Analytical Aspects of Auger Electron Spectrometry of Gases Michael Thompson”’, Paul A. Hewltt, and David S. Woollscroft Department of Chemistry, University of Technology,Loughborough LE1 1 3TU. U.K.
The Auger current obtained from a gas-phase Auger spectrometer has been studied with respect to parameters such as incident electron beam energy and gas pressure. The resolutlon attainable was investigated using argon. Several spin-orbit components of 3Ptransitions of argon were observed. Molecular Auger spectra of acetylene, ethylene, hydrogen sulfide, sulfur dioxide, and ammonia have been tentatively assigned using two empirical calculations of Auger energies. One method Involved use of ionization potentials from UPS and XPS (ultraviolet and x-ray photoelectronspectroscopy) and an estlmate of molecular relaxation energy, and the other an equation which calculates this energy. The potential of Auger spectrometry of gases for the analysis of the spectra of surface adsorbed species is discussed.
Because of its special blend of properties, electron excitation Auger spectrometry (AES) has become established as an important technique for surface analysis. AES offers high spatial resolution and surface sensitivity, fast response, and the ability to give information concerning the chemical state of surface elements. Its range of application has grown to include the fields of metallurgy, corrosion, catalysis, thin-film components, and crystal growth. Despite this interest, relatively little attention has been paid to the analytical potential of the Auger effect in the gas phase. With the latter, it is not necessary to operate the spectrometer in a differential mode. Additionally, solid state broadening problems are avoided ( I ) . Previous studies of atoms and free molecules using electron excitation have been mostly concerned with the noble gases (2-8), some small molecules such as Nz (7,9-12), 0 2 , NO, CO, COz, and H2O (6, 10, I I ) , C302 ( 1 3 ) ,fluorinated (10) and brominated methanes (14). Other papers have referred to germanium ( 1 5 ) and organochlorine compounds (16). Also, theoretical analyses of Auger spectra of hydrogen fluoride ( I 7) and methane ( 1 8 )have been carried out. The present paper is concerned with the potential of electron excited Auger spectrometry of gases in analytical chemistry. From a qualitative point of view, the theory of the Auger effect indicates that the technique can yield an elemental analysis for all elements with the exception of hydrogen and helium. Furthermore, in the gas-phase, it is possible to observe structure associated with valence levels in molecules. Therefore, the valence spectrum observed within the Auger-elemental region can convey information concerning molecular structure. In a sense, the technique is a combination of ultraviolet and x-ray photoelectron spectroscopy. AES has the potential for high sensitivity in the gas-phase due to the high signal generating power of electron excitation. For example, there is an enormous difference between the Auger yield obtained by electron rather than x-ray ionization due to the available flux in the former. Additionally, the inner-shell cross sections for ionization of levels of binding energy less than 500 eV are higher for electron than x-ray excitation. However, despite the greater currents obtained with the former than the latter, the higher secondary electron Present address, Department of Chemistry, University of Toronto,
80 St. George St., Toronto, Ontario, M5S 1A1, Canada. 1336
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ANALYTICAL CHEMISTRY, VOL. 48, NO. 9, AUGUST 1976
background may be a disadvantage, although this is not as serious as in solid analysis. Finally, gas-phase AES offers an opportunity to understand problems associated with Auger chemical shift, ionization cross section, and quantification which are very important in surface analytical chemistry. Moreover, correlation of gasphase results with spectra of gases adsorbed a t surfacesis a distinct possibility. Results in this area may prove useful in the understanding of the nature of bonding of surface adsorbed species. In the present paper, some of the analytical aspects of gas-phase AES outlined above are evaluated using a purpose-built high resolution spectrometer.
EXPERIMENTAL Gases. The gases studied in this work (British Oxygen Co. Ltd.) were of reagent purity and used without further purification. Auger Spectrometry. A schematic diagram of the gas-phase Auger spectrometer (AFMZ Vacuum Generators Co. Ltd., East Grinstead, U.K.) used in this work is shown in Figure 1.The instrument is comprised of an electron gun, bridge piece, gas introduction system, electron energy analyzer, electron multiplier, and pumping equipment. The LEG 32 electron gun which is fitted with an etched thoriated tungsten filament operates at up to 5-keV beam energy and has a controllable emission current and spot size of approximately 15fim. The electron beam passes through a differential pumping system made between the bridge and the gun to allow the latter to operate at a low pressure Torr) while the bridge is at high pressure. A quadrupole arrangement on the gun allows the beam to be centralized through the pumping tubes to the bridge. The bridge piece is situated at the entrance side of the analyzer and contains a gas-inlet block and vertical collision tube. Electrons arising from electron-gas collisions pass out of the tube, through a variable entrance slit, into a 150° hemispherical analyzer which is fitted with an electron multiplier. Additionally, the electrons are retarded to a specific energy prior to analysis and counting. The general mode of operation was to allow gas to leak from the introduction system to the collision tube maintaining an appropriate pressure of Torr in the differential pumping line (with the present instrument design, it was not possible to measure the pressure in the collision area). The electron gun was set with respect to beam energy, filament current, and focusing characteristics to achieve maximum signal-to-background ratio for the most intense peak in the spectrum. The emission current was usually 50 fiA. Auger electron energies were calibrated against the 2p3/23p3p (lD2) [see Nomenclature, Results and Discussion] line of argon at 203.47 eV (19). Experiments on resolution, sensitivity, Auger count, and pressure dependence as well as an examination of the spectra of “3, SOz, HzS, ethylene, and acetylene were carried out.
RESULTS AND DISCUSSION Auger Nomenclature. The Auger electron obtained by ionization of an inner-shell X followed by an electron falling from level Y to fill the hole and release of Auger electron from level Z is normally designated an XYZ electron. For example, a KLlL2.3 Auger electron is one derived from initial ionization of a Is electron; the “down” electron leaves a hole in a 2s orbital and the Auger electron is released from non-split levels 2p1/2 and 2 ~ 3 1 2 For . consistency with UPS and XPS, notation from these two techniques is used with the same overall format, for example, we may label general Auger regions as ls2n2m (n and m representing orbitals) and more specifically transitions as 2p1/23p3p, ls2pub2pubor with symmetry species 2~1/21bilbi.
Ar
Counts Der second (arbitrary1
T
2A
’P
’
8
190.7
190.4
an
191.3
I
1916
192.2
191.9
AI
192.L
192.7
193,3
1936
193.9
19L.2
2ppPi’Pi
Figure 1. Schematic of AFMP gas-phase Auger spectrometer
!Ob4
204.7
2050
205.6
2059
206.2
(A) electron gun power source, (B) base of electron gun, (C)gun filament system, (D)focusing anodes, (E) quadrupoles, (F) variable inlet and slits, (G) target chamber, (H) gas introduction probe, (I)electron multiplier, (J) diffusion pumps, (K) sample rough pumping, (L) backing pump, (M)recorder, and (N) hemispherical analyzer
Resolution. The resolution attainable by the AFM2 instrument is illustrated in Figure 2. In several of the peaks shown which have widths a t half height of -0.2 eV, the spinorbit components of several 3P transitions of argon are observable. These spectra were recorded using 0.5-mm slit width and 10-eV analyzer energy. Sensitivity and Quantitative Aspects. The relationship of Auger current ( I A )for a particular transition XYZ to gas pressure ( P )can be expressed in a similar manner to that used for surface work (20):
-
-
I A = I , G P . r(E,,P) WE,,Ex)
-
YXYZ
S(Exyz,P) (1)
where I , = exciting beam current, G = geometrical factor dependent on the efficiency of the spectrometer in collection of Auger electrons and on the angular distribution of the electrons, E , = exciting beam primary energy, r(E,,P) = Auger transitions caused by scattered electrons, @(E,,Ex) = cross section for ionization of level X of binding energy E x by a n electron of energy E,, y x y z = probability of relaxation by the Auger transition XYZ following ionization of X, S = probability of Auger electron of energy Exyz escaping without an inelastic gas-electron collision. If we are concerned with the Auger count actually detected by the instrument, then factor G will be affected by the luminosity of the energy analysis system. As usual, this is in turn related to the resolution of the analyzer, viz. higher resolution can be achieved by smaller analyzer inlet slits and lower accepted analyzer energy at the cost of Auger count rate. Also contained in G is a geometric factor concerned with the fraction of Auger electrons collected from the target chamber of the inlet slits. The potential of AES for sensitive gas analysis lies in the parameter I,. High electron fluxes can be obtained with commercial electron guns: in the present work, an emission current of 50 PA was used. The nature of the electron gun filament and manner of interaction of the electron beam with the gas are highly critical. Highest count rates were obtained with the etched thoriated tungsten filament. Spreading of the incident beam due to a poor filament or incorrect collimating
Electron energy ieV)
Flgure 2. Auger 3P peaks of argon measured with 0.5-mm inlet slit and 10-eV analyzer energy
and focusing characteristics causes scattering and, therefore, reduced signal-to-background ratio. As an indication of the sensitivity of the technique under the optimum conditions used in this work, the following peak-background count rates were obtained for the most intense transitions in selected spectra (1-mm slits and 20-eV analyzer energy)-argon 30 OOO counts/s, H2S 5000 counts/s, CzHz 1000 counts/s, and NH3 1000 count&. Under similar conditions, 1000 countsh, were obtained for argon present in air. From a quantitative point of view, a typical variation of signal-to-backgroundwith differential pumping line pressure is shown in Figure 3. These curves including the variation with primary beam energy are exhibited by the argon 2p3/23p3p transition. The importance of the parameter S is shown by the scattering of Auger electrons occuring increasingly after a pressure of 2-3 X Torr is reached. Clearly, it will be possible to calibrate the instrument in future in terms of Auger count rate and concentration. Molecular Spectra and Qualitative Aspects. The spectra of the molecules in their particular elemental Auger regions shown in Figure 4-6 are highly characteristic. Therefore, the technique has considerable potential in terms of “fingerprint” qualitative identification. Furthermore, as outlined earlier, the method is capable of imparting both elemental and structural information. The nature of the latter can be evaluated through assignment of spectra of simpler molecules. Interpretation of the spectra is achieved by empirical calculations of possible Auger transition energies and comparison of these values with experimental peak energies. Two approaches are used in this paper both based on the well-known equation: where E x y z is the calculated Auger energy, Ex is the energy ANALYTICAL CHEMISTRY, VOL. 48, NO. 9, AUGUST 1976
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Counts par second
signal to
9000-
background ratio
H,S, S region
8000.
'O'Ol
h
9 0-
8 0-
moo6000-
Eo=L 8 KeV
70-
5000.
6 0-
5 0-
4'oi 131
20
132
133
136
135
134
137
II 139 1LO 141 Electron energy lev1
138
E o . 3 2KeV E o 1 2 JKeV
Figure 5. Auger spectrum of hydrogen sulfide in sulfur,2p3m3n region
NH,, N Auger Pressure (torr)
Figure 3. Auger signal-twbackground ratio for the argon 2 ~ 3 1 2 3 ~ ('02) 3~ transition as a function of differential line pumping pressure and incident beam energy.
b 365
2CQ
220
240
260
3i5
si5
345
3i5
3;s
I
3b5
3%
,
305 395 Electron energy
lev1
280
Figure 6. Auger spectrum of ammonia in nitrogen ls2m2n region
Table I. Assignment of Carbon ls2m2n Auger Transitions of Acetylene Experimental, eV
2 w
220 240 ELECTRON
260 ENERGY (eV).
280
Figure 4. Auger spectra of acetylene and ethylene in carbon ls2m2n
region
of the level where the initial vacancy occurs, and E y and E2 are the energies of the levels from where the "down" and Auger electrons originate, respectively. This equation does not take into account the shifts in bonding energy that occur as a result of the ionization (termed relaxation in this paper). In the first argument used here (A), an estimate of the relaxation energy is obtained by subtraction of the experimental Auger energy from that calculated by Equation 2 for the highest energy peak in a spectrum. Remaining values from all possible combinations of the loss of two electrons are computed with this relaxation energy. The method makes the assumption that all the transitions possible in a given case have constant relaxation energy. This is a patently crude assumption, since one would not expect the same coulombic redistribution for the variety of transitions. 1338
ANALYTICAL CHEMISTRY, VOL. 48, NO. 9, AUGUST 1976
Estimated, eV
Relative intensity
Assignment b
256.9a 256.9 1.00 2pnb 2pnb 252.3 252.0 0.8 2pnb 2pob 250.6 249.9 0.8 2pnb 2sU* 247.3 247.1 0.5 2pub 2pUb 243.5 242.7 0.6 2sa* 2so* 241.9 239.9 0.7 2pob 2sU* 238.4 237.8 0.7 2s0* 2sUb 232.4 23 2.8 0.4 2sub 2sUb amis peak was used as the basis for the calculation of relaxation energy according to method A. b Since 1s is the level of initial vacancy in all these transitions only the
"down" and Auger levels are shown.
In the second procedure (B), Equation 2 is used in the modified form suggested by Schon (21):
E x y z b ) = E x k ) - ( E Y ( z )+ E z b
+ A))
(3)
In this equation z refers to the atomic number of the atom involved in the Auger transition and therefore E z is estimated from the energy of an appropriate level in an atom with atomic number t A. Furthermore, the procedure in this form does distinguish between E x v z = E x - E y - E Z and E X Y Z= E x - E Z - E Y , viz., the orbital origin of the Auger electron is specified. The equation has been used widely for atomic
+
Table 11. Assignment of Carbon ls2m2n Auger Transitions of Ethylene Experimental, eV
Estimated, eV
Relative intensity
Assignment b
Table IV. Calculation of Some 2p3m3n Sulfur Auger Energies of Hydrogen Sulfide by Method B Experimental,
Calculated,
eV
eV
257.90 257.9 0.6 Ib3u Ib,u 253.3 253.5 0.8 1b3, Ib3g 249.8 250.0 1.0 3ag 3ag 245.7 245.6 0.8 3ag 2b,u 241.5 241.2 0.7 2blU 2blU 237.4 236.7 0.5 2bl" 2ag UReference peak for method A calculation. b Only "down" and Auger levels shown.
139.78 148.2 138.55 146.9 137.6 145.6 136.0 144.2 133.9 142.1 132.7 140.5 125.8 133.7 a By equivalence with the equation E,y,(H,S) Ey(H,S) - EZ(HC1).
Table 111. Assignment of 2p3m3n Sulfur Auger Region of Hydrogen Sulfide by Method A
Table V. Auger Energies of 2p3m3n Sulfur and ls2m2n Oxygen Regions of Sulfur Dioxide
Experimental, eV
Estimated, eV
Relative Intensity
Assignmentb
0.4 139.78 139.780 0.2 139.5 139.54 0.1 139.25 139.1 1.0 138.55 138.5 0.4 138.25 138.2 0.1 137.9 137.99 0.2 137.6 137.0 0.1 135.8 136.0 134.9 135.3 133.1 133.9 0.3 132.7 132.0 0.1 131.1 130.9 0.1 127.4 127.4 0.1 126.1 125.8 a Reference peak. b First six assignments involve vibrational levels in H,SZ+. species (solid- and gas-phase) but it becomes more difficult t o use with molecules. An interesting example of an application to a molecular problem was the study by Kawai et al. (22) of ammonia adsorbed t o a molybdenum surface. Both the above methods require values for ionization potentials of the molecules studied in this paper and certain others. The appropriate values were obtained from references 7, 23-29. Acetylene. The characteristic carbon 1s2m2n Auger region of acetylene (Figure 4) exhibits three main bands, the two a t lower energy clearly consisting of several superimposed components. These peaks are partially resolved under higher resolution (not shown). A further feature of the spectrum is the presence of autoionization peaks to the high energy side of the peak a t 256.9 eV. Such transitions are common in molecular Auger spectrometry (12). The acetylene molecule has symmetry Dah and ground state configuration:
=
E s Z p-
Sulfur Auger Relative eV intensity
Experimental, eV
Estimated,
139.30 138.4 137.1 136.0 133.1 131.8 130.7 129.2 126.7
139.3 138.4 137.2 136.5 133.1 132.0 130.7 129.1 126.7
0.2 0.5 0.9 0.7 1.0
Oxygen Auger
505.la 505.1 0.7 504.4 0.6 5 04.4 503.7 0.6 503.0 501.1 0.8 501.5 500.3 1.0 499.9 497.9 0.7 498.1 496.3 496.3 0.5 493.5 493.9 491.0 488.8 0.2 a Reference peak. b For oxygen only "down" and Auger levels shown. CKey - x = 3b1 or la,, y = 3a, or l b , or 2b1. Table VI. Interpretation of ls2m2n Nitrogen Auger Region of Ammonia Experimental, eV
Calculated B, eV
Estimated A,
eV
Assignment b
377.8U 382.1 377.8 3a1 3a1 372.0 376.3 37 2.9 3a1 l e 368.4 368.0 le le 371.4 362.0 365.5 361.2 2a1 3a, 358.8 362.5 361.2 3a, 2a, 354.2 359.7 356.3 2a, l e 343.4 345.8 344.5 2a1 2a1 a Reference peak. b Only "down" and Auger levels shown.
( l s a b ) ~ ( l s ~ * ) 2 ( 2 s a h ) 2 ( 2 s ~ * ) 2 ( 2 p a b ) 2 ( 2 p s b1Zg+ )4
To analyze the spectrum, method A was used. The relatively sharp peak at 256.9 eV is assigned to the ls2pah2psb transition. Calculation of the appropriate energy from UPS and XPS data and Equation 2 gives a value of 268.4 eV. Thus, the relaxation energy is arbitrarily assumed to be 11.5 eV, and this value is used t o compute remaining possible Auger energies. T h e fit of calculated and experimental values for tentative assignments is given in Table I together with an indication of relative peak intensity. Ethylene. The ethylene spectrum (Figure 4) exhibits six clearly defined peaks in the usual ls2m2n carbon region. Autoionization peaks are observed t o the high energy side of the band at 257.9 eV. The ethylene molecule has CzVsymmetry and ground state configuration:
Suggested peak assignments calculated by method A are given in Table I1 (relaxation energy 11.0 eV). The ethylene spectrum provides an interesting opportunity t o contemplate possible applications in the area of catalysis. Since the main gas-phase peaks are separated by approximately 4 eV, spectra of surface adsorbed ethylene may prove fruitful. In this respect, the authors have been able to compare the differentiated gas spectrum with a spectrum of a similar nature from ethylene adsorbed to a copper surface (30). Hydrogen Sulfide. The H2S molecule belongs to point group CpVand has ground state configuration: (la1)2(lb2)2(2a1)2(1b1)2
IA1
I t should, however, be emphasized a t this stage that any folANALYTICAL CHEMISTRY, VOL. 48, NO. 9, AUGUST 1976
1339
lowing discussion involving orbital symmetry terms must be treated as being arbitrary since, on the loss of two electrons, the original molecular symmetry may be severely perturbed. The highly characteristic spectrum of HzS in the 2p3m3n sulfur region is shown in Figure 5. There are two sets of relatively sharp "triplets" to the high energy side of the spectrum and several other peaks are observed in the lower energy region on higher resolution (not shown). The two triplets clearly correspond to the 2p1/2,2p3/2 splitting of the sulfur 2p level where the initial vacancy occurs. As expected, the set of peaks associated with the 2 ~ 3 1 2level is twice as intense as that for the 2p1/2 level. We believe that the peaks within the triplets can be assigned according to the vibrational levels of the HzS2+ ion. A "down" electron and Auger electron removal from H2S+ would result in a progression of three peaks as is observed in the spectrum. This result is consistent with the UPS spectrum of H2S which exhibits three vibrational peaks in its first band which is due to ionization from the l b l orbital. One of these is too weak to be observed readily. The type of vibrational structure observed here is similar to that discussed by Siegbahn (6) and Hurley (31) in relation to the final states of carbon monoxide. An assignment of the H2S spectrum using UPS (including vibrational data) and XPS results and procedure A is given in Table I11 (relaxation energy 10.8 eV). An attempt was made to describe the spectrum of H2S using method B. An estimate of the effect of relaxation energy through the E ( z A) factor, where in this case A = 1,was attempted using data from UPS spectra of HC1. The HC1 211 band is comparable with that derived from the H2S l b l orbital since both involve ionization from a lone-pair-like orbital. In a similar manner 2Pof HCl can be related to the 2al orbital of H2S as they are both due to ionization from H-S or H-C1 cr-bonds. Using the equation Exyz(HzS) = Eszp - Ey(H2S) - EZ(HCl), several Auger energies were calculated; these are shown together with experimental energies in Table IV. Clearly, in this case, the introduction of the E ( z 1)factor does not compensate for the shifts in levels, although the differences between values compare reasonably with the experimental data. Sulfur Dioxide. The sulfur dioxide molecule belongs to the CzVpoint group and has ground state configuration:
+
+
The complete Auger spectrum of SO2 (not shown) consists of two groups peaks, one set being associated with the ls2m2n oxygen region of 490-510 eV and the other with the 2p3m3n sulfur area of 130-140 eV. As one would expect, there are similarities between the two regions since some of the Auger transitions involved are derived from the same molecular orbitals. It is difficult to carry out any more than tentative assignments of the two areas since the peaks are less well-defined than in the other spectra considered here, and the sulfur region is complicated by the splitting of the 2p level. Moreover, the ionization potentials of several orbitals in SO2 as found by UPS are barely distinguishable. However, having made an attempt a t an assignment of the oxygen region by method A (relaxation energy 9.5 eV), the sulfur region (relaxation energy 14.5 eV) can be described by comparison. For example, certain oxygen and sulfur energies have very similar separations, viz., 504.4-136.0=368.4,501.5- 133.1 =368.4,and499.9-131.8 = 368.1. Suggestions for interpretation of the spectra are given in Table V. Ammonia. The ammonia molecule belongs to point group CsVand has ground state configuration (la1)2(2a1)2(le)4(3a1)2. The spectrum of the ls2m2n nitrogen region of ammonia
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ANALYTICAL CHEMISTRY, VOL. 48, NO. 9, AUGUST 1976
(Figure 6) clearly exhibits more than the six bands predicted by method A (ls3a13a1, ls3alle, lslele, ls2a~3a1,Is2alle and l s l a l l a l ) . This result emphasizes the difficulty of such an empirical method since it does not take into account the distinction between, say, ls2alle and lsle2al transitions. However, as pointed out previously, procedure B does predict this distinction and, therefore, it was used to interpret the spectrum of ammonia. For this purpose, the 2a1, l e , and 3al ionization potentials of ammonia were equated with those of the l a l , lbz and l b l orbitals of water, respectively. The experimental and procedure A (relaxation energy 6.0 eV) and B energies are given in Table VI together with appropriate assignments. Again it is worth noting that the E ( z 1) factor does not appear to compensate adequately for the shift in levels undergoing an Auger transition. Gas Mixtures. Mixtures of simple gas molecules can be identified with facility, particularly where the components contain different elements. For example, carbon, nitrogen, oxygen, and argon transitions are detected for air in a wide scan of approximately 100-600 eV. As expected, the molecular spectra can be obtained in 30-eV scans. Where complex mixtures are involved, prior separation by gas-liquid chromatography is envisaged. The interfacing of the spectrometer with GLC equipment will present no serious difficulty since it may not be necessary to remove carrier gas. Also, molecular separators similar to those used in conventional GLC-mass spectrometry are appropriate. Work in this area is in progress.
+
LITERATURE CITED (1)J. D. Nuttall and T. E. Gallon, Solid State Commun., 15, 329 (1974). (2)W. Mehlhorn, 2. Phys., 187, 21 (1965). (3) D. Stalherm, M S c . Thesis, University of Munster, 1968. (4)W. Mehlhorn, 2. Phys., 208, l(1968). (5)W. Mehlorn and D. Stalherm, 2.Phys., 217, 294 (1968). (6)K. Siegbahn, C. Nordling, G. Johansson, J. Hedman, P. F. Heden, K. Hamrin, U. Gelius, T. Bergmark, L. 0. Werme, R. Manne, and Y. Baer, "ESCA Applied to Free Molecules", North-Holland, Amsterdam, 1969. (7)K. Siegbahn, C. Nordling, A. Fahlman, R. Nordberg, K. Hamrin, J. Hedman, G, Johansson, T. Bergmark, S. E. Karlsson, I. Lindgren and E. Lindberg, "ESCA-Atomic, Molecular and Solid State Structure Studied by Means of Electron Spectroscopy", Nova Acta Regiae SOC. Sci. Up., Ser. IV, No. 20
(1967). (8) J. D. Nuttall and T. E. Gallon, Phys. Status Solidi, B, 71, 259 (1975). (9)D. Staiherm. B. Cleff, H. Hillig. and W. Mehlhorn, 2. Naturforsch.. A, 24,
1728 (1969). (IO)W. E. Moddernan. Ph.D. Thesis, University of Tennessee, Knoxville, 1970. (11) T. A. Carlson, W. E. Moddeman, E. P. Pullen, and M. 0. Krause, Chem. Phys. Lett., 5, 390 (1970). (12)W. E. Moddeman, T. A. Carlson, M. 0. Krause, E. P. Pullen, W. E. Bull, and G. K. Schweitzer, J. Chem. Phys., 55, 2317 (1971). (13) L. Karlsson, L. 0. Werme, T. Bergmark, and K. Siegbahn, J. Electron Spectrosc. Relat. Phenom., 3, 181 (1974). (14)R. Spohr, T. Bergmark, N. Magnusson, L. 0. Werme, C. Nordling, and K. Siegbahn, Phys. Scr., 2, 31 (1970). (15) W. 8.Perryand W. L. Jolly, Chem. Phys. Lett., 23,529(1973). (16) F. Meyer and J. J. Vrakking, Phys. Lett. A, 44, 511 (1973). (17)R. W. Shaw and T. D. Thomas, Phys. Rev. A, 11, 1491 (1975). (18)I. E. Ortenburger and P. S. Bagus, Phys. Rev. A, 11, 1501 (1975). (19) L. 0. Werme, T. Bergmark, and K. Siegbahn, Phys. Scr.. 18, 149 (1973). (20)J. C. Riviere, Confemp. Phys., 14, 543 (1973). (21) G. Schon, J. Electron. Spectroc. Relat. Phenom., 1, 377 (1972173). (22) T. Kawai, K. Kunirnori, T. Kondow, T. Onishi, and K. Tamaru, Phys. Rev. Lett., 33,533 (1974). (23)C. Baker and D. W. Turner, Proc. R. SOC.London, Ser. A, 308, 19 (1968). (24)T. D. Thomas, J. Chem. Phys., 52, 1373 (1970). (25)J. E. Collin and J. Delwiche. Can. J. Chem., 45, 1883 (1967). (26)P. Mollere, H. Bock, G. Beker, and G. Fritz, J. Organomet. Chem., 46, 89 (1972). (27) D. W. Turner, C. Baker, A. D. Baker, and C. R. Brundle, "Molecular Photoelectron S ectroscopy", Wiley-lnterscience, London, 1969. (28)C.Fridh, L. Isbrink, and E. Lindholm, Chem. Phys. Lett., 15, 282 (1972). (29) D. R. Lloyd and P. J. Roberts, Mol. Phys., 26, 226 (1973). (30)M. Thompson, H. N. Southworth, P. A. Hewitt, and D. Hall, unpublished work. (31)A. C. Hurley, J. Chem. Phys., 54, 3656 (1971).
RECEIVEDfor review February 2, 1976. Accepted April 13, 1976. We are grateful to the Science Research Council and Loughborough University of Technology for the provision of funds for the purchase of the spectrometer. Also, we are indebted to the S.R.C. for grants to two of us (PAH and DSW).
