682
Anal. Chem. 1982, 5 4 , 682-687
(11) “Internation Tables for X-ray Crystallography, Vol. I V , Physical and Chemical Tables”; The Kynoch Press: Btmlngham, England, 1974; pp 61-66. (12) Brindley, George W. “The Identlflcatlon and Crystal Structures of Clay Mlnerals”; Brown, Q., Ed.; Mineralogical Soclety: London, 1972; pp 489-516. (13) Chung, F. H. J . Appl. Ctystallogr. 1974, 7 , 519-525. (14) McCarthy, Q. W.; Oehringer, R. C.; Smith, D. K.; Injaian, V. M.;
Pfoettsch, D. E.; Kabel, R. L. “Advanes in X-ray Analysis”; Plenum Press: New York, 1981; Vol. 24, pp 253-264. (15) Frevel, Ludo K. Anal. Chem. 1970, 4 2 , 1583-1587.
RECEIVED for review July 27, 1981. Accepted November 24, 1981.
Fourier Transform Infrared Reflection Absorption Spectrometry and Electron Spectroscopy for Chemical Analysis for Surface Analysis Aklra Ishltanl, * Hldeyukl Ishlda, Fusaml Soeda, and Yoshlkatsu Nagasawa Toray Research Center, Inc., Sonoyama, Otsu, Shiga 520, Japan
The combination of electron spectroscopy for chemical analysis (ESCA) and Fourier transform infrared spectrometry (FT-IR) was tested for usefulness in surface analysis. The surface Sensitivity of IR was increased by the use of infrared reflection absorption spectrometry (IR-RAS) coupled with Fourier transform infrared spectrometry (FT-IR). Examinatlon of oxide layers of thickness of about 100 A formed on copper plates with both techniques together with ellipsometry Is described in detail. Growth of oxide layers of different composition corresponding to different heating temperatures was manifested with the analysis. The result verifies effective combination of these two complementary techniques.
Electron spectroscopyfor chemical analysis (ESCA) is now extensively used for various kinds of surface analysis because of its capability for giving information about chemical bonding, ita nondestructiveness, and its moderate surface sensitivity. However, chemical shifts observed in ESCA are usually not large enough to differentiate between the wide range of chemical species present on a surface. Efforts to make use of satellite peaks or valence electron spectra do not seem to fully complement the limitations of structural information obtainable from chemical shifts of core electron binding energy. Combination with other techniques is necessary to utilize the potential of ESCA thoroughly. Infrared spectrometry (IR) seems most promising for this purpose because of the abundant and detailed structural information accumulated in ita long history and also because of its complete nondestructiveness, provided that surface sensitivity could be improved by some means to a level comparable to ESCA. Our main effort has been on increasing the surface sensitivity of IR using various modes of measurement most suited to the variety of samples being measured. The most important point of the work is the use of Fourier transform infrared spectrometry (FT-IR), making use of its high sensitivity, high precision, and data processing capability (1). We have succeeded in measurement of the monomolecular layer of cadmium arachidate of a thickness of 28 8, on a glass substrate with the FT-IR-ATR mode combined with bulk signal subtraction for surface sensitivity enhancement (2). FT-IR-ATR has limitations in its application to such samples as high refractive index substances like metals, metal compounds, and 0003-2700/82/0354-0682$01.25/0
semiconductors, or rough or nonflat surfaced materials, although it is most extensively used to obtain a vibrational spectrum of surface species (3). A new approach in the present work is use of infrared reflection absorption spectrometry (IR-RAS) (4-7) with FT-IR. This combined FT-IR-RAS technique was found to improve the practical usefulness of conventional IR-RAS. In IR-RAS a surface to be measured is illuminated at a grazing angle by polarized infrared radiation which has a polarization plane parallel to the plane of incidence. The perpendicular polarized incident beam or even the parallel polarized one at normal incidence are almost completely canceled near the surface by the reflected light which has a 180° phase difference. On the other hand, a grazing incident beam of parallel polarization, especially around 70 to 85O, interferes constructively with the reflecting light to form an intense standing wave field at the reflection surface. This gives a sensitivity enhancement of up to about 25 times (5) compared with an ordinary transmission absorption mode. This IR-RAS technique has been already widely used for surface analysis of a sample of relatively large area using multiple reflections with a conventional dispersion IR spectrometer (8, 9). But it is hardly an analytical tool of practical importance, especially in industrial applications, because of sample size restriction and the time-consuming measurement due to low sensitivity in a conventional IR technique. This limitation is easily removed by using FT-IRbecause of ita high sensitivity and also extensive data processing capability. A simple attachment which had reasonably small sample illumination area and a single reflection mode for FT-IR-RASwas designed with cooperation of JASCO International Co. We could attain surface sensitivity good enough for measurement of a thin layer of thickness around 100 8, for a wide variety of samples without being restricted by size and number of available specimens. The sensitivity can be extended without much difficulty to the extent of around 10 8, for an easy case where in intense infrared absorber is on a large highly reflective metal surface. Thus FT-IR-RAS can be effectively used in combination with ESCA because its surface sensitivity approaches that of ESCA. The range of applications of the technique is very extensive, covering such problems as impurities, oxides, products of chemical treatment or corrosion, and also thin oil layer on metal surface. We are now extending its application not only 0 1982 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 54, NO. 4, APRIL 1982
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Figure 1. An optical diagram of an attachment for Infrared reflection absorption spectrometry (Fr-RAS): S, sample holder: P, polarizer; M,-M,, mlrrors to metals but also to semiconductors like silicon wafers or carbon materials. A common step of surface analysis is, in the first place, a measurement with ESCA which monitors the existence of a surface layer different from bulk composition and gives information about elements and their rough chemical states. The step could be followed by FT-IR-RAS which gives more detailed information concerning chemical composition and structure. A few examples of this surface sensitive FT-IR-RAS measurement are given for samples of practical importance and then the combination of the two techniques will be shown for oxide layers on colpper plates.
EXPERIMENTAL SECTION Spectrometer and Attachments. Digilab Model FTS-2OB/D with a TGS detectar and a Globar light source was used with an IR-RAS attachment with resolution equal to 4 cm-l and accumulation up to 800 times depending on absorption intensity and thickness of a surface layer. An optical diagram of the IR-RAS attachment is shown in Figure 1. The incident beam of diameter 12 mm from the spectrometer is led into a gold wire grid polarizer (P) by a plane mirror (Ml), The polarized beam irradiates a sample surface on a stage of adjustable position with a grazing angle (70") of incidence. The illuminated area on the sample stage is an oval of axes of 12 and 35 mm. Sample surface area necessary for measurement is about 4 om2which is reasonably small, and it can be easily reduced to about 0.5 cm2in the case of a highly reflective surface. The reflected beam is led into a concave mirror (M3) for collimation with a plane mirror (M,)and then put back to the optical axis of the spectrometer by a plane mirror (M4)with a beam diameter of' 12 mm. A XPS spectrometer,Model ES-200, by AEI-Kokusaidenkiwas used with an A1 Ka (1456.8 eV) source, vacuum of around mmHg and a photoelectron emitting angle of loo. Calibration of electron kinetic energy (KE) was carried out by assuming that of C 1s of hydrocarbon contamination accumulated on surface as 1202.0 eV. Binding energy (BE)was calculated by the equation BE =I 1486.6 - KE. An ellipsometer,Model I)V-36L, of Mizojiri Kogaku, Inc., was used for determinalion of the thickness of thin layers on the surface. Sample Preperation. The spectrum of a steel plate of Japanese Industrial Staridard G3141 treated with zinc phosphate was measured, with a platinum plate used as a spectral reference. Insulating oil was coated on a steel plate evenly and surplus oil was removed with a Kimwipe. Thickness of oil layer on surface for this sample was estimated to be around 0.3 pm. Dioctylphthalateliquid was coated on a tin-plated steel surface in the similar way silicone oil with thickness of about 0.2 pm. An optical reference for this sample was an uncoated plate. Aluminum plates of 3 X 4 cm size were polished to a mirror finish with a metal polish "PIKAL", cleaned with soap solution, washed successively with acetone, n-hexane, and chloroform in an ultrasonic cleaner, and stored in chloroform. Poly(acryr0-
II
2000
683
I
1000
500 c m-1
Flgure 2. FT-IR-RAS spectrum of steel surface treated with zinc phosphate. R, and R are the Intensities of reflected light from reference and sample plates. nitrile-co-styrene)(PAS)with 28 and 72 mol % of monomer units respectively was dissolved to 0.25, 0.5, and 1 wt % chloroform solution. Aluminum plates were dipped into polymer solution and drawn out with variable velocity between 5.7 and 7.6 cm/min. Thickness of a polymer film was controlled with both of polymer concentration and drawing velocity. The resultant thickness of' a film was determined from ultraviolet light absorbance of chloroform solution of the film of a certain area at 260 nm, and also confirmed with ellipsometry for a film thinner than 300 8. A titanium plate with 99.9 % purity produced by Furuuchi Kagaku, Inc., was heated for 1 min at 1000 "C in an electric furnace to produce an oxide layer on surface. y-Aminopropyltriethoxysilane (y-APS) produced by Nippon Unica, Inc., was dissolved in xylene dehydrated by molecular sieves to a 10% solution. The surface of the titanium plate was chemically modified by refluxing it in yAPS solution in air and also under dry nitrogen atmosphere for 6 h. The chemically modified titanium plate surface was cleaned successively with xylene, acetone, and distilled water under ultrasonic vibration and stored in a desiccator. Chemical modification of surface was confirmed by detection of silicon and nitrogen with ESCA. A commercial copper plate of purity of 99.9% was washed with soap and water, polished with alumina abrasive powder, washed successively with acetone and n-hexane and dried in a desiccator. A copper plate was etched with an Ar ion gun under high vacuum to prepare a oxide-free surface as a reference for ellipsometry. The copper plate was heated in air at 80 O C for 0.5-250 h to prepare samples which have oxide layers of various thickness in order to cross-examine the ellipsometry measurement. Copper plates were heated in air for 30 min at 150, 200, and 250 "C to prepare samples for ESCA and FT-IR-RAS measurements. They showed different colors due to the difference of chemical composition and surface layer thickness.
RESULTS AND DISCUSSION Surface Analysis of Steel. Three practically important samples of treated steel surface, which were relatively easy to measure because of the large thickness of the surface layers (-1 pm), were chosen to illustrate the potential of the FTIR-RAS technique. Figure 2 shows a spectrum of a steel surface treated by zinc phosphate for rust prevention and improvement of adhesion to paint, using an untreated steel as reference. It clearly shows the existence of a zinc phosphate layer on the surface. Figure 3 shows a spectrum,of a small amount of insulating oil measured by FT-IR-RAS compared with a thin liquid film on a KBr plate taken by a standard transmission mode. The characteristic vibration bands due to this hydrocarbon are seen without shift or distortion of band shapes in comparison with the transmission spectrum, although there is an uneven background. Figure 4 shows the FT-IR-RAS spectrum of a thin layer of dioctyl phthalate, a common plasticizer, on a tin-plated steel compared again with one on a KBr plate. This kind of sample
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ANALYTICAL CHEMISTRY, VOL. 54, NO. 4, APRIL 1982
r-
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Figure 3. FT-IRRAS spectrum of Insulating oil on steel compared with a transmission spectrum of one coated on a KBr plate: (a) on steel, (b) on KBr
1
3000
4000
2000
1500
1000
500
wavenumbers
Figure 5. FT-IRRAS spectra of poly(acryronitri1e-co-styrene)thin films of thickness (a) 90 A, (b) 260 A, and (c) 2680 A.
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2000
1000
3000
dl1 Fig.4
Reflection- Absorption Spectrum of
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Tin P l a t e d S t e e l
Figwe 4. A FT-IRRAS spectrum of dioctyl phthalate on tln-plated steel compared with a transmission spectrum of one coated on a KBr plate: (a) on tin-plated steel, (b) on a KBr plate.
may be frequently encountered in analysis of an interface between a tin-plated steel and a laminated poly(viny1chloride) film with the plasticizer. Here again, an unshifted and undistorted spectrum good enough for a detailed discussion of adhesion problems as one caused by segregation of a plasticizer in the interface was obtained. Thin Polymer Films on Metal. Figure 5b,c shows FTIR-RAS spectra of PAS on aluminum with thicknesses of 2680 A and 260 A, respectively. All characteristic bands due to acrylonitrile and sytrene monomer units are clearly observed, in good agreement with a transmission spectrum of the same polymer in a KBr disk. Figure 5a shows a spectrum of a film of 90 A thickness. This seems almost the limit of detection because only the more intense bands are discernible in the spectrum. The broad bands in 900-1100 cm-' and 650-800 cm-l regions are considered to come from the hydrated aluminum oxide layer on the substrate. Figure 6 shows the linearity of the relation between film thickness and absorption factor of a characteristic band of PAS at 3020 cm-l verifying the theoretical prediction. The absorption factor used here is defined as 1- (R/Ro),where R and Ro are reflectivity of a sample and a reference, respectively. Chemically Modified Titanium Plate. The spectrum of chemically modified titanium in air shown in Figure 7 has a vCH of the propyl group at 2940 cm-', 6 " of the amino group
Figure 8. Plotting of the absorption factor against PAS film thickness for the band at 3020 cm-'.
I
00
3000
Figure 7. FT-IRRAS
2000 1500 wavenumbers
1000
500
spectrum of a ttanium plate chemically modified
under air. at 1600 cm-l, the TiOz band at 840 cm-l which appears to be deformed and shifted to higher wavenumber due to the measurement mode, and two bands at 1160 and 1040 cm-l of Si-0-Si group. y-APS is considered to be polymerized under this experimental condition because of water in air. A titanium plate modified under dry nitrogen has a very thin layer which shows a discernible spectrum only after taking the difference with an unmodified reference as shown in Figure 8. A band at 1099 cm-' can be assigned to the Si-0-Si group in a chain dimer siloxane or ring oligomers of six or seven members. This is in coincidence with the result of Boerio (10) on y-APS on iron. A band at 925 cm-I may be assigned to the Si-0-Ti bond, when it is compared with spectra of various
ANALYTICAL CHEMISTRY, VOL. 54, NO. 4, APRIL 1982
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885
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Flgure 8. FT-IRRAS spectrum of a titanium plate chemically modifled under dry nitrogen.
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Flgure 10. Variation of ESCA spectra of surface oxidized copper plates with Ar+ ion etching: (a) one treated at 150 OC for 30 min, (b) one treated at 200 OC for 30 min.
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60
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Figure Q. Diagram of correlatlon between measurement for copper oxide thin layers.
I 70
+ and A in ellipsometry
organosiloxytitaniums observed by Zeitler (11). The thickness of modified layer was estimated by using the measured absorption factor 0.0178, n = 1.421 (refractive index of y-APS monomer), absorption coefficient of the Si-0-Si band at lo3, and concent,rationof y-APS a t 4.3 mol/L, to be about 50 A. This3 is close to a value of the y-APS layer on SnO2 determined with ESCA by Murray (12). Oxides on Copper. (1) Ellipsometry. The Ar+ ion etched copper plate immediately before measurement showed a complex refractive index; n = 0.79 - 3.0i in accordance with literature (13). Plotting (tan is intensity ratio between reflected beams which have parallel and perpendicular components with the ]plane of incidence) against A(phase difference between the two components) gives a curve shown in Figure 9. It corresponds to a complex refractive index of n = 2.1 - 0.li. Here, refractive indexes of Cu20 and CuO are close together, 2.71 and 2.63, respectively, and they are not differentiated by ellipsometry. Open circles in the figure are values determined for thin oxide layers prepared by heating copper plates at 80 "C for different times in order to crossexamine appropriateness of the curve. Satisfactory agreement is obtained between them. The three samples of different heat treatment temperatures at 150,200, and 250 OC indicated as closed circles in the figure were determined to have oxide layers of 155 f 5, 320 f 10, and 470 f 5 8, thickness, respectively. (2) ESCA. Measurement and analysis of ESCA spectra were carried out utilizing all standard techniques of the state of the art. Result is summarized in Table I and typical spectra are shown in Figure 10. Three kinds of Cu samples were measured, one left, a t room temperature for long time, and
+
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KINETIC
ENERGY
(eV
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Flgure 11. Variation of PIES spectra of surface oxidized copper plates wlth Ar+ ion etching: (a) one treated at 150 OC for 30 min, (b) one treated at 200 OC for 30 min.
two samples heated at, 150 and 200 "C, respectively, in air for 30 min. Emitting angle of photoelectrons was set to loo against the normal for most of the samples. Auger spectra with X-ray photoexcitationwere also recorded to complement the ESCA data. Measured elements are copper and oxygen coming from the main composition, carbon due to hydrocarbon contamination, and also small amount of nitrogen and silicon on unetched surfaces. Elements due to contaminant are seen notably with a grazing angle measurement of 60' on the outermost surface. They decreased rapidly with Ar+ ion etching. Oxygen also decreased with etching indicating variation of cornpositon of thin layers although preferential sputtering of oxygen over copper should be taken into account. Composition of copper surface is discussed from spectra due to copper and oxygen. The ratio between the integrated intensities of Cu 2psIzand 0 1s gives a good measure for the elemental composition of' the oxide film, especially when a component of 0 1s due ta copper oxides is used for calculation after curve resolving of a 0 1s spectrum as indicated in Table I. The three stable chemical states expected in the system, namely, metal, cuprous, and cupric oxides, are differentiated by combining ESCA and X-ray induced AES spectra. In a ESCA spectrum Cu(I1) can be easily differentiated from Cu(1) and Cu(0) with a chemical shift and also the observable satellite peaks as shown in Figure 10, although it is difficult to differentiate Cu(1) from Cu(0). On the other hand, Cu(0) can be distinguished from Cu(1) and Cu(I1) in an AES spcetrum as indicated in Figure 11,by making use of different features in the spectral profile, although Cu(1) and Cu(1I) are hardly distinguishable from each other in the spectrum (14, 15). Figure 10 shows the variation of the Cu 2pIl2and Cu 2p3l2 peaks in the ESCA spectra of copper plates heated at 150 and
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ANALYTICAL CHEMISTRY, VOL. 54, NO. 4, APRIL 1982
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200 "C for 30 min due to the Ar+ ion etching for 1and 4 min. The spectrum of the 150 OC heated sample does not show much difference upon etching, except the quick disappearance of weak satellites and the small shoulder in the higher BE side of the main peaks. This comes from inability of ESCA to discriminate Cu(0) and Cu(1). On the other hand, the sample heated at 200 "C indicates striking difference after etching for 1min. The outermost surface of the plate is characterized with a broad main peak due to Cu 2p3,2 in a chemical shift range for typical Cu(I1) species and intense satellite peaks at 7.3,9.0, and 10.1 eV distance from the main peak, indicating existence of Cu(I1) on surface (16). Elemental composition limits the probable Cu(I1) chemical species on the surface to oxides, hydroxide and carbonate. The satellite components at 7.3 eV and 9.0 eV can be assigned to CuO and the 10.1-eV component may be due to hydroxide or carbonate (16). These spectal features are smoothed out with 1min of etching and a spectrum characteristic of Cu(0) and Cu(1) appears, indicating thinness of the cupric oxide layer on the surface. The Auger spectra shown in Figure 11give complementary information about surface composition. The spectra of both samples indicate the presence of oxides on the surface and change into those of metal upon etching. They prove the existence of CuzO on the surface of samples treated at 150 "C. The thickness of the oxide layer on the surface of the sample heated at 200 "C also seems larger than that of the one treated at 150 "C. With 4 min of etching both show an identical copper metal spectral pattern with the characteristic three components. A semiquantitative estimation of stoichiometry of the surface layer composition is carried out in the following procedure; a detailed description of the calculation will be given later (17). First the relative amount of Cu(I1) is calculated from a curve-resolved Cu 2p3I2 peak, taking the standard spectra of pure CuO and Cu metal into account. Then the relative amounts of the Cu(0) and Cu(1) Cu(I1) components are estimated from Cu LMM Auger peaks using a scale empirically obtained for Cu metal and CuO mixture independently. The relative ratios between copper and oxygen are calculated from the above result of Cu(O), Cu(I), and Cu(I1) compositions and compared with the values obtained from O* ls/Cu 2p3p ratios in XPS spectra with reasonable agreement as indicated in Table I. The untreated copper has very thin ( 20 A) Cup0 surface layer on the metal substrate. The copper heated at 150 OC has a layer of CuzO about 100 A thick (--8O%), while one treated at 200 "C has a CuO overlayer about 50 A thick with a 200-250 8, thick CuzO layer as schematically indicated in Figure 12. The result gives features of metal surface composition good enough to compare the samples of different thermal treatment although preferential sputtering of oxygen over copper should be taken into account for a more quantitative estimation. (3) FT-IR-RAS. The same surface oxidized copper plates heated at 150,200, and 250 OC for 30 min were measured by the FT-IR-RAS technique (1%21). Measurement was carried out with a resolution of 4 cm-l and accumulationof 600 times. The obtained spectra shown in Figure 13 were smoothed by using a Lorentzian weighting function With a program supplied by Digilab. The sample heated at 150 "C shows clearly a band at 650 cm-l due to CuzOalthough the thickness of the oxide layer is merely 150 A. The shift of the band around 40 cm-' to higher wavenumber compared with one in a transmission spectrum of CuzO powder is due to anomalous dispersion of refractive index (18). The sample heated at 200 "C or 250 "C shows an additional band at 560 cm-l due to CuO, indicating that further oxidation to cupric oxide takes place a t higher temperature.
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ANALYTICAL CHEMISTRY, VOL. 54. NO. 4, APRIL 1982
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Rgun 14. A dlagram for surface wmposltion of oxldhed copper plates heated at 150 OC. 200 OC. and 250 OC. calculated from FTIR-RAS spectra.
Figum 12. Dapth pmWs of suface compc&bn of oxldlred copper plates a1 (a) 150 OC and (b) 200 OC. fw 30 mln.
FT-IR-RASgives the overall composition of oxides layers without the ambiguity present in ESCA depth profiling due to preferential sputtering or a misunderstanding brouhgt by surface sensitivity of ESCA which gives only several scores of A thickness information when it is not accompanied with Ar* ion etching. Besides, it provides far more detailed structural information in a case of organic compound thin f h which are more difficult to differentiatefrom each other by ESCA. We believe that these data establish that the surface sensitivity e n h a n d FT-IRRAS technique can be utilized quite successfully in combination with ESCA for a wide range of materials. Further development in other modes of FT-IR measurement such as diffuse reflectance, infrared emiasion and photoacousticdetection will add more variety in obtaining vibrational spectrum of surface to complement the conventional surface analytical techniques like ESCA and AES. LITERATURE CITED
Jk '.. .. '..
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Flgum IS. FT-IR-FIAS spectra of surface oxldized copper plates at (a) 150 OC ard (b) 200 OC for 30 mln.
Relative composition of Cu& and CuO was cnlculated with observed absorption factors and intensity coefficients determined by Poling (19). Figure 14 shown a schematic diagram of the oxide layer structure obtained by combiningthe relative composition from FT-IR-RAS, total thickness of oxide layer from ellipsometry and order of the oxide layers from ESCA The approximate fh thickness of the oxides is in reasonable agreement with ESCA depth profiling.
(1) (mffha. P. R. "Chsmlcsl Inhared Fwiat Transform Spectmsmw". Why: New Y u k , 1975. (2) Ohnlshl. T.; Ishltanl. A,: Ishiis. H.: Yamamdo, N.; Tsubomua. H. J. mp.chan.1978. 82. 1989-1991. (3) Jakobasn. R. J. '"FourierTransform Inhared Spectmsmw. AppxCa&ns Io Chemical Systems"; Femtm. J. R.. Baslle. L. J.. E&.; Acadernlc press: New Ycwk. 1979: Vd. 2. Chapter 5. (4) Frands. S. A,; Ellim. A. H. J . Opt. Soc.Am. 1959. 49. 131-138. (5) Oreenk, R. G. J . Chem. Fhp. 1900,44.310315. (6) McIntyre. J. D. E.; Aspens. D. E. S v t . Sei. 1971,2 4 , 4 1 7 4 . (7) Oleenler. R. 0.; Rahn. R. R.; Schwa&. J. P. J . Catal. 1071. 23, 42-48. (8) Mclntyre. J. D. E.; Aspnr. D. E. S v t . Scl. 1071,24,417-434. (9) Muller, R. H, Ed. "Advance in EL5clrsxhem$by and ElecbDchemlcal Enalneerlm": Wllev: New Y u k . 1973 Vd. 9 ~o;tria.~ 3S&lein. .f L. H.: eelvenkamp. J. E. J . w .ponvm. Sci. 1078.22, 203-213. Zenler. V. A,: Brawn. C. A. J . phy8. WWm. 19SI. 81,1174-1177. Unlereken. D. F.; Lennox. J. C.: Wier. L. M.: Moses. P. R.: Mvray. R. W. J . EkImanBf. Chem. 1077. 81. 309-318. Yorhida. K.: Klshiml, K.: Nagata. S. Mem. Fac. Eng., Kobe W . 1975.m . 12. 131. (14) Sch& G. S>. Scf. 1973. 35. 98-108. (15) Mclntyre. N. S.:Rummety, T. E.; Cook. M. 0.; OmM. D. J . EL9cDo&m. Soc.1978. 123, 1164-1170. (t6) Frost. D. C.: Ishltanl. A,; Md)owell. C. A. Mol. phys. 1072. 24, 861-877. (17) soeda. F.; lahltanl, A.. unpub1)shed data. (18) Cleenler. R. G.:RBhn. P. R.; Schwa&. J. P. J. Catai. 1071. 23,42. (19) Poling. G.W. J . Elscbodxm. SOC. 1960. 118,958-963. (20) Boerio. F. J.; Armogan. L. Appf. Spec!nx. 1078.32,509-510. (21) k l o . F. J.; Chen, S. L. Am/. Spectmsc. 1070. 33. 121-128.
R e c m for review August 26,1981. Accepted December 28, 1981.
