same magnitude). Furnace Design IT, which had a larger heated surface area and allowed a longer residence time for the sample particles, was a more efficient atomizer than Design I. Because its operational temperature was lower than that of Design I and yet similar results were obtained, better heat transfer must have been occurring. However, Design I1 could not be operated above 1100 "C without producing a decrease in the fluorescence S/N ratio. Modifications are needed that will incorporate a n effective heat transfer without the adverse effects of the previously proposed viscous flows and radial sample injections. These changes are now being studied. Even so, the present platinum furnaces are effective atomizers for atomic fluorescence siudies of volatile elements. Advantages offered by this technique of atomization over previous nonflame methods are the simplicity of sampling, the precision of measurement, and the relative freedom from chemical interferences. With slight modifications of the furnace and the use of an ultrasonic nebulizer, this nonflame method should be applicable in the trace analysis metals in real samples such as biological fluids and jet lubricating oils.
cadmium without excess interferents, are given in Table IV for two different furnace conditions for both furnace designs. All the results indicate the interferents depressed the fluorescence signal, When the Design I furnace temperature is increased from 1350 OC to 1600 OC, the interference effects are decreased considerably; the fluorescence depression due t o the 100-fold excess interferents is almost completely removed. Also detection limits of all analytes studied with Design I furnace a t 1600 "C are about 20-fold lower than for the same furnace a t 1350 "C (see Table 111). The recoveries of 1 pg/ml C d with Design I1 furnace a t 1100 "C are approximately identical with those of Design I furnace a t 1600 O C (see Table IV). When the Design I1 furnace temperature was increased t o 1400 "C, the recoveries were increased further with a concomitant decrease in the S/N ratio for the 1 pg/ml C d solution. CONCLUSIONS Atomization of the sample is a function of heat transfer efficiency. This was illustrated when furnace Design 11, operated at 1100 O C , produced the same, and in some cases lower detection limits than obtained with furnace Design I operated a t 1600 O C (the interference effects were also of the
RECEIVED for review May 7, 1971. Accepted July 19, 1971. This work was supported by AF-AFOSR-70-1880B.
X-Ray Photoelectron Spectroscopy of Molybdenum Corn pounds Use of ESCA in Quantitative Analysis William E. Swartz, Jr., and David M. Hercules Department of Chemistry, Unicersiry of Georgia, Athens, Ga. 30601 The Mo (3d,2-3d512)electron binding energies have been measured as a function of oxidation state for a series of molybdenum compounds. A linear relationship is found to exist between the binding energies and oxidation state. The binding energy shift between the Mo(3d) electrons in Mooz and Moo3 was large enough (1.7 eV) to allow measurement of one oxide in the presence of the other. This permitted development of a quantitative analytical procedure for bulk analysis of MoOrMoOs mixtures. The resulting analysis shows a relative standard deviation of 2%.
X-RAY PHOTOELECTRON SPECTROSCOPY (ESCA) is a technique for determining the binding energies of core-electrons. ESCA measures the kinetic energies of electrons ejected from a molecule by a mono-energetic beam of X-radiation ( I ) . The binding energies of the core-electrons are dependent upon the oxidation state of the atom. Few ESCA data have been reported for the transition metals. Fadley et al. have studied multiplet splittings of the core-electron binding energies for several of the transition metals (2, 3). Fadley and Shirley have also studied the densities of states and coreelectron energy levels of Fe, Co, Ni, Cu, and Pt ( 4 ) . N o extensive investigation has been reported on a given transition (1) K. Seigbahn et d.. "ESCA 'Atomic, Molecular and Solid State Structure Studied by Means of Electron Spectroscopy,' " Almquist and Wiksells, Uppsala, 1967. (2) C. S . Fadley, D. A. Shirley, A. J. Freeman, F. S. Bagus, and J. V. Mallow, Phys. Reo. Lett., 23, (24), 1397 (1 969). (3) C. S. Fadley and D. A. Shirley, Phys. Ret. A, 2, 1109 (1970). (4) C. S . Fadley and D. A. Shirley, Phys. Rer. Lett., 21, (14), 980 (1968). 1774
metal although Cook et al. (5) have recently reported a detailed study of platinum compounds. We wish t o report an investigation of molybdenum (3d3/2-3dS,2) core-electron binding energies as a function of molybdenum oxidation state for ca. 20 compounds. ESCA has been shown to offer possibilities for quantitative chemical analysis. Siegbahn et al. ( I ) have reported some quantitative studies in which they were able to determine the C:Cl:S ratios in a number of amino acids and insulin. Siegbahn et a/. ( I ) have also analyzed brass samples containing zinc (10-50%), copper (50-90x), tin (ca. 0.7%), and lead (ca. 0 . 8 z ) . Kramer and Klein (6) have attempted quantitation of frozen solutions containing K3Fe(CNh, K4Fe(CN)a, and NaC1. For concentrated solutions (ca. l M ) , a reasonable calibration was demonstrated. The analysis of mixtures of MOOSand Moos has always been accomplished using time consuming wet chemical analyses. An instrumental technique has not yet been reported for such oxide mixtures, since no established technique has been able to distinguish one oxide from the other. We wish t o report development of an analytical technique which employs ESCA t o analyze mixtures of Moo3 and MoOZ. EXPERIMENTAL Apparatus, The electron spectra were obtained with a 30-cm, double focusing iron-free electron spectrometer of the __-
Y.Wan, U. Gelius, K. Hamrin, G. Johansson, E. Olsson, H. Siegbahn. C. Nordling, and K. Siegbahn, .I. Ameu.
( 5 ) C. D. Cook, K.
Chem. SOC.,93, 1604 (1971). (6) L. N. Kramei and M. P. Klein, J. Cliem. Phys., 51, 3620 (1969).
ANALYTICAL CHEMISTRY, VOL. 43, NO. 13, NOVEMBER 1 9 7 1
Table I. Mo(3d3/2-3djp) Binding Energies as a Function of Oxidation State Mo oxidation MO(3daip) Eb, MO(3dxz) Eb, state eV eV +6 $6 +6 +6 +6 +6 +6 +fl
+6 +6 +6 +6 +6 +6 +5
+4
+4 +6 0 0 0 0
235.6 i 0 . 1 235.2 i 0 . 2 235.2 i 0 . 2 235.3 i 0 . 1 234.2 rt 0 . 1 234.6 i 0 . 1 234.3 i 0 . 1 234.7 i 0 . 1 234.7 rir 0 . 1 234.3 i 0 . 1 234.2 i 0 . 1 234.7 i 0 . 2 234.7 i 0 . 1 234.3 i 0 . 1 234.5 i 0 . 2 233.9 i 0 . 1 233.8 i 0 . 1 233.8 i 0 . 3 233.7 i 0 . 1 233.8 i 0 . 1 229.6 i 0 . 1 229.7 i 0 . 2
232.5 =k 0 . 1 232.1 i 0 . 2 232.1 =k 0 . 1 232.2 i 0 . 1 231.1 z t 0 , l 231.6 =k 0 . 1 231.2 zt 0 . 1 231.1 f 0 . 1 231.6 i 0 . 1 231.1 i 0 . 1 231.1 f 0 . 2 231.5 i 0 . 2 231.5 f 0 . 1 231.2 i 0 . 1 231 . O i. 0 . I 230.9 + 0 . 1 230.8 i 0 . 1 230.8 i 0 . 1 230.6 i 0 . 1 230.4 i 0 . 1 226.1 i 0 . 2 226.6 i 0 . 2
Run on cryogenic probe. in the MoOz (C ,HiO& complex: however, acetylacetone is known to complex as the neutral species. This would make M o a + 4 species as the data indicate. Data obtained after one-month exposure to air. Data obtained on freshly cleaned foil.
* M o is formally +6
split coil solenoidal type. A detailed description of the apparatus has been previously reported (7). Reagents. All of the molybdenum compounds studied were obtained from the Climax Molybdenum Company. No further purifications were performed. Other metal oxides were obtained from Fisher Scientific Company. Procedure. The spectra were all obtained a t an instrumental resolution of 0.04 %. Instrumental resolution does not consider the natural widths of the exciting radiation o r the width of the core-electron level itself but is a n indication of only spectrometer aberration. The aluminum Kcul/2 X-ray line at 1486.6 eV was used for excitation in all cases. The X-ray power supply was run at 25 kV and 25 mA. The Bendix Channeltron Electron Multiplier detector was operated at 2800 volts. All samples except for the molybdenum foil were run as powders dusted onto a backing of doublebacked cellophane tape. Since the Mo(CO)& and MOO, (C,Hi02)? sublimed in vacuuo, it was necessary to employ a cryogenic probe to obtain electron spectra. The work function of the spectrometer was not determined since only relative binding energies were measured. In calculating the binding energies, the spectrometer work function was assumed to be 4.0 eV, a reasonable value for this type of spectrometer (I). The binding energies were measured relative t o the Mo(3dj ?) electrons in M o o 3 . The binding energies for the Mo(3da ?-3d5 ,) electrons were initially determined relative t o the P(2p) binding energy of 133.3 eV in sodium pyrophosphate Na4P?07,reported by Pelavin et a/. (8). In every case, the M o (3d3 2-3d5 .) binding energies were measured immediately after the Mo (3d5 2) energy in Moo3. All measurements were referenced to the C(1s) line. Data were obtained on several monomeric species in which molybdenum had formal f 6 , f4, and 0 oxidation states. (7) J . J. Jack and D. M. Hercules, ANAL.CHEW, 41, 729 (1971). (8) M. Pela\iii, D. N. Hendtickson. J. M. Hollander, and W. L. Jolly, J . Plivs. Chem., 74, 1 1 16 (1970).
2000
I
I \ t I
I
I
I
I
2.5500
I
I
I I 2.5600
SPECTROMETER CURRENT (AMPS) Figure 1. Molybdenum (3d3jz-3ds,2)electron spectrum for Moo3 Binding energies for only one molybdenum + 5 compound were determined. Several polymeric compounds in which molybdenum was present in the + 6 state were also studied. RESULTS AND DISCUSION
The binding energy data for the molybdenum compounds investigated are shown in Table I. The reported binding energies are the average of three replicate measurements, having the indicated standard deviations. A representative spectrum for the M o (3d3,2-3d5/2)electrons in MOO3 is shown in Figure 1. The separation between the electron lines is only 3.1 eV, yet they are almost completely resolved. The 3.1 eV separation between the M o (3d3/2) and M o (3dsiz) electrons is characteristic of all the compounds investigated in that the energy separation in all cases was 3.0 i 0.2 eV. Binding Energies as a Function of Oxidation State. As expected, the binding energies increase as the oxidation state of molybdenum becomes more positive. In addition, the general trend in binding energies as a function of oxidation state is identical for both the M o (3d312)and Mo (3dsjs) electrons. This can be seen from Figure 2 where the binding energies are plotted as a function of oxidation state. There is no theoretical reason t o expect the plot of binding energy cs. oxidation state t o be linear. The data obtained for molybdenum metal powder and foil are indicative of the sensitivity of the ESCA technique t o surface oxidation. The observed binding energies for the powdered M o sample (233.7 and 230.6 eV) match closely those of the + 4 oxide (233.9 and 230.9 eV) indicating significant surface oxidation. Normally, if oxidation has occurred o n the surface, one might expect t o find Moos (the most highly oxidized form of molybdenum) rather than the MoOz observed here. I n fact, no indication of the fully oxidized species appeared in the spectrum. This indicates that oxidation o n the surface of the molybdenum powder does not
ANALYTICAL CHEMISTRY, VOL. 43, NO. 13, NOVEMBER 1971
1775
237
1
3200
-
L
2000
-
v)
D
z
8
2400-
u
v)
% + z
-
\
v)
0
+2
+’
+5
+3
OXIDATION
+6
3
2000 -
0
STATE
0
Figure 2. Molybdenum (3d3,2-3d5,2) binding energies as a function of oxidation state __
MO(3d3d
_ - _ _ _MO(3dein) proceed significantly beyond the f 4 oxidation state. In addition, no signal corresponding to elemental molybdenum was observed. Since it has been previously shown that the ESCA technique normally samples only the first 20-100 or less of surface ( I ) , the oxide layer on the molybdenum powder must be at least 20-100 thick. The binding energies recorded for molybdenum foil depended upon pretreatment of the sample. If the surface was cleaned with emery paper just prior t o obtaining a spectrum, values of 229.6 eV and 226.1 eV were observed for the M o (3da12)and (3d512) electrons, respectively. These values agree well with the values reported for elemental molybdenum by Siegbahn et al. ( I ) and also with the measurements for M o (CO)6in Table I. O n such a sample only a very weak oxygen peak could be detected, probably from traces of the emery paper. However, when the Mo foil was allowed t o equilibrate with the air for one month, M o binding energies of 233.8 eV and 230.4 eV were observed for the (3d3,2)and (3d5,?)electrons, respectively. These agree very closely with the values observed for M o o s and M o powder, indicating that complete surface oxidation t o M o o n had occurred. O n this foil sample, no peaks corresponding to elemental M o could be detected, and a strong oxygen peak was observed. These results cannot be due to differences in charging effects between the M o foil, powder, and oxide, because the C(1s) electron peak was constant for all samples. Figure 3 is the M0(3d~,~-3ds,s)spectrum obtained for a commercially available sample of MoOz. It is obvious that surface oxidation to M o o 3 has occurred. The spectrum contains one rather well defined peak and both a low and high energy shoulder. The high energy shoulder corresponds to a binding energy of 230.9 eV and is assigned as the MO (3d512) electron line for MOO?. The low kinetic energy shoulder corresponds to a binding energy of 235.6 eV and is assigned t o the M o (3d3/2)electrons of MOO,. The central peak is a combination of the M o (3d5,J electrons of MOO, (EO= 232.5 eV) and the Mo(3dsj2)electrons of Moon (Ea = 233.9 eV). This is supported by the fact that the peak appears to be somewhat wider than the peak in Figure 1 assigned t o the MO(3dsiz)electrons in MOO,. The energy of the Mo(3d3,2) peak in M o o t was arrived at by adding 3.0 eV t o that of the Mo(3dSi2)binding energy. This is a valid operation since all of
A
A
1776
2.5500
2.5600
2.5700
SPECTROMETER CURRENT (AMPS) Figure 3. Molybdenum (3d3,2-3d5,2) electron spectrum for M o o 2 the Mo(3d3,2) and M 0 ( 3 d ~ , binding ~) energies in Table I were separated by 3.0 f 0.2 eV. The spectrum from MoC14 (C5HsN)2was identical t o that for MOO?,indicating that this compound also was subject to surface oxidation. All of the other M o ( f 6 ) compounds investigated yielded nicely resolved doublets comparable to that from M o o 3 . An attempt was made to remove the surface layer of MOO, from the M o o s . It is known that MOO, is soluble in basic solution while MOO? is not. Purification of Moor contaminated with Moos can be accomplished by treatment of the sample with base (9). The MOO* was treated with 2 M NaOH for varying lengths of time, by putting several grams of the Moon into 1 0 ml of 2 M NaOH. The solution was continuously stirred with a magnetic stirrer. The spectra obtained on the treated samples showed a gradual decrease of MOO, content with time. The spectrum in Figure 4 shows a n untreated sample and a sample that had been treated with base for approximately 100 hours. From the marked decrease in the intensity of the low kinetic energy (low spectrometer current) shoulder due t o the Mo (3d3,1) electrons in MOO3 and the central peak due t o a combination of the M o (3d3:2) electrons in M o o s and the M o (3d5,?)electrons in MoOa, it can be seen that the M o o 3 contamination of the surface of M o o t has been reduced. However, there must still be a considerable amount of MOO, present since the intensity ratio of the 3d3,?:3d5,2lines for pure Moo2 would be 2:3. The shoulder on the high kinetic energy (high spectrometer current) side of the spectrum is that due to the Mo(3dj,.) electrons of Moo?. (9) W. J. Tschudi, Climax Molybdenum Company, Greenwich, Conn., private communication.
ANALYTICAL CHEMISTRY, VOL. 43, NO. 13, NOVEMBER 1971
4600
290C
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3800
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I
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4
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ti
f
t
1 ( I I I ( I I I I I I
I 2.5700
2.5600
2.5500
SPECTROMETER
(AMPS)
(AMPS)
Figure 5. Molybdenum (3d3,2-3ds,2) electron spectrum for a 25 :75 (MOO3: MOO?)mixture
Figure 4. Mo (3dJ 2-3d5,2)electron spectra for untreated MOO? (000)and MOO? treated for 100 hours with 2 M NaOH (0.0) This peak undergoes little change upon treatment with 2M NaOH. The molybdenum atoms in the polymeric species exist in different environments, yet the data show no appreciable difference in the molybdenum binding energies. There is, however, a shift of approximately 1.0 eV between the monomeric and polymeric Mo(+6) species. Such a difference could possibly be the result of the difference in the Madelung energy contribution to the binding energy (the lattice energy) in going from the monomer t o the polymer. Sample charging effects are known t o be quite serious in some instances. Charging effects have been ruled out in this study since the C(1s) electron lines due t o hydrocarbon contamination and the O(1s) lines had identical energies for the molybdenum powder mounted o n an insulating cellophane tape and for the molybdenum foil which was mounted maintaining electrical contact with the spectrometer. Quantitative Analysis of M o o R M : o o l Mixtures. An instrumental method for quantitatively analyzing mixtures of M o o Band MOO:!has not been developed until now because there has been no way of distinguishing one oxide from the other. It can be seen from Table I that there is a 1.7 eV shift in the 3d electron binding energies between M o o 3 and M o o 2 . This shift should be large enough to enable one t o distinguish between Moo3and MOO:!in mixtures of the two oxides. To satisfy the need for the rapid analysis of Moo3MOO:!mixtures, a n attempt was made t o use ESCA t o develop an analytical procedure for quantitative analysis of bulk mixtures. The need for a rapid quantitative analytical technique for mixtures of the molybdenum oxides was desired for commer-
CURRENT
cially produced Moo2(9). Such samples would be expected to contain the M o o 3 surface contaminant as discussed above. Use of MOO?contaminated with M o o 3 o n the surface in the calibration mixtures renders the spectra more difficult t o interpret and, therefore, lowers the precision of the analysis. However, use of pure MOO? would yield an analytical procedure for the ideal case, one seldom encounters. The use of M o o 2 bearing the MOO, surface contamination would also introduce a small error on a weight per cent basis. Since the 3d5/?electrons from MOO?are detectable, the layer of MOO,, must be less than 100 thick. Therefore, the error introduced will be very small. For these reasons, use of contaminated Moo2 is justified. A typical spectrum of the Mo(3d) electrons for a mixture ( M o 0 3 : M o 0 2 = 25:75) of the oxides is shown in Figure 5. The electron peak designated as E in the spectrum is that due t o the M o (3d3:?) electrons in M o o 3 while the shoulder a t E 4.7 eV is due t o the M o (3dSp)electrons in Moos. The peak counting rates a t E and E 4.7 eV were found t o be dependent on the bulk percentage of M o o ppresent in the mixture. The ratio of these peak counting rates can therefore be used t o quantitate mixtures of MOO, and MOO?. The necessary calibration curve was derived by measuring 4.7 eV) ratios for a series of the (counts E)/(counts E synthetic mixtures of M o o o and MOO?. The mixtures were prepared by weighing given amounts of each oxide and thoroughly mixing them in a mortar and pestle. The MOO3 was green while the M o o e was black; therefore, any gross inhomogeneities could be detected. The mixtures used t o determine the calibration curve contained from 5 t o 95z MOO?. The data obtained for mixtures of the oxides are tabulated
A
+
+
+
ANALYTICAL CHEMISTRY, VOL. 43, NO. 13, NOVEMBER 1971
1777
+
‘t.
loo/-
$ 40 20
3
2
I
5
4
6
COUNTS E / COUNTS (E +4.7eV) Figure 6. Calibration curve for quantitative analysis of MoOs-MoOn mixtures
+
Table 11. Counts E/Counts E 4.7 eV for Mixtures of Mooz and Moo3 Counts E,’Counts
7Z
MOO^,
100 95 90 80 75 60 55
so
45 40 25 20 10 5 0
N 3 5 5 6 6 3 4 5 3 4 3 3 5 3 3
Ij(MoV1) 2.5565 2.5561 2.5562 2.5558 2.5557 2.5558 2.5559 2.5558 2.5558 2.5557 2,5555 2.5555 2.5555 2.5555 2.5557
E
+ 4.7 eV
1 . 3 0 i 0.10 1.41 i 0 . 2 0 1.70 i 0.10 2.10 i- 0.20 2.30 ?! 0.20 2.95 i 0.05 2 . 9 9 =t0.20 3.31 i 0.10 3.46 i 0 . 0 6 3.60 i 0.17 4.17 i 0.05 4.53 i 0.20 5.00 i 0.30 5.05 i- 0 . 2 0 5 . 0 7 + 0.05 ~
Table 111. Percentage of MOO?Determined for Unknown Mixtures of MooLand Mooa Unknown MOO., % MOO?Determined. ”.;, A B C
I> E
51 46 47 80 20
5 3 i 1 52 i 6 50 =t3 82 f 2 23 & 2
in Table 11. The indicated ratios are averages of a t least three replicate measurements having the standard deviations indicated. I n order t o check the effect of sample preparation, several mixtures were synthesized more than once. In Table I1 these are the mixtures for which N (number of determinations) is greater than three. There is apparently some dependence upon sample preparation; however, the resulting standard deviations indicate that sample preparation does not introduce large errors. The resulting calibration curve is shown in Figure 6. The slope, intercept, and errors in the plotted line were determined by the method of least-squares. In the least-squares analysis, the ratio of peak counting rates, (counts E)/(counts 1778
E 4.7 eV), was taken as the known variable while the percentage of Moonin the sample was taken as the measured variable. This procedure should best determine the precision of the quantitative analysis. The least squares analysis yielded a slope of -25.28 with a standard deviation of 0.43. The standard deviation in Y , percentage of Moon, determined by the analysis was 2.1 %. Thus, the quantitative analysis of mixtures of Moo3and MOO*is good to within one standard deviation of f 2 %. The calibration curve shows the greatest deviation from linearity for oxide mixtures having less than 10% M o o s . The least-squares analysis calculated an error of 4 x for the 10% MOO*mixture. Therefore, it is possible that the major contribution to the 2% error determined for the entire range of 0 t o 100% arises from the large error in the low MOO* mixtures (1.0 eV) and the lowest oxidized species (referred to as the -ous oxide)
ANALYTICAL CHEMISTRY, VOL. 43, NO. 13, NOVEMBER 1971
is stable t o further oxidation, similar procedures can be used for the general quantitative analysis of mixtures of Commercial oxides. The core-electron binding energies were measured for several commercial oxides in order t o determine those which can be analyzed by this method. The resulting data are tabulated in Table IV. Of the six oxide pairs investigated, the data indicate that three can be analyzed (PbO--Pb02, Cr203-Cr03, and As@As205)and three cannot (W02-W03, Cu20-Cu0, and Vz04V205). For the Pb, Cr, and As oxides, the shifts are all larger than 1.0 eV. The C r oxides should be the easiest t o analyze since the shift is 2.8 eV between Cr(II1) and Cr(V1); therefore, the electron lines should be almost completely resolved as are the Mo(3dj/2-3d5/2) electron lines in Moos. Since the shifts between Pb(I1) and Pb(1V) and As(II1) and As(1V) are only 1.2 eV, spectra o n the mixtures will not be completely resolved. The spectra would be comparable t o those for the molybdenum oxide mixtures. The data for W, Cu, and V indicate that the -ous oxide in each case is subject t o further oxidation (to the -ic oxide) since the binding energies are essentially equivalent for the -ous and -ic oxides. Hamrin et al. (10) have reported that spectra of the vanadium oxides are dominated by VZOS,the most highly oxidized species. Only after deconvolution of the spectra were they able t o speculate o n the binding energies of the lower oxidation states. They have also reported that the W(4fi:,) electrons in WOn and W O a are separated by 3.0 eV. One oxide could easily be identified from the other without deconvolution. This indicates that the analytical procedures should be applicable t o the tungsten oxides. The discrepancy in our results must be due t o the WO? samplei.e., our sample of W 0 2 was oxidized t o a greater extent. Nordling et al. ( I / ) have reported a shift of +0.7 eV for the binding energy of the Cu(1s) electrons between CuO s n d (IO) K. Hamrin. C. Nordling, and L. Kihlborg, Institute of Physics,
University of Uppsala, Report UUlP -692 (1970).
Table IV. Electron Binding Energies of Commercial Metal Oxides Binding Compound Electron energy, eV Can do'? PbO Pb(4f;d 139.8 1.2 Yes PbOI Pb(4fiin) 141 .O Cr201
Cr(3p)
CrOa AszOj ASSO:,
CUlO
Cr(3p) As(?d) As(3d) W(4f;/?) W(4fiid CU(2PJiS) CU(2P3d
VI05
V(2P3/2)
WO?
w 03 cuo
v,o4
V(2PO/Z)
45.6 48.4
2.8
Yes
45.6 46.8 36.1 36.3 954.6 954.4 516.6 516.6
1.2
Yes
0.2
No
0.2
No
0.0
No
CUZO. They did not use commercial powdered samples, but selectively oxidized metal samples in the spectrometer sample chamber. Therefore, it appears that the shift would not be large enough t o permit analysis even if the CUSOwere stable t o oxidation. ACKNOWLEDGMENT
W. J. Tschudi and the Climax Molybdenum Company must be gratefully acknowledged for supplying all of the molybdenum compounds investigated here.
RECEIVED for review May 24, 1971. Accepted July 20, 1971. This work was supported in part through funds provided by the United States Atomic Energy Commission under Contract AT-(38-1)-645. One of us (W. E. S.) would like t o thank the National Institutes of Health for a predoctoral fellowship during the term of this research. (11) C. Nordling, E. Sokolowski, and K. Siegbahn, A r k . Fys.,
13, 483 (1958).
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