environment around an atom. Splittings have also been observed with paramagnetic molecules and transition metal ions (86 87). The reason for splitting in paramagnetic species is that in systems with unpaired electrons the exchange interaction affects core electrons with a (spin up) and B (spin down) spins diierently. A brief description of the equipment used in photoelectron spectroscopy is presented in this section. Greater detail is provided in some of the references (8, 8830). The basic components of the X-ray photoelectron spectrometer are shown in Figure 7. The ultraviolet photoelectron spectrometer will be the same with the exception of the incident energy source. Aluminum, magnesium, copper, and chromium are commonly used as anodes in the X-ray tubes. X-ray
Figure 7.
Basic components of X-ray photoeledron spectrometer.
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eter due to requirements of the spectrometer geometry and non-magnetic materials in construction. The X-rays produced at the anode pass through the beryllium or aluminum window of the X-ray tube and into the sample chamber. X-rays impinging upon solid samples cover a few millimeters squared in sample area thus providing an averaged sampling of the specimen surface. Solid samples are conveniently studied as a crushed powder adhering to conducting tape or as a flat plate or wafer. Gases are studied a t low pressure (. Hmm~&x.S.. NeuDzQTB, C., sari 81008*HN,K., Phyk I d , 9, %W
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Solutions
To date, liquids at roam temperature have been studied by vaporizing or free&& This method has now been used with solutions of me(CN)s, K$e(CN)@ and NaCl (38). X-ray photoelectron apeotra of fmsen solutions were obtained. It was observed that the binding energie~of the Fe 3p electrons of We(CN)s and me(CNJ6 in solution diffe~by about 0.2 eV from the corresponding wlid samples,
Some of the inherent limitations of photoelectmn spectroscopy are implied in previous sectiom. Not d i s m d was the problem of linewidth in photoelectron spectra, whiCh is governed by the width of the irradiating line, spedrometer broadening, and natural energy width of the electron level being studied. The first two parameters may be judiciody controlled to a certain extent. Broad lines mean the binding energy slrifts for some chemical statas of interest may be small compared with the linewidth. For example, if an atom iapemnt in two oxidation states in a sample, the electron peak fop one state might appear as a shoulder on the second peak. If the lines are broad, reaolu718
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A M h m r . D. A., S~(ancs,157, 1671 (1967). (22) J o ~ Y , W. L.. an.~ I U D L W D~.N ,.. J. Am?. CIrom.dm.,92,1863 0870). (z?) Jomx, W. L.,J. A m . C h 5w.. 92, !35!60 (1870). (24) NoVd~ov,T., iYim HOLIIMDDB, J. M.. Phm. Bb)l. L*, 21. 1133
(1968). T., aw H o m m , J. M.. Bun. Am. Pfrys. &a,, 14, Sal (as) N~YAXOT, tieas). (38) HBDWN. J., H6rimrr. P. F.. Noun~mo,C., AND 6meaaaa. K.,P k w LO&, zsA, 178 (isas).
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