3838
J. Phys. Chem. 1993,97, 3838-3841
Evidence for the Formation of a Delocalized Anion: CARS Spectroscopy of 2,4-Pentadione Adsorbed on Planar ZnO Optical Waveguides W. M. K. P. Wijekoon,'*t W. M. Hetherington, 111, and W. R. Salzmant Department of Physics, Oregon State University, Corvallis, Oregon 97331 -6507 Received: December 28, 1992 The C-C-C deformation vibrational frequency of adsorbed 2,4-pentadione on a planar ZnO (0001) surface was obtained by using waveguide surface coherent anti-Stokes Raman scattering spectroscopy. The CARS signal from the adsorbed layer was enhanced by establishing an interference condition within the ZnO waveguide to minimize the CARS contribution from the waveguide and from the substrate. Several different forms of surface-bound species, namely, delocalized anion, diketone, and enol form, were identified. Diffusion of chemisorbed species among the chemisorption sites was observed as the surface coverage was reduced. Zn2+ ions on the surface seem to facilitate the chemisorption sites for pentadione molecule.
Introduction A thorough understanding of the interaction of adsorbates with metal oxide surfaces is important in many aspects of surface chemistry.' Catalysis, integrated optical circuitry, and semiconductor technology are a few branches of scientific research where adsorbate-substrate interaction plays a predominant role. Vibrational spectra of adsorbates provides the most conclusive information about the nature of adsorbatesurface interaction.2 A number of viable techniques have been developed in the past two to threedecades to identify the vibrational spectra of surfacebound species, but many are not surface specific or do they apply to a variety of chemical situations.) The problems encountered are 3-fold. First, both IR absorption and spontaneous Raman scattering havesufficient spectral resolution but lack the necessary sensitivity for monolayer detection. For powder samples surface roughness prevents characterization by Auger or photoelectron spectroscopies as well as cleaning by ion sputtering. Electron energy loss spectroscopy is very sensitive, but it lacks spectral resolution and must be performed under UHV conditions. Second, IR and Raman suffer from interfering signals from the bulk oxide. Third, these techniques are not sufficiently surface specific to be generally useful over the full range of interesting chemical environments. Waveguide surface coherent anti-Stokes Raman scattering (WSCARS) is an attractive solution to many of the above problem^.^ The concept involves three-wave mixing with the evanescent fields above an optical waveguide. By use of the planar optical waveguide geometry, efficient generation of CARS signals has been observed in polymer and metal oxide waveguide^.^ The anticipated surface sensitivity and spectral resolution were demonstrated by the detection of a WSCARS spectrum of ethylene adsorbed on a ZnO surface.6 Identification of adsorption sites for pyridine on hydroxylated ZnO surface clearly showed the value of WSCARS as a surface spectro~copy.~WSCARS has also been used to monitor live chemical reactions and photogenerated transient molecular specie^.^.^ In this work, we utilize the unique attributes of WSCARS to obtain the vibrational frequencies of 2,4-pentadione adsorbed on a planar ZnO (0001) surface. The chemistry of 2,Cpentadione makes it an interesting candidate for WSCARS investigations.I0 The molecule can be retained on a surface in several ways. The neutral diketone can interact with the surfacevia two keto groups, acting as a bidentate
* Addresscorrespondencetothisauthor at Photonics Research Laboratory, Department of Chemistry, The State University of New York at Buffalo, Buffalo, N Y 14214. Department of Chemistry, University of Arizona, Tucson, A Z 85721.
ligand." If the enol form binds with a substrate, the molecule acts as a unidendate ligand.I2 Also the molecule can bind with a metal atom through the y-carbon atom, forming a metal-carbon bond.') In addition, 2,4-pentadione can be retained on a surface in theformofthedelocalizedanion. In thissituation thediketonic counterpart is attached to the substratevia two oxygen atoms.l+'6 At room temperature, about 80% of 2,4-pentadione exists as the enol in liquid." However, on a substrate the keto/enol ratio changes drastically, depending on the nature of the substrate as well as on the surface coverage. For example, on fused silica surfaces both the keto and enol isomers exist, but the majority of the adsorbates are in the keto form.18-20The keto/enol ratio is always higher for the molecules adsorbed initially than for thoseadsorbed later, indicating that chemisorption is predominant at low surface coverages.19 The selection of the C-C-C deformation vibrational frequency of 2,4-pentadione molecule for WSCARS investigations is based on the following considerations. The vibrational spectra of 2,4pentadione and zinc acetyl acetonates are well-known, and, therefore, the assignment of WSCARS band can be made very easily.1° The structural changes of the molecule (different binding modes) upon adsorption are reflected in the variation of this vibrational frequency.21.22Also the vibrational frequency occurs in an energy range (-600-700 cm-I) where many of the other existing surface vibrational probes suffer from interfering signals from the bulkoxides.2.) Furthermore, thechangein thevibrational energy of the mode for different adspecies, such as enol, diketone, and anion, falls within a relatively narrow spectral range that could be investigated by a single spectral scan without changing either the coupling angles or the phase matching angle. Theory The WSCARS geometry in which laser beams are coupled into and out of a thin-film optical waveguide is shown in Figure 1. The waveguide structure consists of a silica substrate of refractive index nsubs, a film (thickness = 1 pm) of index nnl,,,,and a superstrate of index nsup.When nhlm is greater than nsubs and nrup,a field will be constrained by total internal reflection and will propagate through the film as a guided wave. The films can be made of transparent materials such as polymers and oxides. To obtain a CARS spectrum of the monolayer or the surface, fields at ul and o2are coupled into the film through an opticallycontacted prism and mixed to excite a dipole in the molecule at the beat frequency, A = w I-02. Thesecond w~ photon isscattered by this induced dipole and the field at @CARS = A + wl,which is generated as a guided wave, is coupled out of the film through another optically contacted prism. Coherent Raman scattering benefits from the waveguide geometry to a much greater extent
o o ~ ~ - ~ ~ s ~ ~ ~ ~ ~ 0 ~ 1993 o ~ American ~ - ~ ~Chemical ~ a ~ Society o 4 . o o ~ o
The Journal of Physical Chemistry, Vol. 97, No. 15, 1993 3839
Formation of a Delocalized Anion Y
f fa'
0.
1
Y
z
f
by the proper choice of modes for the fields of W I , w2, and @CARS. When the susceptibilities for the three regions are written as xFilm, (3) and xlibs for the monolayer, film, and substrate, xmono, (3) respectively, F = Frxrr becomes
By choosing TE modes for the three frequencies such that the product fJJcARS is approximately antisymmetric across the film, (4)
X
Figure 1. Planer waveguide geometry used for this experiment. The optical waveguide structure consists of a 0.6-pm ZnO film on a fused silica substrate. The prisms are used to couple the laser fields into and out of the film.
than spontaneous Raman scattering. This geometry intensifies the laser fields by virtue of the small waveguide thickness, a fact that allows CARS to occur with relatively little laser power. Furthermore, since CARS is quadratic in the interaction distance of the two beams, the long overlap region (>I cm) within the waveguide leads to very high conversion efficiencies. The theory of general three-wave mixing in optical waveguide structures has been developed thoroughly, and only the details pertinent to this study will be p r e ~ e n t e d .On ~ the basis of the theory of guided waves, there are discrete allowed modes of TE (transverse or x-polarized electric field, in our experimental geometry) and T M (transverse or x-polarized magnetic field) polarization^.^^ Guided waves are characterized by an internal reflection angle 8, an effective refractive index nGlmsin 8, and an effective wave vector 8 = w/c(nmmsin 8). The coordinate system for a waveguide of thickness 2h is defined such that the substrate extends from -- to -h, the film from -h to h, and superstrate from h to+=,although in this workthethicknessofthesuperstrate is measured in angstroms. The evanescent fields can extend above the film to a distance of several hundered angstroms, and they can be used for interfacial nonlinear optical spectroscopy. Using coupled mode theory, the intensity in the guided wave at wCARS is
and the field at WCARS arises mainly from the monolayer and the substrate only. In practice, the contribution from the substrate (usually fused silica) is also small compared to that from the monolayer. When modes TEI, T E I , and TE2 are used for the fields at wI,02, and WCARS, respectively, the selection is referred to as the 112 combination. Ignoring for the moment all but the 1F12 factor in eq 1, the total WSCARS signal is ZCARs
= IAI2 + AB*
+ A*B + lB12
(5)
where
When A is small, the surface signal may appear through the AB cross terms. If the thickness of a waveguide is carefully chosen, the ratio of the surface to waveguide background signals, R = (A2 2AB)/B2, can be larger than 10006.The effect of a phase mismatch isgiven by the function (2 ~ i n ( 4 / 2 ) / 9 ) ~The . relatively slow variation in this function usually does not introduceconfusing modulations into a spectrum, especially when the crossing angle can be adjusted during a spectral scan. It is important to include the dispersion of the waveguide material in the calculation of this function. When the beams are focused into the coupling prisms, the spread in wavevector of the two input beams will significantly reduce the depth of modulation. The effect of phase mismatch has been discussed previously.6
+
Experimental Aspects
where L is the interaction distance within the waveguide, Cis a collection of constants and approximate constants, and 4 = AP-Lz is the phase mismatch where A8 = &ARS - 281 + 82. The polarization vectors of the three fields are t l , t?, and €CARS. Labeling the distribution function for the field at wi as f;, the tensor F is
=~
~ ~ ~ ' 3 ' ( z ~ ~ : ( Z ~ f 2 ( z dz ~ f ~ A R S ( (2) Z )
The third-order susceptibility, ~ ( ~ contains 1 , vibrationally resonant and nonresonant (electronic background) terms. When T E modes are used for frequencies wI, w?, and WCARS, only x ~is measured. ~ ~ x Since only T E modes were used for the WSCARS spectra presented here, all future references to this scalar are labeled as ~ ( 3 ) . Other tensor elements can be measured by using a combination of T E and TM modes. For the following discussion of WSCARS signals, constant (independent of A = 01 - 02) nonresonant values of are assumed for the substrate and the film, and a large resonant value is assumed for a superstrate consisting of a single adsorbed monolayer. When the total WSCARS signal from the waveguide structure is calculated, it is found that, although the monolayer signal is large and detectable, the signal from the nonresonant susceptibility of the film is generally much larger. The contribution to the WSCARS signal from the film can be suppressed
The ZnO optical waveguide of thickness of 0.60 f 0.03 pm was deposited onto a polished fused silica substrate using RF magnetron ~ p u t t e r i n g .The ~ ~ input and output SrTiO3 coupling prisms were held against the surface of the waveguide by means of a pressure applicable aluminum structure that could be adjusted to a desired tightness to obtain a good optical contact. Both input beams were focused with a 300-mm lens onto the area of optical contact at the edge of the input prism. The beams at W I and w2 were incident on the prism with small angular separations in both xz and y z planes. The coupling angles in the xz plane (defined as the angle below the normal to the appropriate prism surface) for the modes used in this work were 17' for the TEI mode for wI, 16' for the TEI mode for w2, and -1.5' for the TE2 mode for WCARS. The coupling angles did not need to be changed during a scan over as much as 100 cm-I. This 1 12 mode combination was chosen on the basis of the geometrical constraints of the vacuum chamber system and the anticipated favorable ratio of the monolayer to background signals. The angle between the two input beams in the yz plane was -4'. A calculation predicted that the surface CARS process could not be phasematched with any choice of angle in the y z plane. Hence, the measured signal was considerably lower than for a phase-matched situation. The laser system used for this investigation consisted of two dye lasers that were synchronously pumped by the second
-
-
3840 The Journal of Physical Chemistry, Vol. 97, No. 15, 1993
-
I
015
032
649
060
083
700
RAYAN SHIFT (Au): cm-I
Figure 2. (a) WSCARS spectrum of a ZnO (0001) bare surface. (b) WSCARS spectrum of a ZnO (0001) surface after exposure to 30 mTorr of 2,4-pentadione. The sample was introduced at 7.5 X Torr.
t
015
AI....
632
649
660
RAMAN SHIR (Aw) cm
063
700
-I
Figure 3. (a) WSCARS spectra of
2,4-pentadioneadsorbed on a ZnO (0001) surface (a) with immediate active pumping, (b) after evacuation of the system for a period of I O h, at 8 X Torr, and (c) after evacuation followed by heating of the waveguide in the presence of oxygen at 70 O C and at 6.9 X 10 Torr. harmonic output of a cw Nd3+:YAG laser that was mode-locked and Q-switched at 500 Hz. The individual pulses within each pulse train were approximately 50 ps in duration and had a spectral line width of less than 2 cm-I. The approximate laser wavelengths used were 568 nm for wI (fixed) and 590 nm for w2 (tuned). The surface CARS signal generated in the desired mode was coupled out of the waveguide a t a specific angle, spatially filtered, passed through interference filters, and detected with a photomultiplier tube. When the power density at the surface of the waveguide was less than 10 MW cm-2, approximately 2000 photons per pulse train were detected. The CARS signal was normalized by dividing by Iil (using the second harmonic of wI generated in a KDP crystal) and Iu2. The spectrum presented here was obtained by averaging 1600 pulse trains every 0.2 cm-I. ZnO waveguide films are composed of densely packed crystallites oriented with all c-axes perpendicular to the film.26 This (0001) polar waveguide surface has the Zn2+ions on the surface, which are coordinatively unsaturated and more outwardly positioned than the layer of 02-ions, resulting in an outward pointing dipoles2' The surface roughness of the waveguide is approximately i 2 . 0 nm rms. The ZnOsurface wascleaned briefly by RFplasmaetching beforeit was placed in thevacuum chamber. Once in the chamber, the surface was cleaned by heating with IR and UV radiation from both the laser and lamps. Before introduction of the samples the system was evacuated to 7.5 X Torr while heating in the presence of oxygen, during which process the waveguide temperature reached as high as 100 OC. 2,CPentadione was purified by vacuum distillation and dried over molecular sieves. The WSCARS spectrum of 2,4-pentadione adsorbed on ZnO, Figure 2b, was obtained at a pressure of 30 mTorr. Spectrum a in Figure 3 was taken with immediate pumping. After evacuating the system for a period of 10 h at 8 X 10-6 Torr, spectrum b was generated. After the waveguide was heated in the presence of oxygen at 70 OC for a period of 2 h, spectrum c was obtained. This basic sequence is reproducible with respect to the spectral features and approximate time dependence for
Wijekoon et al. each feature. However, we were unable to regenerate the WSCARS spectrum of ZnO bare surface (Figure 2a) even after evacuation followed by heating of the system in the presence of oxygen. Both the extent of waveguide heating and the location of interaction area on the waveguide surface influenced the absolute and relative amplitudes. All the WSCARS spectra are drawn on the same vertical scale so that the relative surface densities of all the adspecies can be compared. All the spectra reported here had no dependence upon the laser intensity on the surface.
Observations and Discussion The vibrational spectra of 2,4-pentadione and its coordination complexes with metals have been well established.10~21 The C-C-C deformation vibration of 2,4-pentadione appears as a very strong band at 627 cm-I in the Raman spectrum, while it appears as a weak peak at 623 cm-I in the infrared spectrum. In the case of the 2,4-pentadione anion and its enol, this vibration is shifted toward higher energy and takes place at 630 cm-I and at 634 cm-I in the Raman spectrum, respectively. In the coordination compounds of 2,4-pentadione this vibration is shifted toward higher energy.I0 Figure 2b shows the WSCARS spectrum of 2,4-pentadione adsorbed on ZnO (0001) surface a t a pressure of 30 mTorr. The spectrum clearly indicates the presence of several different adsorbed species on the surface. The Raman peak centered at 670 cm-l arises from the u(Zn-0) G(ring) [u4] vibrations and the peak centered a t 654 cm-1is due to the u(C-CH3) u(Zn-0) [ u l J vibrations of the adspecies.I0 Appearance of these two Raman peaks in the spectrum confirms the formation of the delocalized anion of 2,4-pentadione on the ZnO surface. In addition to the delocalized anion, existence of the surface-bound keto and enol isomers and physisorbed adsorbates is evident in the WSCARS spectrum. The spectrum shows substantial inhomogeneous broadening. At large surface coverages the adlayer is not well ordered and the adsorption sites are evenly occupied. This condition leads to an inhomogeneous distribution of distances between interacting oscillators, resulting in asymmetric vibrational peaks.28 Further, vibrational coupling of adsorbates can bring additional broadening of the vibrational features.29 Vibrational coupling is restricted to the nearest neighboring resident and originates due to interaction of molecular wave functions of oscillating moieties. Both these phenomena can be eliminated or diminished by removal of surplus adsorbates. It is interesting to notice the vibrational activity in the higher energy edge of the WSCARS spectrum. Raman activity around 685 cm-l, which disappears after a brief evacuation, falls in the characteristic region for the G(ring) u(M-0) vibration of 2,4pentadione metal coordination compounds. For example, in the Raman spectrum of Al(CH3COCH2COCH3)3,the b(ring) u(M-O) vibration appears at 690 cm-I, whereas in Ga(CH3COCH2COCH3)3 it appears at 685 cm-1-30 In the case of C U ( C H ~ C O C H ~ C O C this H ~ )mode ~ appears at 684 cm-I in the infrared spectrum.31 This vibrational feature reappears in the WSCARS spectrum as soon as a new dose of 2,4-pentadione is introduced into the sample chamber. Therefore the peak at 685 cm-I in the WSCARS spectrumindicates the presenceof a weakly bound cyclic structure. This is not unusual since, at relatively higher surface coverages, both chemically and physically adsorbed species are present on the surface. This cyclic structure may originate from the interaction of diketone with residual surface hydroxyls.20 There is no quantitative estimation of the surface hydroxyl content on the ZnO waveguide surface that was employed for this experiment, but the effect of a surface-bound water layer on adsorbates has been previously addressed.32 WSCARS spectra in Figure 2 show that most of the weakly bound surface species leave the surface as soon as the chamber is evacuated. The coverage dependence of thevibrational features
+
+
+
+
Formation of a Delocalized Anion as well as the diffusion of the adspecies among the chemisorption sites was observed from the intermediate spectral scans. Survival of the vibrational features, even after evacuation of the chamber for 10 h at 8 X Torr (Figure 2, spectrum b), indicates that thechemisorbed molecules are firmly attached to the ZnOsurface. We assign the Raman peaks at 671 and 657 cm-1 in Figure 2b to the u4 and u 1 2 vibrations, respectively, of the 2,4-pentadione anion adsorbed of the surface. This assignment is based on the fact that in zinc acetyl acetonate complexes the corresponding vibrations occur at 666 and 651 cm-I, respectively. In addition, these vibrational frequencies are consistent with the u4 and u I 2 vibrational frequencies of 2,4-pentadione anion found on other metal oxide surfaces. On the A1203surface above two vibrations have been detected at 688 and 657 cm-I, respectively, and on the MgO surface those two vibrations has been reported a t 698 and 649 cm-’,respectively.16 The Raman peakat 637 cm-i is assigned to the C-C-C deformation vibration of chemisorbed enol, and the Raman peak at 624 cm-1 is assigned to the same vibration of surface bound diketone. The corresponding vibrations occur at 634 and 626 cm-I in the Raman spectrum of liquids. The existence of the delocalized anion indicates that the chemisorption takes place at a Zn2+ site on the surface. The formation of the delocalized anion removes a proton from 2,4pentadione. The released proton can combine with a surface hydroxyl group to form molecular water, which can be removed from the surface by subsequent evacuation. The removal of surface hydroxyls exposes more and more Zn2+sites available for chemisorption. Therefore, as evacuation proceeds, diffusion of the adspecies among the adsorption sites takes place, resulting in a well-ordered adlayer. Even though the coexistence of surface bound anion, enol, and diketone of 2,4-pentadione on the ZnO surface is clearly seen in the WSCARS spectra, the orientation of the adsorbates could not be determined since all three laser beams were polarized parallel to the waveguide surface. Zinc acetyl acetonate complexes have been studied t h ~ r o u g h l y . ~ ~In” [~Zn(CH3COCH2COCH3)2]3 complexes, the Zn atom is in an octahedral environment and is six coordinated, whereas in the case of Zn(CH3COCH2COCH3)2the Zn atom is in a tetrahedral surrounding.33~~~ In the former case, two distinct Zn-0 distances have been observed, namely, 2.108, for bonds from zinc to bridging oxygen atoms and 2.02 8, for bonds from zinc to nonbridging oxygen atoms. In addition to the above structures, fivecoordinated Zn atoms are found in the case of Zn(CH3COCH2COCH3)2.H20, where the Zn atom is approximately in a tetragonal pyramidal e n ~ i r o n m e n t .In~ ~this situation all the Zn-O distances differ from each other. The smallest distance is the same as the Zn-0 distance found in zinc oxide crystals (1.95 A) in tetrahedral arrangements. The morphology of the planar waveguide used for this experiment prohibits for formation of the above-mentioned structures on the surface. However, in the cases of delocalized anion and diketone, a single molecule of 2,4-pentadione can bind with a Zn2+ site as a bidentate ligand. Whether the resulting chelate possesses the CzVsymmetry cannot be conclusively determined from these data, since only T E modes were used. We believe that the chelate ring most probably is distorted due to the perturbations caused by the residual surface-bound hydroxyls.
Conclusions We have demonstrated the utility and the applicability of the WSCARS technique in studying the relatively lower vibrational frequencies of surface-bound molecules. The vibrational spectra of adsorbates can be obtained without interference from the substrate. Further the technique provides the flexibility to investigate the orientation of the adsorbates on a substrate if one performs WSCARS using permutations of the mixed modes. Also, one can learn about surface dynamics and the dephasing time of adsorbates by performing time-dependent and time-resolved WSCARS.
The Journal of Physical Chemistry, Vol. 97, No. 15, 1993 3841 Acknowledgment. We thank E. W. Koenig, R. M. Fortenberry, and G. I. Stegeman for helpful discussions. This work issupported by the National Science Foundation. References and Notes ( I ) (a) Henrich, V. E. f r o g . Surf. Sci. 1979, 9, 143. (b) The Physics of Solid Surface and Heterogeneous Catalysis; King, D. A,, Woodruff, D. P., Eds.; Elsevier: Amsterdam, 1983. (c) Chemistry and Structure at Interfaces: New Laser and Optical Techniques; Hall, R. B., Ellis, A. B., Eds.; VCH Publication Inc.: Dearfield Beach, FL, 1986. (d) Sam0rjai.G. Chemistry in Two Dimensions: Surfaces; Cornell University Press: Ithaca, NY, 1981. (e) Shen, Y. R. Principles of Nonlinear Optics; John Wiley & Sons: New York, 1984. (f) Shen, Y. R. Nature 1990,337,519. (8) Shen, Y. R. J. Vac. Sci. Techno/. 1985, 83, 1464. (2) (a) Vibrational Spectroscopy of Adsorbates; Willis, R. F.,Ed.; Springer: New York, 1980. (b) Little, L. H. Infrared Spectra of Adsorbed Species; Academic Press: London, 1966. (c) Bolger, B. In Surface Studies with Lasers; Ausseneges, F. R., Leitner, A,, Lippitsch, M. E., Eds.; Springer: New York, 1983. (d) VibrationalSpectroscopiesfor AdsorbedSpecies; Hair, M. L., Bell, A. T., Eds.; American Chemical Society: Washington, DC, 1980. (3) (a) VibrationalSpectroscopyof MoleculesonSurfaces; Yates, J. T., Jr.; Madey, T. E., Eds.; Plenum: New York, 1987. (b) Spectroscopy of Surfaces; Clark, R. J. H., Hester, R. E., Eds.; John Wiley & Sons: New York, 1988. (4) Stegeman, G.I.;Fortenberry, R.; Moshrefzadeh, R.; Hetherington, W. M.; Van Wyck, N. E. Opt. Lett. 1984, 9, 88. ( 5 ) Hetherington, W. M.;Van Wyck,N. E.;Stegeman,G. I.; Fortenberry, R. Opt. Lett. 1984, 9, 88. (6) Wijekoon, W. M. K. P.; Ho, Z. Z.; Hetherington, W. M. J . Chem. Phys. 1987.86, 4384. (7) Hetherington. W. M.; Ho, Z. Z.; Koenig, E. W.; Stegeman, G. 1.; Fortenberry, R. Proc. SPIE. 1986, 620, 102. (8) (a) Wijekoon, W. M. K. P. Ph.D. Dissertation, UniversityofArizona, 1988. (b) Wijekoon, W. M. K. P.; Hetherington, W. M. J . A m . Chem. Soc.. in press. (9) (a) Hetherington, W. M.; Koenig, E. W.; Wijekoon, W. M. K. P. Chem. Phys. Lett. 1987, 134, 203. (b) Ho, Z. Z.; Wijekoon, W. M. K. P.; Koenig, E. W.; Hetherington, W. M. J. Phys. Chem. 1987, 9/, 751. (IO) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds, 4th ed.; John Wiley & Sons: New York, 1986. ( I I ) (a) Nakamura, Y.; Kawaguchi, S.Chem. Commun. 1968,716. (b) Van Leeuwen, P. M. W. N . R e d . Trau. Chim. 1968,87,396. (c) Nakamura, Y.; Isobe, K.; Morita, H.; Yamasaki, S.; Kawaguchi, S . Inorg. Chem. 1972, 1 1 . 1573. (12) Koda, S.;Ooi, H.; Kuroya, H.; Nakamura, Y.; Kawaguchi, S. Chem. Commun.1971, 280. (13) (a) Lewis, J.; Long, R. F.;Oldham, C. J. Chem. Soc. 1965,6740. (b) Gibson, D.; Lewis, J.; Oldham, C. J. Chem. Soc. A 1966, 1453. (c) Behnke, G. T.; Nakamoto, K. Inorg. Chem. 1968, 7, 330. (14) (a) Nakamoto, K.; Martel, A. E. J . Chem. Phys. 1960, 32, 588. ( I 5 ) Allara, D. L. In Vibrational Spectroscopies for Adsorbed Species; Hair, M. L., Bell, A. T., Eds.; American Chemical Society: Washington, DC, 1980. (16) Brown, M. D.; Norman; Nelson, J. W.; Turner, R. J.; Walmsley Gorge, D. J. Chem. Soc., Faraday Trans. 2 1981, 77. 337. ( I 7) Morrison, R. T.; Boyd, R. N. Organic Chemistry, 2nd ed.; Allyn and Bacon: Boston, 1970. (18) Yoshino, T. J . Chem. Phys. 1955, 23, 1564. (19) Kendall, R. S.; Leyden, D. E.; Burggraf; Pern, F. J. Appl. Spectrosc. 1982, 36, 436. (20) Cross, S. N. W.; Rochester, C. H. J . Chem. Soc., Faraday I 1978, 74, 2 130. (21) Ernstbunner, E. E. J . Chem. Soc. A 1970, 1558. (22) Nakamoto. K.; McCarthy, P. J.; Martel, A. E. J . Am. Chem. Soe. 1961, 83, 1272. (23) Kogelnic. H. In Integrated Optics; Tamir, T., Ed.; Springer: Berlin, 1979. (24) Stegeman, G. I.;Fortenberry, R.; Moshrefzadeh, R.; Hetherington, W. M.; Van Wyck, N. E.; Sipe, J. Opt. Lett. 1983, 8, 295. (25) Fortenberry, R. Ph.D. Dissertation, University of Arizona, 1986. (26) Hicknell, F. S. Proc. I E E E , Ultrasonic Symp. 1984, 309. (27) Akther, S.; Lui. K.; Kung, H. H. J. Phys. Chem. 1985, 89, 1958. (28) Willis, R. F.; Lucas, A. A.; Mahen, G. D. In The Physics of Solid Surface and Heterogeneous Catalysis; King, D. A,, Woodruff, D. P., Eds.; Elsevier: Amsterdam, 1983. (29) (a) Moskovits. M.; Hulse, J. E. Surf. Sei. 1978,78,397. (b) Hollins, P.; Pritchard, J. Progr. Surf. Sei. 1985, 19, 275. (c) Hollins, P. Surf. Sei. 1981, 107, 75. (30) (a) Hester, R . E.; Plare, R. A. Inorg. Chem. 1964.5, 13. (b) Ogoshi, H.; Nakamoto, K. J. Chem. Phys. 1966,45,3113. ( c ) Junge, H.; Musso, H. Spectrosc. Chim.Acra 1962, 24A, 1219 and references therein. (3 I ) Makami, M.; Nakagawa, I.;Shimanouchi, T. Spectrosc. Chim. Acta 1967, 23A. 1037. (32) (a) Mattman, G.;Oswald. H. R.; Schweizer, F. Helu. Chim. Acta 1972. 55, 1294. (b) Artherton, K.; Newbold, G.;Hockley, J. A. Discuss. Faraday Soc. 1971, 52, 3 3 . (c) Morimoto, T.; Nagoa, N. Bull. Chem. Soc. 1972, 43, 3146. ( 3 3 ) Kosaku, K.; Shigero, I.; Kozo, H. J . Phys. Chem. 1967, 71, 4384. (34) Barret, M. J.; Cotton, F. A.; Eiss, R. Acta Crystallogr. 1968. B24, 904. (35) Shuzo, S.; Masashi. 0. J. Mol. Struct. 1981, 77, 265.
