2408
Anal. Chem. 1980, 60,2408-2412
aqueous standard reference solutions. Registry No. Ti, 7440-32-6; Fe, 7439-89-6; stainless steel, 12597-68-1;steel, 12597-69-2.
LITERATURE CITED (1) (2) (3) (4)
Adams, D. F. Anal. Chem. 1948, 2 0 , 891. Beevers, J. R.; Breyer, B. J . €lectroenal. Chem. 1959, 1 , 39. Hoff, H. K.; Jacobsen, E. Anal. Chim. Acta 1971, 5 4 , 511. Donoso N. G.;Chadwick W. I.; Santa Ana V. M. A. Anal. Chim. Acta 1975, 77, 1. (5) Yamamoto, Y.; Hasebe, K.; Kambara, T. Anal. Chem. 1983, 5 5 , 1942. (6) Wibms, G. J.; Palmans, R. A.; Coiard, J.; Ducheyne, P. Analusis 1984, 12. 443. (7) Rat, J. C.; Burnel, D. Ann. falslf. Expert. Chim. Toxicol. 1985, No. 834, 13. (8) Gemmer-Colos, V.; Neeb, R. Naturwissenschaften 1988, 7 3 , 498.
(9) Thomerson, D. R.; Price, W. J. Analyst (London) 1971, 96, 825. (10) Walsh, J. N. Analyst (London) 1977, 102, 972. (11) Boumans, P. W. J. M.; De Boer, F. J. Spectrochim. Acta, Part B 1972, 278, 391. (12 ) Inductively Coupled Plasma Emission Spectromeby; Fuwa, K.; Haraguchi, H., Eds.; Nankodo: Tokyo, 1980; p 115. (13) Spectrochlm. Acta. Part B 1978, 338, 219. (14) Locatelll, C.; Fagioli, F.; Blghi, C.; Garai, T. Talanta 1985, 32,539. (15) Locatelll. C.; Fagioli, F.; Bighi, C.; Garal, T. Ann. Chim. (Rome) 1986, 76, 255. (16) Locatelli, C.; Fagioli, F.; Garai, T.; Bghi, C.; Vecchietti, R. Anal. Chim. Acta, In press. (17) Lleanu, C.; Popescu, I . C.; Hopirtean, E. Anal. Chem. 1976, 4 8 , 2010. (18) Riley, J. P. Anal. Chim. Acta 1958, 19, 413.
RECEIVED for review January 20,1988. Accepted June 1,1988.
Further Developments in Photoacoustic Fourier Transform Infrared Spectroscopy: Temperature Studiest Marek W.Urban* North Dakota State University, Department of Polymers and Coatings, Fargo, North Dakota 58105 J a c k L. Koenig Case Western Reserve University, Department of Macromolecular Science, Cleveland, Ohio 44106
Fourler transform Infrared photoacoustlc spectroscopy has been applied to determine the temperature effect on the photoacoustic slgnai in slllca. The lntenslty of the slgnai due to SCQ-SI vibrations Is not affected by temperature. Only the lntenslties of the vlbratlonal bands of the surface species adsorbed on the slllca surface become weaker. This effect Is attributed to smaller surface coverage of the specles adsorbed on the surface. These studles suggest that between 5 and 65 O C carbon monoxide Is adsorbed on the slika surface and oxldires to carbon dioxlde at higher temperatures. Possible sources of enhancement of the photoacoustic signal are also discussed. The presence of highly polarizable xenon as a couplkrs gas in the phatoacowrtk cell does not cause the resonance effect,and the enhancement d I R bands Is related to the orlentatlon of the surface species.
polarizable xenon coupling gas was used. In some cases the enhancement exceeded intensities of the vibrational bands of the bulk. For example, in the PA FT-IRspectra of the silica powder, the band at 1991 cm-' becomes more intense than that due to the Si-0-Si vibrations a t 1100 cm-' (5). The former was assigned to carbon monoxide adsorbed on the silica surface. In several papers the approach of a highly polarizable inert gas as a coupling medium in the photoacoustic cell was applied to estimate the orientation of small and large molecules on surfaces (6-10). Although we have tried to justify this approach by using Dignam theory (11,12),there are still some experimental issues to be tested. One of these issues is the possibility of a resonance effect in the photoacoustic cell when highly polarizable xenon is used as a coupling gas. In this work we examine possible sources of enhancement and how temperature affects the intensity of the bands enhanced in the presence of xenon.
Although there are several well established surface Fourier transform infrared (FT-IR) techniques, the application of photoacoustic (PA) FT-IR spectroscopy for certain sampling situations can be advantageous. Improved signal-to-noiseratio and other overwhelming advantages such as capability of surface depth profiling or capability to study opaque samples make this technique a useful tool in both basic and applied research. A future possibility of using photoacoustic detection with Hadamard transformation is even more promising (1). Several advantages of PA FT-IR spectroscopy were summarized in various review articles (2-4). One of the recently reported applications of PA FT-IR was the capability of determining the orientation of surface species (5). Perhaps a major advantage of this approach was its simplicity and the fact that the intensity enhancement of the modes due to the species parallel to the surface was very strong when highly
EXPERIMENTAL SECTION Silica gel powder (28-200 mesh, chromatographic grade) was purchased from J. T. Baker. Photoacoustic FT-IR spectra were recorded on a Digilab FTS-1OM spectrometer equipped with a photoacoustic cell (Digilab). Each spectrum was obtained with a moving mirror velocity of 0.3 cm/s and by averaging 400 scans at a resolution of 4 cm-'. The frequencies were calibrated to an accuracy of better than 0.01 cm-' with an internal He-Ne laser. All spectra were ratioed against a carbon black powder (Fisher Scientific Co.) single-beam spectrum, recorded with the same mirror velocity and at the same temperatures as the sample spectra. The sample was placed in a 3 mm deep sample cup and purged inside the cell with high-purity (99.999%)helium or xenon gases (Union Carbide Corp.) for 10 min before scanning at a gas pressure of about 0.2 psi. The photoacoustic cell was stabilized at the temperature of the sample compartment for approximately 1h prior to Fist use. Photoacoustic FT-IR spectra at elevated temperatures were obtained by using a photoacoustic cell specially designed for that purpose. Figure 1 shows a schematic of a custom-built sample compartment with a built-in heater (Watlow, Inc.) and a ther-
Work performed at North Dakota State University, Department of Polymers and Coatings. 0003-2700/88/0360-2408$01.50/0
0 1988 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 60, NO. 21, NOVEMBER 1, 1988
IRI
A
2409
n
PA-FT-IR COUPLING GAS: HELIUM
D
C
Flgure 1. High-temperature photoacoustic FT-IR cell (modified Digilab cell): A, KBr window: B, sample compartment; C, brass cube; D, gas inlet; E, microphone coupling; F, thermocouple; G, heater; H, cooling system.
mocouple. In order to avoid overheating of the microphone compartment and to achieve good temperature stability, the sample cube was water-cooled. The cooling system was built underneath the heating element, as shown in Figure 1. The temperature control unit allows coverage of a temperature range from 0 to 600O C with an accuracy of A 1 "C. A detailed description of the sample cube is given in the figure captions. Prior to each run a thermostat was set at a desired temperature and the cell was equilibrated for approximately 15 min.
RESULTS AND DISCUSSION Figure 2 shows a series of photoacoustic FT-IR spectra of silica obtained at various temperatures ranging from 5 to 103 "C by utilizing helium as a coupling gas. Trace A of Figure 2 shows the spectrum recorded a t 5 "C. Two intense bands at 1100 and 3500 cm-' are observed. Because the amount of the surface adsorbed species is temperature-dependent and consequently will affect the band intensities in the infrared spectrum, it is necessary to make a distinction between the modes associated with molecules adsorbed on the surface and those due to the bulk. The band at 1100 cm-' is assigned to the Si-0-Si lattice mode (bulk) whereas the 3500 cm-l band is due to water molecules adsorbed on the silica surface. Because of the high thermal stability of silica between 5 and 103 "C it is appropriate to use the Si-0-Si lattice mode as a probe to examine the temperature effect on the intensity of the photoacoustic signal generated from the bulk. Figure 3A depicts integrated intensities of the Si-0-Si band plotted as a function of temperature. The intensity remains constant in the examined temperature range. This result is somewhat surprising, since the RG theory predicts that the temperature increase should lead to a decrease of the photoacoustic signal (13). This process is mathematically described by (PA)
TO
where X' is the gas thermal conductivity, p o is the pressure in the photoacoustic cell, and Tois the temperature of the gas. This equation is valid only for the gaseous species. However, at higher gas temperatures, a pressure builds up in the cell. Examination of eq 1 shows that both the temperature and pressure effects cancel one another out, and therefore the net photoacoustic signal of the Si-0-Si lattice vibrations does not change. The situation might vary however if the solid sample had phase transitions in the studied temperature range. A broad OH stretching mode dominates the 3500 cm-l region. As the temperature of the sample increases, this band decreases in intensity. Apparently, elevated temperature leads to the removal of water and surface hydroxyl groups from the silica surface, resulting in weaker IR absorption bands. The spectrum recorded at 103 O C shows negligible traces of water
. 3600
2800
.
. 2000
I200
WAVENUMBERS (cm
Flgure 2. PA FT-IR spectra of silica (coupling gas, He) recorded as a function of temperature: A, 5 OC; B, 21 OC; C, 33 OC; D, 45 OC; E, 55 OC; F, 88 OC; 0, 103 OC.
TEMPERATURE Figure 3. Integrated intensity as a function of temperature: A, 1100 cm-' Si-0-Si band; B, 3500 cm-' water band.
(Figure 2G). It should be mentioned that no gas-phase H 2 0 is observed in the spectra, since prior to each measurement the cell was flushed with helium. Integrated intensity of this band, plotted as a function of temperature, is shown in Figure 3B. It is interesting to note that at temperatures between 20 and 70 "C, there is a distinct drop of intensities, indicating desorption of weakly bonded water molecules from the surface. The intensity drops again above 100 "C, where almost all hydroxyl and water molecules are released from the silica surface. Recently, it has been shown that the use of highly polarizable xenon gas as a coupling medium in the photoacoubtic cell makes it possible to detect and determine orientation of
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ANALYTICAL CHEMISTRY, VOL. 60, NO. 21, NOVEMBER 1, 1988
the species adsorbed on the silica surface (5). We have demonstrated that the band at 1991 cm-' is enhanced in the presence of xenon and diminishes in intensity when silica powder is outgassed at elevated temperatures. This band was assigned to carbon monoxide adsorbed on the silica surface. The assignment was based on the vibrational frequency at which this mode absorbs in the infrared region as well as on previous studies. For example, Blyholder et al. (14) have found that when COz was admitted to a silica-supported iron, a new band a t 1960 cm-' was found in the spectrum, which the authors assigned to carbon monoxide chemisorbed on the surface as a result of dissociation of C02. Similar to our previous experiments, the cited data were obtained a t room temperature. If the 1991 cm-l band is due to the species adsorbed on the silica surface, one would expect both the intensity and vibrational frequency to be temperature-dependent. Thus, in an effort to understand the origin of this band it is useful to perform in situ measurements and determine the temperature effect on the 1991 cm-' band. In addition, one would like to examine other origins of the enhancement of this band in the presence of xenon coupling gas. Postponing the discussion of the temperature effect on the xenon-coupled PA FT-IR spectra until later, let us consider other possible sources of enhancement of the photoacoustic signal. A question is raised as to whether the enhancement of the 1991 cm-l band in the presence of highly polarizable xenon is indeed related to the orientation effect of the surface species. Although there are several factors that may influence the photoacoustic signal intensity in a given frequency range, two categories can be distinguished: (1)factors that affect the photoacoustic signal in the entire spectral range and (2) factors that selectively enhance only a narrow region, usually about 500 cm-'. Within the first category the most important factor is the internal cell volume. As has been shown before, the magnitude of the photoacoustic signal is inversely proportional to the cell volume (15). Therefore, it is beneficial to work with small cell volumes. Another important factor is the distance between the window and the sample. If this distance is less than the thermal diffusion length of the gas, heat generated a t the sample will be lost to the window, resulting in a decrease of the photoacoustic signal. Usually, separation of the window and the sample by a distance of 2 mm is sufficient to prevent this effect with most coupling gases for modulation frequencies above 10 Hz. A major cause of the selective signal enhancement (category 2) is usually related to the geometry of the cell. If a cell design is such that the sample compartment is separated from the microphone compartment, signal intensity can also be lost due to the acoustic damping in small channels connecting the two chambers. This damping results from viscous friction in the gas and heat losses to the walls of the connecting channel during sound transmission. A cell used in our current and previous studies incorporates this feature. Such a design (two chambers: the sample and the microphone connected by a small-diameter tubing) may produce a resonance effect that leads to the enhancement of the photoacoustic signal in a certain frequency range. This enhancement is due to a socalled Helmholtz resonance and is a function of the cell parameters as well as the coupling gas used in the photoacoustic experiment. Figure 4A shows the PA FT-IR spectrum obtained by using xenon as a coupling gas. As was shown in our earlier work, strong enhancement of the bands in the 1991 cm-' region is observed. One might attribute this effect to a Helmholtz resonance, since the velocity of acoustic waves propagating through xenon is different from that in helium. Thus, in order to examine the enhancement due to the Helmholtz resonance, the cell resonance frequencies must be estimated.
n
PA-FT-IR COUPLING GAS: XENON
, )600
,
,
,
2800
. 2000
,
,
,
I200
WAVENUMBERS (an-')
spectra of silica (coupling gas, Xe) recorded as a function of temperature: A-G defined in Figure 2. Figure 4. PA FT-IR
The Helmholtz resonance in PA cells has been discussed by several authors (16-18), and its frequency is related to cell parameters by
where C is the speed of sound, Vl and V2are the volumes of the sample and the microphone chambers, and L and r are the length and radius of the connecting tube. In order to estimate the F, values for the spectra recorded in helium and xenon, one needs to know the cell parameters and the speed of sound for the respective gases. Since the cell parameters are not provided by the manufacturer, we measured the Vl, V,, L , and r values. They are as follows: V , = 1 cm3, V2= 1 cm3, L = 2 cm, and r = 0.1 cm. While the speed of sound experimentally determined for helium is 1238 cm/s (CRC), its value for xenon is not available. However, it can be estimated by using the probable speed equation
(3) where R is the gas constant, T i s temperature (K), and M is atomic mass. At room temperature, the estimated velocity of sound in xenon is about 194 cm/s. For comparison purposes, the estimated value for helium is 1567 cm/s. Although both values suffer an error since eq 3 is derived with the assumption of an ideal gas behavior, they can be used to estimate the Helmholtz resonance frequencies in the respective gases. Using eq 2, we find that the Helmholtz frequency for helium is about 42 Hz whereas for xenon it is about 5 Hz. The spectra presented here and in our previous work were obtained
ANALYTICAL CHEMISTRY, VOL. 60, NO. 21, NOVEMBER 1, 1988
2411
-
P A FT-IR
A
I
0
P
ro
W
80
'
.-
im
'a
m A T U R E Figure 6. Integrated intensity (B) and a frequency shitt (A) of the 1991
cm-' band plotted as
00
?ZOO
2UOO
I600
.
ao
WAVENUMRERS
Figure 5. PA FT-IR spectra: A, He; B, He (long connecting tube); C,
Xe;
D,Xe (long connecting tube).
with a mirror velocity of 0.3 cm/s, so that the wavenumber range (4000-600cm-') corresponds to the acoustic frequency range 360-2400 Hz. The F,values obtained for helium and xenon do not fall in this region. Therefore, the observed enhancement in the presence of xenon is not caused by a resonance effect. In order to provide experimental evidence for a nonresonance effect in the studied frequency range, the geometry of the photoacoustic cell was modified in such a way as to increase the length of tubing connecting the sample and microphore chambers by 20 ( L value in eq 2). According to eq 2, such geometry should effectively shift the Helmholtz frequency approximately by a factor of 3. Thus, if the enhancement of the 1991 cm-' band in xenon is due to Helmholtz resonance, one would expect to observe a shift of this band. Figure 5 shows photoacoustic FT-IR spectra obtained with ordinary and modified cell geometries when helium (traces A and B) and xenon (trace C and D) are used as the coupling gases. A comparison of the spectra obtained with helium indicates that the only apparent difference between spectra recorded with ordinary and modified geometry is a very poor signal-to-noiseratio of the latter. A poor signal-to-noiseratio results from a larger cell volume and the external vibrations to which the modified cell is sensitive. Nevertheless, the general shape of the bands characteristic of silica is maintained. A similar observation is made for the spectra obtained with xenon as a coupling gas (Figure 4, traces C and D). Again, a modified geometry of the cell gives very poor signal-to-noise ratio, but both spectra obtained with xenon as a coupling gas are the same. In particular, the enhancement of the 1991 cm-' band is maintained, indicating that the presence of highly polarizable xenon in the photoacoustic cell is not responsible for the resonance effect.
a function of temperature.
At this point, the temperature-induced changes of the 1991 cm-' band as well as the Si-0-Si vibrations at 1100 cm-' in the presence of xenon (Figure 4) can be reexamined as a function of temperature. Similar with respect to the heliumcoupled spectra, the intensity of the Si-0-Si band remains unaffected by elevated temperature. On the other hand, examination of the PA FT-IR spectra recorded at higher temperatures (Figure 4) indicates that the 1991 cm-' bnd shifts to a higher vibrational frequency and, at the same time, successively diminishes in intensity. These effects are interrelated: as temperature is raised, it is expected that this band would become weaker because of a lesser amount of the surface species present and, as a result, different interactions between the species. In view of the conclusions from the previous section, the above observation suggests that this band is indeed due to the species adsorbed on the silica surface. Figure 6 illustrates the relationship between integrated intensities and wavenumber of the 1991 cm-' band plotted as a function of temperature. While the observation that the intensity of this band diminishes with temperature can be justified, a frequency shift is somewhat more complicated; in particular, it is difficult to rationalize the possibility of the CO species at vibrational energies corresponding to COz. In an effort to explain the wavenumber shift of CO adsorbed on the surface of ZnO, it has been postulated that the high-energy bands may be due to species such as CO+ or C 0 2 + (19). The basis for such an assignment was attributed to the process of the thermal treatment of oxides, which above 100 "C induces defect structure on the surface, producing a high concentration of conduction electrons. They, in turn, may lead to an electron-transfer-adsorption mechanism resulting in the formation of CO+ and C 0 2 + species. In addition, the formation of twofold bridges or linear structures on the surface is possible, since the observed vibrational bands fall within the spectral region corresponding to these structures. Perhaps one would adopt the above structures if the band originating at 1991 cm-' shifted as high as 2170 or 2115 cm-l, since these wavenumbers correspond to R- and P-branches of the gaseous carbon monoxide. As shown in Figure 6, the 1991 cm-' band shifts upward to the wavenumbers corresponding to physisorbed carbon dioxide. While at low temperatures the band originating at 1991 cm-' can be assigned to some form of carbon monoxide, at higher temperatures there are other species present on the SiOzsurface. Close inspection of Figure 6 shows that at about 65 O C the intensity of the band drops drastically, the band broadening is observed, and the vibrational energy shifts upward by approximately 150 cm-'. The intensity decrease and broadening are perhaps related to a smaller amount of the species on the surface and a thermally induced broader population of the surface vibrational states. On the
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ANALYTICAL CHEMISTRY, VOL. 60, NO. 21, NOVEMBER 1, 1988
other hand, the wavenumber shift, even above that of CO, suggests the presence of carbon dioxide on the surface. The 1991 cm-’ band shifts successively up to the energy corresponding to free COz,and a further temperature increase does not affect its vibrational energy. Instead, its intensity is diminished, which suggests a thermally activated desorption process. In light of the above considerations these data indicate that in the temperature range 5-65 “C carbon monoxide is adsorbed on the silica surface whereas at higher temperatures carbon dioxide is present. Although several mechanisms could be proposed, oxidation of CO to COz seems to be the reasonable one. A similar mechanism was suggested for adsorption of CO on Rh/AlZ0, catalyst (20-22). It should be noted that the transition at 65 “C corresponds to a break point in Figure 3B where the intensity of the 0-H stretching mode of water is plotted as a function of temperature. Although one could speculate whether oxidation of CO and desorption of water molecules are interrelated effects, more experimental data is needed to understand and correlate both processes with one another. There are several factors that may contribute to the overall mechanism. In an effort to understand temperature- and surface-coverage-dependent effects, several theoretical and experimental (23-26) studies showed that not only dipole-dipole coupling between neighboring dipoles contributes to the interactions on the surface but the electrostatic and chemical effects may play a role. At this point it is certain that COzadsorbed on the surface is not in the form COO- since one would observe symmetric and asymmetric vibrational modes in the 1600-1400 cm-’ region. In the temperature range 5-65 “C carbon monoxide is adsorbed on the surface and a t higher temperatures undergoes oxidation to form COz. These results are in agreement with the studies by Yates et al. (21).
CONCLUSIONS This work demonstrates that photoacoustic FT-IR measurements above r c ” temperature provide useful information about the species adsorbed on the surface. Elevated temperatures do not influence the signal-to-noise ratio, which is maintained throughout the studied temperature region. Apparently, the emission from the sample surface is not a frequency-modulated signal and therefore is not detected by the instrument. It is expected that at higher temperature the Brownian motion would diminish the signal-to-noise ratio. The band intensities of the Si-0-Si lattice modes are not affected by temperature. Only vibrational modes due to the species adsorbed on the surface are diminished. These studies suggest that below 65 “C carbon monoxide is adsorbed on the
silica surface, whereas at higher temperatures carbon monoxide undergoes oxidation to form carbon dioxide. The Helmholtz resonance in the photoacoustic cell has no effect on the enhancement of bands when xenon is used as a coupling gas.
ACKNOWLEDGMENT Alan P. Bentz of the USCG Research Labs (Groton, CT) is acknowledged for providing an FTS-1OM spectrometer. Registry No. SOz, 7631-86-9; CO, 630-08-0; COz, 124-38-9; Xe, 7440-63-3; He, 7440-59-7. LITERATURE CITED (1) Hammaker, R. M.; Graham, J. A.; Tillottaand, D. C.; Fateley, W. G. VihNOnel Spectm end Struckre; Dwig. J. R., Ed.; Elsevier: Amaterdam, Holland, 1986; Vol. 15. (2) Urban, M. W.; Koenig, J. L. Vi6ratbnel Spectra and Structure; Durig, Ed.; Elsevler: Amsterdam, Holland, 1987; Vol. 18. J. I?., (3) Graham, J. A.; Grim, W. M., 111; Fateley, W. 0. Fowler Transform Infrered Specb-oScqDy,App~Uonsto cimnlcal Systems;Ferraro. J. R., and Baslle, L. J., E&.; Academic: 1985; Voi. 4. (4) Urban, M. W. J . Coatings Tedmd. 1987, 50(745), 29. (5) Urban, M. W.; Koenig, J. L. Appl. Spectrosc. 1985, 39, 1051. (6) Urban, M. W.: Koenlg, J. L. Appl. Speclrosc. 1888. 4 0 , 513. (7) Urban, M. W.; Koenlg, J. L. Appl. Specirosc. 1988, 4 0 , 851. (8) Urban, M. W.; Chatzl. E. G.; Perry, B. C.; Koenig. J. L. Appi. Spectrosc. 1988, 40. 1103. (9) Chatzi. E. G.; Urban, M. W.; Koenig, J. L. Macromol. Chem. fhys., Suppl. 1988, 5 , 99. (10) Chatzl, E. 0.; Urban, M. W.; Ishlda, H.; Koenig, J. L. Pokmer 1988, 27, 1850. (11) Roth. J. 0.; Dignam, M. J. Can. J . CY”. 1978, 5 4 , 1388. (12) Dig”, M. J.; Reo, B.; Roth, J. R. J . Chem. Soc., Faraday Trans. 2 1973, 60, 80. (13) Rosencwaig, A. photoacoustlcs andphotoacousfk Spectroscopv; Wiley: New York, 1980. (14) Slyholder, Q.; Neff, L. D. J . f h y s . Chem. 1982, 66, 1684. (15) Aamcdt, L. C.; Murphy, J. C.; Parker, J. G. J . Appl. fhys. 1977, 48, 927. (16) Ouincy, R. S.; Seizer, P. M.; Yan, M. W. Appl. Opt. 1977, 16, 223. (17) Fenellus, N. C. Appl. Opt. 1979, 78, 1784. (18) Nordhaus, W. J . Appl. fhys. 1081, 25, 221. (19) Tayler, J. H.; Amberg, C. H. Can. J . Chem. 1861, 39, 535. (20) Yang. A. C.; 5ahnd, C. W. J . phvs. Chem. 1957. 67, 1504. (21) Cavanagh, R. R.; Yates, J. T., Jr. J . chsm. fhys. 1981, 74, 4150. (22) Smtth, A. K.; Hugues, F.; Theoller, A.; Basset, J. M.; Ugo, R.; Zanderighi, G. M.; Bllhou. J. L.; Bllhou-Bougnd, V.; Graydon, W. F. Inorg. Chern. 1979, 18, 3140. (23) Pritchard, J. Vibrations h AdsorbedLayers, Conferem Records Series of KFA; Ibach, H., Lehwald. S., Eds.; KFA: Juelich, FRG, 1978; p 114. (24) Persson, B. N. J. J . fhys. C 1978, 7 1 , 4251. (25) Brhrlo, 0. P.; (Limby, T. B. J . pnvs. C 1977, 70, 2351. (26) Persson, B. N. J.; Persson, M. SOlM State Commun. 1980. 36, 175.
RECEIVED for review October 22,1987. Accepted March 28, 1988. M.W.U. expresses sincere thanks to the 3M Company and the National Science Foundation (North Dakota EPSCOR program) for partial support of this work.
