Anal. Chem. isaa, 60, 1404-1408
1404
(5) Wittmaack, K. Nucl. Insbum. Methods 1977, 143, 1. (6) Yu, M. L.;Reuter, W. J . Appl. Phys. 1981, 53, 1478-1488. (7) Reuter. W.; Yu, M. L. J . Appl. Phys. 1982. 53, 3764-3786. (8) Benninghoven, A.; Ruedenauer. F. G.; Werner, H. W. Secondary Ion Mass Spectrometry; Wiley: New York, 1987. (9) Coburn, J. W.; Winters, H. F.; Chuang, T. J. J . ~ p p l Phys. . 1977. 4 8 , 3532.
(10) Boudewijn. P. R.; Akerboom, H. W. P.; Kempeners, M. N. C. Spectrochim. Acta, Part 8 1984, 398, 1567-1571. (11) Reuter, W. Nucl. Instrum. Methods 1986. 815,3-175.
RECEIVED
for review July 2% 1987. Accepted February 28,
1988.
Secondary Ion Emission and Sputter Yields from Metal Targets under F,+ Bombardment Wilhad Reuter* and J. G . Clabes
IBM T. J. Watson Research Center, Yorktown Heights, New York 10598
Sputter yields and relative Ionization probabllltles have been determlned after Saturation bombardment of 13 elements using a massgeparated 10-keV F,' primary beam generated in a cold cathode ion gun operated at a pressure of 5.3 Pa of pure fluorlne gas. Sputter yleids are larger than those obtained under 0,' bombardment and are In good agreement wlth those obtalned from an available formallsm developed to give the best flt to experlmentai data on sputter ylelds of the element under Inert ion bombardment. IonIration probabllltles are hlgher by up to a factor of 50 for those elements glvlng poor ion yields under 0,' bombardment. Partlally fluorlnated metal surfaces are formed after bombardment to steady-state Ion emlsslon condltlons wlth a fluorlne uptake of about 10-20 atom %.
I t has long been recognized in secondary ion mass spectrometry (SIMS) that the sensitive detection of electropositive elements in a metallic target requires bombardment with primary ions that form strong ionic bonds with the target atoms. Traditionally, ion sources are operated with oxygen gas to produce abundant amounts of 02+ions. Excellent secondary ion yields are obtained for those elements that can be completely oxidized and that form strong ionic bonds with oxygen. It has been shown in a static SIMS study of positive metal ion (M+) emission that for such elements as Mg, Al, Cr, Si, and Fe, more than 10% of all sputtered particles are emitted in the M+ state ( I ) . Secondary ion yields, however, may be smaller by up to several orders of magnitude for those elements (e.g., Cu, Ni, Zn, Pd, Cd, Ag) that form only weak bonds with oxygen or that are only partially oxidized under 02+ bombardment (2). Since the ionicity of metal-fluoride bonds is considerably larger than that in metal oxides, one would expect higher secondary ion yields for these elements under F2+bombardment for comparable projectile uptakes in the target. Anticipated problems associated with the high reactivity of fluorine gas with ion source materials led initially to the use of carbon tetrafluoride as the ion source gas. With the addition of nitrogen or oxygen, we produced a stable discharge and extracted several hundred nanoamperes of a mass-filtered CF3+beam ( 3 ) . However, neither F2+nor F+ is formed with sufficient abundance for analytical applications. Ionization probabilities relative to those obtained under 02+ bombardment increase by an order of magnitude for Ni, Cu, and Ag (3),i.e., for those elements giving relatively poor secondary ion yields with the standard 02+ source. The fluorine loading
of the sample after saturation bombardment measured with X-ray photoemission spectroscopy (XPS) is smaller (10-20 bombardment, atom %) than the oxygen uptake under 02+ yet ionization probabilities can be larger under CF3+bombardment. The carbon uptake under CF3+bombardment is about 30 atom 70 with the carbon present mostly as metal carbides. In addition to Si,F,+, molecular ion species of the Si,C,+ type are formed; Le., mass interference problems are more likely to occur with this ion source as compared to 02+ operation. In view of these results, it appears to be of much interest to study whether under F2+bombardment the fluorine uptake and the secondary ion yields can be increased further with the additional advantage of reduced molecular interference probabilities. EXPERIMENTAL SECTION The analytical system combines XPS-SIMS capabilities under ultrahigh vacuum (UHV) conditions and has been described earlier ( 4 ) . The cold cathode ion gun (5) was operated under an F2 pressure of 5.3 Pa. From the known conductance of the ion source aperture we calculated a daily consumption of 56 cm3 of F2 gas at 1-atm foreline pressure. With the addition of a 1-L balast tank in the gas foreline we could maintain constant gas flow within 10% during 8 h of operation without resupplying F2gas from the cylinder. The gas foreline and balast tank were constructed of Monel. We did not use a pressure regulator valve to avoid potential leakage in such devices. Instead we used a double valving system generating a pressure of 1/2 atm in the foreline, monitored by a Datametric Barocell gauge constructed of Monel. The gas cylinder and foreline were encased by a shroud connected to the house exhaust system and equipped with a photohelic alarm system. On the basis of the experience of Matheson Gas Products, we passivated the inner Monel surfaces of the gas foreline by exposure to atm of a 20180 F2/N2gas supply. The foreline pressure rapidly increased to about 3/10 atm, probably due to the reaction of fluorine with the absorbed water layer. This pressure rise is only observed in this start-up procedure. The ion source pressure of 5.3 Pa was maintained through a variable-leak Granville Phillips valve without any leakage upon closing over a 6-month operating period. With these precautions, no problems were encountered in the safe operation of the ion source. With the gun in operation, the sample chamber pressure increased from about 1 X to 7 X Pa. Additional peaks appear in the residual gas spectrum at masses 69, 50,31, and 19 with a cracking pattern very similar to that of CF4. This means that most if not all of the fluorine gas reacts with components in the primary beam section before entering the sample chamber. Probably for this reason we have not observed any unusual deterioration of the electron multiplier with time. After ignition of the plasma the discharge current stabilizes in about 1 h at about 3 mA and drops finally to near zero after 2-6 days of operation. The anode cylinder exhibits an insulating
0003-2700/88/0360-1404$01.50/0 ?Z 1988 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 60,
NO. 14, JULY 15, 1988
1405
Table I. Sputter Yields S in Atoms Per Primary Ion Determined Experimentally or Calculated with the Formalism Given in Ref 6
Ge Si
* 53 Pa F2 - - - - I O N SOURCE ON Ihr
I O N SOURCE ON 4hn
JT-
IO0
200
300
P L A T E POTENTIAL ( V )
Figure 1. Primary beam mass spectra of the extracted ion beam accelerated to 10 keV. The cold cathode ion source was operated at a pressure of 5.3 Pa of pure F, gas.
ring in the vicinity of the tip of the aluminum cathode and also an insulating layer near the top of the anode. Auger spbctroscopy done in these sections reveals the presence of aluminum fluoride in the former and of chromium and iron fluoride in the latter coating. We did not systematically investigate techniques to reduce or eliminate this problem, for example, by the choice of other construction materials. The primary ion beam extracted from the ion source was mass-analyzed in a low-mass-resolutionWien filter by measuring the target current as a function of the electrostatic plate voltage. The result is shown in Figure 1 after 1 and 4 h of operation. In the early phase of the operation, primarily 02+and HF+ are extracted and no F2+is detectable, the primary reaction products of fluorine reacting with a water layer formed during source cleaning. After about 4 h of operation primarily F2+and F+ are formed. This represents the typical operating condition used in this study. Although considerably reduced, 02+ may contribute up to 10% to the extracted F2+ beam. Consequently we also observe in the mass spectra of the secondary ions emitted from metal targets M,O,+ components with an intensity lower by about 1 order of magnitude than that of the M,F,+ components. Likewise, XPS studies after saturation bombardment indicate some oxygen uptake. Obviously this problem can be eliminated by prior bake-out of the ion source and/or by the use of a magnetic sector type of mass separator with better mass resolution. The XPS work was done in a Hewlett-Packard 5950B XPS spectrometer connected to the SIMS chamber via a vacuum interlock system. The instrument is equipped with a monochromatized A1 K a source. The sample holder allows on-axis rotation of the specimen for studies of the compositional homogeneity with depth. The energy resolution was determined on the Au 4f core levels to 0.8 eV. RESULTS AND DISCUSSION Sputter Yields. The samples used in this study were prepared by electron-beam (e-beam) evaporation of a 1-pm metal film onto a silicon substrate except for silicon, for which we used a silicon water. A focused raster-scanned 10-keV F2+ beam of 100-nA target current under normal incidence was used to remove 0.3-0.9 pm of the metal films. Talystep tracings of the craters yield data on the number of atoms sputtered with an accuracy of about *lo%. The actual beam current was estimated from sample current measurements a t variable positive bias potentials on the target to suppress secondary electron emission. This was done only on five selected targets. On the basis of these data, we reduced the sample current data by 20% as our best estimate for the beam
cu Cr Ni
Fe Zr Ag
Pd Mo
Pt W Ir
SF*+ (10 keV, exptl)
( 5 keV, calcd)
3.6 2.5 5.3 2.8 4.3 2.9 1.4 8.9 4.0 2.4 3.8 1.6 2.7
3.0 1.9 5.2 2.3 3.4 3.2 1.4 6.6 4.0 1.6 2.6 1.5 2.6
2 SFf
current. We expect the overall error in the determination of the sputter yield S (atoms per primary F2+ion) not to exceed 20%. Also shown in Table I are sputter yield data calculated from the empirical equation developed by Yamamura e t al. (6). In the approach of Yamamura the functional dependence of S on the stopping power and the surface binding energy developed by Sigmund (7)were used with fitting parameters to obtain good agreement with experimental data for S of pure elements under inert ion bombardment. The sputter yields calculated for 13 elements obtained by using 5-keV F+ bombardment and the sublimation energies of the metals for the surface binding energy have a mean deviation from our experimental data of only 14%. Such a good agreement implies that the fluorine uptake in the metal films is small and that the sublimation energies of the metals fairly well represent the surface binding energies. The only sputter yield measurement found in the literature is that of silicon @), for which a value of 1.3 silicon atoms13 keV F+ ion was reported, which is in fairly good agreement with our result of 1.8 silicon atoms15 keV F+. Relative Ionization Probabilities. The secondary ion intensity is,Min counts per second of the isotope of abundance f area the element M is given by
is,M= p+&STf where i, is the primary beam current in ions per second, S is the sputter yield in atoms per primary ion, T i s the overall transmission for the selected isotope, and p+iis the ionization probability. Since ipsis the number of atoms N removed per second, by rearranging eq 1we obtain for the useful yield Y+M of element M
For elements close in atomic number, T i s constant and hence
For a given element M and the same isotope but different primary beams (F2+and 02+), the relative ionization probability is given by (4)
N was determined from the Talystep tracings of 0.3-0.9 pm deep craters. is,M+is the measured secondary ion current integrated over the energy distribution. After an initial optimization of the transmission of the secondary ion optics and the observation that this setting is independent of the primary
1406
ANALYTICAL CHEMISTRY, VOL. 60,NO. 14,JULY 15, 1988
Table 11. Ionization Probabilities (Eq 4) for 13 Elements for Normal Incidence 10-keV F2+Bombardment Relative to the Ionization Probability for 10-keV OZcn
Ge Si
cu Cr Ni
Fe Zr A&!
Pd
Mo
Pt W Ir
1.7 0.022 7.9 1.6 25.0 3.0 1.8 28.0 33.0 1.9 13.0 3.8 44.0
'I
I
5.6, 3.2 17, 760 63, 8 1100, 710 170, 6.9 570, 190 105, 59 2.2, 0.078 41, 1.2 340, 180 0.11, 0.0083 13, 3.4 2.1, 0.048
a In the last column the useful yields Y+ (eq 2) are given for the elements for the respective primary beams.
projectile Oz+or F2+,no changes were made in the ion optics and the sample position in order to ensure equivalence in conditions. The results are summarized in Table 11. The trends are very similar to those observed in our study ( 3 ) of the ionization probabilities of the elements under CF3+bombardment vs 02+bombardment. Again we find that for those elements (Cu, Ni, Ag, Pd, Pt, Ir) giving relatively low secondary ion intensities a t constant target current (2) under oxygen bombardment, there occurs a significant enhancement of the relative ionization probability for F2+. However, this enhancement falls short of the goal of this study, to raise the ionization probabilities of these elements to the level achievable for metal oxides giving high secondary ion yields. This becomes apparent if we extract from Table I1 the useful yields Y+N~,C",F~+ normalized to Y+Cr,O2+, giving 0.09 and 0.24, respectively, and Y+A,Pd,Fz+ normalized to Y+Mo,o2+,yielding 0,012 and 0.22, respectively. We selected Cr(OZ+)and Mo(OZ+) for the normalization in order to satisfy the conditions of eq 4 of nearly equal mass and also because the absolute secondary ion yields (M+ per sputtered atom) approach unity (I) with a sputter yield correction of S = 3 applied by us to the data in ref 1. When ion yields are high under 02+ bombardment (Cr, Fe, Zr, W), Fz+offers no significant advantage relative to 02+.Again, the only exception is Si, with a relative ionization probability far lower than 02+ bombardment and even significantly lower under CF3+ bombardment (3). Saturation Bombardment for XPS Study. The generation of a target for the XPS study is not trivial, since the acceptance area for the XPS signal is about 1.5 X 6 mm2, which is much larger than the raster-scanned 1-mm2 ionbombarded area. This required 18 overlapped areas bombarded to saturation as judged by the simultaneously monitored SIMS signal. For this reason, only Ni, Cr, and Si were selected for the X P S study. In Figures 2 and 3 the SIMS signals are shown for l60+ and , the respective metal ions during saturation bombardment of Si and Ni, respectively. both The primary beam current was 100 nA for Fz+and 02+, a t 10 keV. The raster scan width was not changed. For 02+ bombardment of silicon under normal incidence the wellknown behavior is observed that the Si+ signal rapidly drops and then recovers to a steady-state value equivalent to the near-surface intensity (Figure 2). This is understood in terms of a native oxide layer at the surface and the formation of a converted layer of SiOz after saturation bombardment (9). Under Fz+bombardment steady state is reached much faster due to a steady-state sputter yield for Si higher by a factor bombardment. The near-surface of 4 than that under 02+ intensity is given by the ionization probability of S i 0 2 and bomis by a factor of 2 higher than the intensity under 02+
'w,
I
I
i e\./-
',
I
= L
I/
1 9 +( ~F ~ + )
c
0
2r
.. .... .\*. .. .. .. . .. .... ... ... ... \.
I
I
1
200 400 600 SPUTTER TIME ( s e d Flgure 3. Same condltions as in Figure 2, but for a nickel target. 0
bardment, probably due to the increased sputter yield of Si02 with the increasing mass of the projectile. As the oxygen intensity decreases and fluorine accumulates, the 30Si+signal goes through a minimum and finally reaches steady state a t an intensity lower by about an order of magnitude, consistent with the much lower ionization probability we found for F2+ vs 02+.For Ni under 02+ bombardment, we find the 64Ni+ signal decreases by a about a factor of 6 from the surface to the steady-state signal as the native NiO layer is removed, until a partially oxidized (10) converted layer is formed a t steady state. Under F2+bombardment the B4Ni+signal rapidly rises by about 2 orders of magnitude as the matrix oxide is removed and the fluorine reaches a saturation concentration. This again is consistent with the large increase in p p 2 +vs pi,oZ+. XPS Study. After saturation bombardment of pure element targets with 10-keV F2+under normal incidence, we used XPS to gain a better understanding of the chemical-state
ANALYTICAL CHEMISTRY, VOL. 60, NO. 14, JULY 15, 1988
Table 111. Composition in Atomic Percent of a Si, Cr, and Ni Target after Normal Incidence IO-keV Fz+Saturation Bombardment Obtained with a Photoelectron Take-Off Angle at 38'
M F 0
atom % atom % atom %
Si
Cr
Ni
0.87 0.10 0.03
0.64 0.27 0.09
0.86 0.05 0.09
1407
I.o
>
c z" W 52
E
0.5 0.0 1.0
0.5
J
9
Cr 2P3/2
- 10keV -.-.PURE
~
0.0
[L
F2'
Cr
0.5 l
0.0
-502
-500
-570
-576
-574
-572
BINDING ENERGY ( e V )
Figure 4. Cr 2p3,, XPS spectra after normal incidence saturation bombardment of Cr with 10-keV F,+ and after in situ e-beam evaporation of a pure Cr film.
changes induced by reactive ion bombardment and the resulting changes in the ionization probabilities. The sample was transferred into the XPS chamber via a vacuum interlock to study chemical-state changes and to determine the atomic concentrations. Data were taken at the standard take-off angle of 38" with respect to the sample surface and at 13" to increase the surface sensitivity. The atomic concentration CA of the element A was calculated from
where I is the background-corrected integrated photoelectron intensity and u, A, and T a r e the photoionization cross section, the mean free path, and the instrument transmission for the respective elements and photoelectron energies. On the basis of experience with a wide variety of targets of known composition, we expect that the concentrations should be accurate within *30%. Some of the XPS results'for Fz+bombardment are summarized in Table 111. All targets show a few atomic percent of oxygen, which at least in part is introduced by some 02+ contamination in the extracted Fz+beam (see Experimental Section). Chromium exhibits the highest uptake of F, giving identical composition for the grazing (13") and the more normal (38")emission angle. In contrast, Ni shows a preference for oxygen incorporation over fluorine. This, as well as an increase of the oxygen concentration for Si in the very near surface region, is indicative of oxidation after F2+saturation bombardment. More importantly we find that the fluorine uptake is comparable to those observed under CF3+ bombardment. Again the projectile uptake is much smaller bombardment, yielding atomic concentrations than under 02+ of 60%, 66%, and 30% in Cr (converted layer CrzOS), Si (converted layer SiO,), and Ni (converted layer partially oxidized to NiO), respectively. In Figure 4 the Cr 2pSjzrange is shown after Fz+bombardment and also for a pure Cr layer generated by in situ e-beam evaporation. As to be expected from the compositional data, a large fraction of the near-surface region is present in the metallic state of chromium. Very similar to our data
.
O -J
/
'. '
= . \
1
\
] /
' d
SPUTTERING TIME ( a r b units) Figure 5. Cr and Ni depth profiles obtained from a 150 A Cr/150 A Ni muttihyer structure by using the respective primaty ion beams under normal Incidence bombardment.
obtained for CF3+ (3), Fz+bombardment leads to a long tail extending to higher binding energies. We again interpret this tail to be the formation of chromium-fluorine bonds of the type CrF, CrF2, and CrF3 with decreasing abundance. This is also indicated by the asymmetry in the F 1s core level exhibiting a broadening toward lower binding energies (BE'S). While Cr under oxygen bombardment is fully oxidized to Cr203,the associated binding energy shift with respect to CrO is only 2.1 eV vs 6 eV for CrF3 (11). We presented in ref 3 qualitative arguments rationalizing from these XPS data the experimental result that the ionization probabilities are similar under CF3+ (F2+)and 02+bombardment. The Si 2p spectrum after Fz+bombardment is virtually identical with that of pure silicon, whereas under OZf bombardment SiOz is formed with a binding energy shift of 5 eV partially enhanced by charging. The binding energy difference between F 1s and Si 2p of 586.7 eV is in full agreement with that found for dissociative adsorption of SiF, or XeF, on Si (12). At the low concentration of fluorine in this target, only SiF is likely to be formed, which causes a shift of 1.0 eV toward higher B E with respect to elemental Si (13). A pronounced tail in the F Is spectrum toward higher BE seen at the 38.5" emission angle may indicate interstitial fluorine in the lattice. The expected energy shift of 0.3 eV (14) of the Si 2p core level cannot be resolved with our instrument, however. Since the uptake of fluorine and the charge shift to higher BE are much bombardment, it is not smaller under Fz+vs those under 02+ surprising that the ionization probability for Si+ formation is much smaller in the former case. For Ni the fluorine uptake is even smaller than for Si, and hence the Ni 2~312spectrum after F2+bombardment is indistinguishable from that obtained from the pure metals, yet the ionization probability is higher by a factor of 25 for F2+ vs 0,'. The BE of F Is was found a t 684.8 eV, in perfect agreement with the BE found in NiF, (15). The charge shift from Ni metal toward higher BE for the Ni 2p is far larger (+5.5 eV) for NiF, (16) than for NiO (1.7 eV). Even if at this low concentration fluorine is dispersed in the nickel lattice, the ionicity of the NiF bond should still be significantly higher than that of the Ni-0 bond, offering an admittedly highly vs speculative reasoning for the large increase of p+,,Ni,F2+ p +i,Ni,02+. . .
APPLICATION We have applied the F2+ion source for the depth profiling of the same 150 A Cr/150 A Ni oscillating layer structure for which we reported our results ( I 7) for N2+,Oz+,Ar+, and CF3+
1408
ANALYTICAL CHEMISTRY, VOL. 60, NO. 14, JULY 15, 1988
bombardment. In Figure 5 the depth profiles normalized to the maximum signal are shown for 5-keV Fz+,CF3+,and 02+ bombardment. The double peak structure in the Ni layer under 0,' bombardment has been discussed by us (3,17)and is related to changes in the ionization probability of Ni' in the intermixed interface region. This is not observed under CF3+bombardment for reasons discussed in ref 17. Under F2+bombardment a depth profile is generated, very similar to that obtained under CF3+bombardment. Common to both CF3+and F2+bombardment is the relatively small uptake of fluorine in the target. Under such conditions, matrix effects in the ionization probability are minimized (3,17). Again we find that Cr sputters considerably more slowly than Ni, probably due to a higher surface binding energy in the partially fluorinated Cr surface vs the surface binding energy of essentially pure Ni. We refrain from a comparison in the achievable depth resolution since we have not established equivalence in the primary beam sizes in this study. CONCLUSION The motivation in this study and in our prior work with CF3+was to explore whether, under bombardment with the most electronegative element, ionization probabilities of those elements that give relatively poor ion yields under 02+bombardment can be raised. This is an important issue in view of recent development to enhance ionization by postionizing processes (18)using high-power laser pulses, an electron beam, or a hot electron gas plasma. Laser pulse ionization at power levels now available nearly fully ionizes most elements in the sputtered flux and employs time-of-flight mass spectrometry, recording the entire mass spectrum under high transmission conditions, thus yielding excellent absolute detection sensitivities. The technique suffers from the low duty cycle of about due to the pulsed operation. In contrast, both e-beam and hot electron gas postionization are not subject to this problem and ionize the sputtered flux less efficiently (about and, at least in their present configuration, employ relatively poor ion transmission systems. Many SIMS systems are designed for high transmission ( - 0 . 1 ) and will give excellent absoiute detection sensitivities if the ionization probabilities for the target constituents are large. For those
elements inefficiently ionized under 02+ bombardment, our studies show that ionization probabilities can indeed be increased by the use of either CF3+or F2+primary beams. Large differences in the ionization probabilities however remain, compared to the nearly equal ionization probabilities achieved with postionization. Even if one succeeds in increasing the fluorine uptake in the sample by bleeding either F, or XeFz into the sample chamber, it is doubtful that the ionization probabilities can be enhanced by such a large factor. ACKNOWLEDGMENT We thank M. L. Yu for valuable discussions and D. S. Yee for the metal films used in this study. Registry No. Ge, 7440-56-4; Si, 7440-21-3; Cu, 7440-50-8; Cr, 7440-47-3; Ni, 7440-02-0; Fe, 7439-89-6; Zr, 7440-67-7; Ag, 744022-4; Pd, 7440-05-3; Mo, 7439-98-7; Pt, 7440-06-4; W, 7440-33-7; Ir, 7439-88-5; F2+,12184-86-0. LITERATURE CITED (1) Benninghoven. A. Surf. Sci. 1975, 53,596-625. (2) Storms, H. A,; Brown, K. F.; Stein, J. D. J . Anal. Chem. 1977, 49, 2023-2030. (3) Reuter, W. Anal. Chem. 1987, 59,2081-2087. (4) Frisch. M. A.; Reuter, W.; Wittmaack, K. Rev. Sci. Instrum. 1980, 51. 695-704. . . ~ . (5) Wittmaack, K. Nucl. Instrum. Meth. 1977, 743,1. (6) Yamamura. Y.; Matsunami, N.; Itoh, N. Radiat. E f f . 1983, 71,65. (7) Sigmund, P. Phys. Rev. 1969, 383-416. (8) Tachi, S.;Okudaira, S. J . Vac. Sci. Techno,., 8 1986, 4 ,459-467. (9) Reuter, W. Nucl. Instrum. Meth. Phys. Res.. Sect. 8 1988, 875, 173-175. (10) Yu, M. L.; Reuter. W. J . Appl. Phys. 1981, 53, 1478-1488. (11) Wagner, C. D.; Riggs, W. M.; Davis, L. E.; Moulder, J. F.; Muilenberg, G. E. Handbook o f X-ray Photoelectron Spectroscopy; Perkin-Elmer Gorp.: Eden Prairie, MN, 1978. 12) Chuang, T. J. J . Appl. Phys. 1980, 57,2614-2619. 13) McFeely, F. R.; Morar, J. F.; Himpsel, F. J. Surf. Sci. 1986, 165,277. 14) Seel, M.; Bagus, P. S. Phys. Rev. 8 : Condens. Matter 1983, 28, 2023. 15) Gaarenstroom, S. W.; Winograd, W. J . Chem. Phys. 1977. 6 7 , 3500. 16) Matienzo, L. J.; Yin, C. 0.; Grim, S. 0.; Schwartz, S.E. Inorg. Chem. 1973, 72, 2764. 17) Reuter, W.; Scilla, G. J. Anal. Chem., preceding paper in this issue. 18) Reuter, W. Chemical Physics; Benninghoven, A,, Colton, R. J., Simons, D. S.,Werner, H. W., Eds.; Springer-Verlag: Heidelberg, FRG, 1986; Vol. 44, pp 94-101. I
RECEIVED for review October 14, 1987. Accepted February 28, 1988.
