17166
J. Phys. Chem. B 2004, 108, 17166-17170
An Ordered Retinoate Monolayer Prepared on Rutile TiO2(110) Taka-aki Ishibashi,*,† Hiroshi Uetsuka,‡ and Hiroshi Onishi§ Surface Chemistry Laboratory, Kanagawa Academy of Science and Technology (KAST), KSP East 404, 3-2-1 Sakado, Takatsu, Kawasaki, 213-0012, Japan ReceiVed: May 30, 2004; In Final Form: August 13, 2004
An all-trans retinoate ion monolayer was prepared by a substitution reaction of all-trans retinoic acid with trimethyl acetate ion adsorbed on rutile TiO2 (110) in solutions. The prepared monolayer was assessed by infrared-visible sum-frequency generation (SFG) spectroscopy, X-ray photoelectron spectroscopy (XPS), and scanning tunneling microscopy (STM). Two strong vibrational SFG bands were observed at 1580 and 1405 cm-1. The strong intensities of the bands were due to electronic resonance of the SFG signal. The 1580 cm-1 band was assigned to the predominantly in-phase stretching vibration of the conjugated CdC double bonds of retinyl chromophore, and the 1405 cm-1 band to the symmetric -COO stretching vibration of carboxylate ions attached to TiO2 (110) with the two oxygen atoms on bridge-bonded 5-fold coordinated Ti atoms. The assignments suggest that retinoic acid is dissociatively adsorbed with its -COO group pointing to the substrate. Precovering TiO2 (110) substrate with trimethyl acetate was found to be crucial to the high quality of the monolayer based on control experiments. STM observation showed that no lateral order of retinoate existed on the monolayer.
1. Introduction Immobilizing desired organic compounds on desired substrates with controlled molecular densities and orientations is an ideal way to make useful optoelectronics and molecular electronics devices because a variety of combinations of organic functional groups and inorganic substrates allow researchers to design them flexibly. Toward this goal, several methods for making such films on substrates have been developed and are being elucidated. Langmuir-Blodgett films, thiolate selfassembled monolayers on rare metals, organosiloxanes on oxide silicon, and 1-alkenes on silicon are representative examples.1,2 Studies on organic layers on TiO2 are rather scant,3 despite attractive features of the substrates, such as high dielectric constant, controllable conductivity, and the stability of the material. Nanocrystalline TiO2 film with adsorbed dye molecules is currently used for the most efficient photoelectrochemical solar cells that utilize the photoelectron transfer from dye molecules to the nanocrystals.4 However, details on adsorption of dye molecules are not yet known. The efficiency of solar cells depends on uncontrollable factors such as the distributions of various trap-sites for charge carriers that mainly originate from defects in the nanocrystals. Ordered organic layers on a clean and flat single-crystal TiO2 surface have the possibility of creating efficient optoelectronic devices without such uncontrollable factors. It is established that the ordered monolayer of a light carboxylate ions (RCOO-) is prepared on an atomically flat * To whom correspondence should be addressed. E-mail: taib@ hiroshima-u.ac.jp. † Present address: Department of Chemistry, Graduate School of Science, Hiroshima University, Kagamiyama, Higashi-Hiroshima 739-8526, Japan. ‡ Present address: Functional Materials Team, Technology Research and Development Department, General Technology Division, Central Japan Railway Company, 1545-33 Ohyama, Komaki 485-0801, Japan. § Present address: Department of Chemistry, Faculty of Science, Kobe University, Rokko-dai, Nada, Kobe 657-8501, Japan.
rutile TiO2(110) exposed to the vapor of the corresponding carboxylic acid.5 However, this method cannot be applied for heavy calboxylates including sensitizer dyes due to their low vapor pressure. We propose in this paper a method to synthesize the monolayer of the heavy carboxylates. A monolayer of retinoate (R ) C19H27) is prepared and characterized on the TiO2(110) surface. This method should be able to be extended usefully to preparation of a number of other systems of interest. 2. Principle of Monolayer Preparation Light carboxylic acid (RCOOH) with R ) H, CH3, C(CH3)3, etc. dissociates on the rutile (110) surface to give the conjugate carboxylates (RCOO-),
RCOOH (g) f RCOO- (a) + H+ (a) where (g) and (a) represent the gas phase and the adsorbed phase.5-14 Each carboxylate is chemically adsorbed in a bridge fashion with the oxygen atoms bridge-bonded on two 5-fold coordinated Ti atoms. The COO plane is parallel to the [001] direction. A long-range ordered monolayer of the carboxylates is spontaneously formed at room temperature. When a RCOO monolayer on TiO2(110) is exposed to another carboxylic acid (R′COOH) vapor, the preadsorbed carboxylates are exchanged at room temperature15,16
RCOO-(a) + R′COOH (g) f RCOOH (g) + R′COO-(a) We expect a similar exchange reaction at a solution-TiO2 interface and utilize it to synthesize the monolayer with a heavy R′. An atomically flat TiO2(110) is prepared and covered with trimethyl acetate (R d C(CH3)3) in a ultrahigh-vacuum (UHV) chamber. The covered surface is removed from the vacuum into the air and dipped in a solution of the R′COOH of interest. Covering the surface with trimethyl acetates is assumed to
10.1021/jp047662l CCC: $27.50 © 2004 American Chemical Society Published on Web 10/09/2004
Ordered Retinoate Monolayer
Figure 1. Illustrations of (a) all-trans retinoic acid (C20H28O2) and (b) all-trans retinoate adsorbed on TiO2(110).
protect the chemically active 5-fold coordinated Ti sites from water, CO2, and uncontrollable organic molecules in the air. Retinoic acid was chosen to test this method. It contains four conjugated CdC double bonds in the polyenic backbone with a partially unsaturated six-member ring at one end (Figure 1a). A theoretical calculation predicted the bridge adsorption of the rodlike retinoate without a serious steric confliction. The molecular part of the model in Figure 1b was an energyoptimized geometry of a (retinoate)(HCOO)TiO2(OH)2 cluster with Gaussian 98.17 The energy optimization was performed with the B3LYP method of density functional theory with the 6-31G(d,p) basis set. The cluster structure (not shown) was almost the same as that of (HCOO)2TiO2(OH)218 except for the retinoate part. 3. Experimental Section Three types of molecular films, types I-III, were prepared for SFG measurements. The type I films were prepared by dipping a TiO2(110) wafer precovered with trimethyl acetate ions into a solution. The procedure of the film preparation contained four steps. In the first step, a polished rutile TiO2 (110) wafer of 16 × 10 × 0.4 mm3 (Earth Chemicals Co.) was cleaned with cycles of Ar+ sputtering and vacuum annealing in a UHV chamber. The base pressure of the chamber was 1 × 10-7 Pa. A well-contrasted (1 × 1) LEED pattern was observed on the surface. In the second step, a (2 × 1)-ordered monolayer of trimethyl acetate ions was prepared on the surface by exposing the surface to trimethyl acetic acid vapor at room temperature. Formation of a (2 × 1)-ordered monolayer of trimethyl acetate ions was confirmed by LEED. In the third step, the wafer was removed from the UHV chamber and submerged into an acetone solution of all-trans retinoic acid (Sigma-Aldrich Co.). The time required for the transportation of the wafer from the chamber to a container of the solution was typically 5 min. Retinoic acid was used as received. The concentration of the solution was 10 mmol/dm3. The wafer was dipped into the solution for 2-5 h. A longer dipping time of up to 30 h was examined and was found to give no effect on the SFG spectra of the samples. In the last step the wafer was removed from the solution and rinsed by pouring 100 mL of acetone on it. Type II and III films were prepared for control experiments. In the preparation of type II and III films, a bare TiO2(110) wafer was used instead of a precovered wafer. The
J. Phys. Chem. B, Vol. 108, No. 44, 2004 17167 dipping and rinsing processes were common for the three types of films. For the type II film, the bare wafer was cleaned in the UHV chamber with the same procedure used for the type I film. For the type III film, the bare wafer was heated at 730 K in air for 2 h and cooled at room temperature before dipping in the solution. SFG measurements were performed in a box filled with N2 gas using a multiplex SFG spectrometer with a 1-kHz laser system.19,20 A picosecond narrow-bandwidth (7 cm-1) visible pulse and a femtosecond broad-bandwidth (200 cm-1) IR pulse were used as the probe lights. The visible and IR probe beams were overlapped spatially and temporally on a sample with incidence angles of 70 and 50° from the surface normal of the sample, respectively. Sample wafers were set so that the [001] direction was parallel to the incidence plane of probe beams. The SFG signal generated in the direction of reflection was observed. Polarizations of the signal and probes were selected to be p-polarized (ppp-polarization). The signal was analyzed by a spectrograph with an 8 cm-1 slit width and a multichannel CCD detector. Structureless weak luminescence was emitted at a wavelength region shorter than that of the visible probe even when the sample was irradiated with only the visible probe. The signal without the IR probe was subtracted from the signal with IR to deduce SFG intensity. Sensitivity of the spectrometer was calibrated by dividing the SFG spectrum of a sample by the spectrum of a GaAs(110) wafer with the ppp-polarization at the azimuth angle where the angle between [001] direction and the probe incidence plane was 55°. XPS measurements were performed on an ESCA 2704 spectrometer (Surface Science Instruments) using an aluminum anode (Al KR ) 1487 eV) to analyze composition of the films. A takeoff angle of 35° from the surface plane was used. STM observations were carried out using a UHV-compatible microscope (JEOL, JSPM4500S). A one-side polished TiO2 wafer (6.5 × 1 × 0.35 mm3, Earth Chemicals Co.) was used for the microscope measurement. The same procedure described above was employed to prepare type I film. STM images were measured in the constant-current mode. 4. Results and Discussion Vibrational Spectrum of the Monolayer. We exploited the electronic resonance enhancement of SFG signal intensity to conduct a high-sensitivity measurement. The retinyl chromophore, the ring and polyenic backbone, has a characteristic CdC double bond stretch band at around 1580 cm-1 that is strong in both Raman and IR spectra21 and thus is suitable for detection of the molecule by SFG spectroscopy. The SFG intensity of the band can be enhanced when the SFG signal is in or near resonance with a one-photon allowed electronic transition of retinoate. The enhancement enables us to detect retinoate ions on the surface sensitively. SFG in such conditions was called doubly resonant SFG,22 because the optical process is resonant with the vibrational state and the electronic states simultaneously. The absorption spectrum of the retinoate films on the TiO2(110) wafer could not be measured because the substrate itself has a strong absorption due to a band-gap transition below the 400 nm wavelength. Therefore, we referred to absorption spectra of retinoic acid solutions and a spin-coated film of retinoic acid on fused silica. Retinoic acid in acetone has a strong electronic absorption band in the near-UV region (Figure 2, dash-dotted line). The peak of the band was positioned at 354 nm with a molar absorption coefficient of 47000 dm3 mol-1 cm-1. To estimate the absorption of retinoate, we measured the
17168 J. Phys. Chem. B, Vol. 108, No. 44, 2004
Figure 2. Electronic absorption spectra of all-trans retinoic acid: (a) acetone solution at a concentration of 1.6 × 10-4 mol/dm3 (dashdotted line); (b) triethylamine solution at a concentration of 1.6 × 10-4 mol/dm3 (dotted line); (c) spin-coated film on fused silica (solid line). The film was prepared by spin-coating a 0.3 mmol/dm3 ethanol solution of retinoic acid on a fused silica substrate for 2 min at 2000 rpm. The area intensity of retinoic acid molecules is estimated to be 1.0 × 1014 cm-2, if the molar absorption coefficient of retinoic acid on the film is assumed to be the same as that of retinoic acid in acetone.
electronic absorption of retinoic acid in triethylamine. Retinoic acid is expected to dissociate in the basic solvent. The observed peak shift, a 9 nm blue shift, (Figure 2, dotted line) was not large. The spin-coated film of retinoic acid also exhibited similar absorption (Figure 2, solid line), though its peak position was shifted to 370 nm and the band broadened nearly two times. Retinoic acid chemisorbed onto anatase TiO2 nanosol in an aqueous solution exhibited two electronic absorption bands at 420 and 387 nm.23 The long wavelength band has been assigned to partly deprotonated retinoic acid and the short wavelength band to protonated acid. This observation suggests a possibility that the electronic absorption of retinoate also shows a substantial red-shift on rutile TiO2 (110). In Figure 3a is shown the SFG spectrum of retinoate film on TiO2 (type I) measured with a 480 nm visible probe. The spectrum consists of two relatively large vibrational bands at 1580 and 1405 cm-1 on a flat vibrationally nonresonant background. Broad and weak vibrational bands were also recognized at 1500-1430 cm-1, where degenerate methyl deformation vibrations are expected to appear. The vibrational band shapes of the two strong bands were relatively symmetric because of the low background signal level. The intensity of the strongest 1580 cm-1 band was 0.0068 times that of GaAs(110), which gives strong SFG signal as a reference. The band intensity was of the same order of magnitude as the vibrationally nonresonant SFG intensity of a gold film. This strong intensity suggests that the vibrational SFG bands were attributed to retinoate and were enhanced by an electronic resonance effect. The wavelength of the SFG signal of 1580 cm-1 corresponds to 446 nm. This wavelength is on the longer wavelength-sided tail of the electronic absorption band of the spin-coated film of retinoic acid. The SFG intensity of the 1580 cm-1 band relative to the SFG from GaAs(110) was decreased 6 times when the visible probe wavelength was changed to a wavelength more off-resonant with the near-UV band of retinoate, i.e., from 480 to 535 nm. (Figure 2b) The decrease supports the idea that the SFG signal with the 480 nm probe was enhanced by an electronic resonance effect of retinoate. Because the type I film was rinsed with solvent, we expect that retinoate was adsorbed on TiO2 not physically but chemically, via dissociative adsorption of the carboxylic acid group. We
Ishibashi et al.
Figure 3. SFG spectra of retinoate films: (a) type I film with 480 nm probe; (b) type I film with 535 nm probe; (c) type II film with 480 nm probe; (d) type III film with 480 nm probe. The ppp-polarization was used. Visible and infrared laser power was 0.2 and 2 mW. The intensity of the spectra was normalized to the vibrationally nonresonant SFG signal of GaAs(110).
failed to detect retinoate on the type I film using spontaneous Raman spectroscopy with a 457.9 nm probe. An obstacle to detection was the broad Raman band of rutile TiO2 located at 1600 cm-1. Vibrational assignments of the SFG spectra of type I film are discussed referring to the vibrations of all-trans retinal. Reasonable vibrational assignments thus obtained support the idea that retinoate bridge-adsorbed on the TiO2(110) surface. Retinal has an all-trans retinyl chromophore with -CHO terminal instead of -COO-. The electronic24 and vibrational21 structure of all-trans retinal has been extensively studied. We assume that the nature of retinyl chromophore vibrations of the retinal is valid for retinoate. The 1577 cm-1 band of retinal in solution is strong both in IR and Raman spectra.21 This band is the second strongest in IR and the strongest in Raman among the bands below 1700 cm-1. Its Raman intensity is greatly enhanced by electronic resonance when the probe wavelength approaches the electronic absorption of retinal at 370 nm.25 The band has been assigned to the predominantly in-phase stretching vibration of the conjugated CdC double bonds. The SFG intensity of a vibrational band is roughly proportional to the product of its IR intensity and Raman intensity of the band.26 On the basis of the IR and Raman intensity of retinal, we assigned the 1580 cm-1 SFG band of the type I film to the predominantly in-phase stretching vibration of the conjugated CdC double bonds. In the 1350-1500 cm-1 range, the retinal has several vibrational modes mainly contributed from methyl deformations. The bands have medium IR intensities, but are very weak in preresonance and resonance Raman spectra. Therefore, the 1405 cm-1 SFG band cannot be attributed to the retinyl chromophore of retinoate. On the other hand, the band is reasonably assigned to the symmetric COO stretch. The symmetric COO stretching wavenumber of carboxylate salts is 1420 cm-1 with a notable exception of formate.27 The symmetric COO stretching wavenumber of formate is 1360 cm-1. The lowwavenumber shift of 60 cm-1 (1420-1360) is attributed to the high wavenumber of the C-H bond attached to the carboxylate group.27 The symmetric COO stretch bands of formate on rutile TiO2(110) have been investigated with high-resolution electron
Ordered Retinoate Monolayer
J. Phys. Chem. B, Vol. 108, No. 44, 2004 17169
Figure 4. X-ray photoelectron spectra of retinoate films. (a) C(1s) region; (b) Ti(2p) region.
energy loss spectroscopy (HREELS)8 and reflection-absorption IR spectroscopy (RAIRS).9 For the symmetric COO stretch vibration, a single band was identified at 1365 cm-1 via HREELS, whereas two IR bands at 1363 and 1393 cm-1 were reported. The 1363 cm-1 IR band was assigned to the formate adsorbed in a bridge fashion with the OCO plane parallel to the [001] direction with C2V symmetry (species A), whereas the 1393 cm-1 IR band was assigned to formate with the OCO plane perpendicular to the [001] direction with Cs symmetry (species B). The coverage of species A and B at 300 K was estimated to be 0.4 and 0.2 ML, respectively. Species B was thought to be adsorbed at the vacancies in the bridging oxygen row.9 Taking into account the reported wavenumbers of the COO stretch of formate on rutile TiO2(110) and a general trend of the low wavenumber shift from heavy carboxylate salts to light formate salt, we assigned the 1405 cm-1 band of type I film to the symmetric COO stretch mode of retinoate adsorbing in a bridge fashion; the oxygen atoms were chemisorbed on a pair of 5-fold coordinated Ti atoms with the OCO plane parallel to [001]. Role of Preadsorbed Trimethyl Acetate. We have conducted control experiments to elucidate how the orientation and order of retinoates depend on the substrate. As described in the Experimental Section, we examined two additional films without precovering by trimethyl acetate. The preparation of rutile TiO2(110) was different for the two films. The substrate of type II was prepared as a sputter-annealed (1 × 1) surface while that of type III was a surface heated in air at 730 K. SFG spectra of type II and III films measured with a 480 nm visible probe are shown in Figure 2, parts c and d, respectively. The SFG intensity largely depended on the pretreatment of the substrate. SFG intensities of type II and III films were 70-80 times smaller than that of type I film. The SFG intensity depends on the order of a film as well as the density of molecules on the film, because SFG is a second-order process. For example, a film with randomly orientated molecules gives no vibrational resonant signal in the dipole approximation.28 Therefore, the number density of retinoate and the order of the molecules should be separately considered to interpret the different SFG intensities of the three films. XPS was used to evaluate the atomic contents of the three films. The observed spectra for the C(1s) and Ti(2p) regions are shown in Figure 4. The main peak of the C(1s) signal
Figure 5. STM images of molecule-covered TiO2(110): (a) trimethyl acetate covered surface; (b) surface of sample a exposed to air for 10 min; (c) retinoate-covered surface (type I film). Conditions: (a) 30 × 30 nm2, sample bias voltage (VS) ) +2.0 V; (b) 30 × 30 nm2, VS ) +2.0 V; (c) 80 × 80 nm2, VS ) +2.5 V.
appeared at 285.2 eV with shoulders at 286.8 and 288.8 eV for type III film. The main peak was assigned to nonoxidized organic carbon and the minor peaks to carbons in various oxidized states. The area intensity of the minor peaks was 1030% of that of the total C(1s) signal. The Ti(2p) signal also consisted of twin peaks at 459.1 and 464.9 eV. The atom number ratio of total carbon over titanium was estimated from the total C(1s) and Ti(2p) intensities. The ratios were 1.6, 3.0, and 1.8 for type I, II, and III films, respectively. If we assume that the C(1s) signal was due to retinoate and the sampling depth of the Ti(2p) signal was 1.15 nm, the number densities of the retinoate were estimated to be 2.9 × 1014, 5.5 × 1014, and 3.3 × 1014 cm-2, respectively. The order of the estimated densities is consistent with a monolayer level thickness. The ideal density of a (2 × 1)-ordered carboxylate monolayer is 2.7 × 1014 cm-2. The result of the XPS measurements suggests the molecular densities on type II and III films to be no less than that of type I film. Therefore, the weak SFG intensity of type II and III films is attributed not to the decreased density of the retinoate but to the insufficient order of type II and III films. The order of the films depended on pretreatment of the TiO2(110) substrates. The wafer for type III film that is oxidized in an extreme condition may have been roughened. The surface structure of TiO2(110) is sensitive to the history of the crystal.29 When a sputter-annealed (1 × 1) surface was heated in a lowpressure (10-8 Pa) O2 gas, for example, nanometer-sized protrusions covered the surface.30 It is interesting that the orders of type I and type II film were considerably different, although the difference between the wafers for type I and II films was subtle in a sense. Both wafers should be equally flat and have active sites for a bridge adsorption at least in the UHV chamber, though the sites were covered with trimethyl acetate on the wafer for type I film but not for type II film. The difference in the order of the films strongly suggests that removing a clean and flat surface in a (1 × 1) symmetry in air damages the surface quickly. Trimethyl acetate ions with the bulky tert-butyl group may protect the flat surface with the active sites from oxygen
17170 J. Phys. Chem. B, Vol. 108, No. 44, 2004 and water molecules, the probable source of the damaging process. STM Topography of the Monolayer. Type I film, in which a sufficient molecular ordering was suggested, was observed by STM. Figure 5a shows the STM image of a TiO2(110) surface fully covered with trimethyl acetates prepared in the UHV-STM chamber. Bright particles ordered in a (2 × 1) symmetry represent individual trimethyl acetate ions. To evaluate what happens on the trimethyl acetate monolayer exposed to the air, it was removed from the chamber, kept in air for 10 min, and observed under the UHV microscope. The obtained image shown in Figure 5b suggests that the trimethyl acetate monolayer remained intact. An STM image of a type I film similarly observed is shown in Figure 5c. Particles of a uniform size, 1 nm in diameter and 1 nm in height, covered the surface. The uniform size of the particles is reasonable for retinoate chemisorbed perpendicular to the surface in the bridge form as shown in Figure 1b. The lateral order of the retinoate, such as a (2 × 1) structure observed for trimethyl acetate, did not exist in the film. In summary, we developed a new method for the preparation of a monolayer of large molecules on rutile TiO2(110) surface. The target molecules with a carboxylic acid group, all-trans retinoate, were adsorbed on the surface in solution phase. SFG spectroscopic studies showed that preparing a clean flat surface precovered with trimethyl acetate is crucial for the order of the monolayer. This method can be applied to other molecules with a carboxylic group, if the molecules are soluble in liquids. Acknowledgment. This work was supported by Grants-inAids for Scientific Research from the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) of the Japanese Government [No. 13555008 and 15033277 from Priority Area (417)] and also by Core Research for Evolutional Science and Technology (CREST) of the Japan Science and Technology Agency (JST). References and Notes (1) Ulman, A. An Introduction to Ultrathin Organic Films from Langmuir-Blodgett to Self-Assembly; Academic Press: San Diego, CA, 1991. (2) Linford, M. R.; Fenter, P.; Eisenberger, P. M.; Chidsey, C. E. D. J. Am. Chem. Soc. 1995, 117, 3145. (3) Matsuura, A. Y.; Obayashi, T.; Kondoh, H.; Ohta, T.; Oji, H.; Kosugi, N.; Sayama, K.; Arakawa, K. Chem. Phys. Lett. 2002, 360, 133. (4) Gra¨tzel, M. Nature (London) 2001, 414, 338. (5) Onishi, H. In Chemistry of Nanomolecular Systems-Towards the Realization of Molecular DeVices; Nakamaura, T., Matsumoto, T., Tada, H., Eds.; Springer: Berlin, 2003.
Ishibashi et al. (6) Onishi, H.; Aruga, T.; Iwasawa, Y. J. Catal. 1994, 146, 557. (7) Thevuthasan, S.; Herman, G. S.; Kim, Y. J.; Chambers, S. A.; Peden, C. H. F.; Wang, Z.; Ynzunza, R. X.; Tober, E. D.; Morais, J.; Fadley, C. S. Surf. Sci. 1998, 401, 261. (8) Henderson, M. A. J. Phys. Chem. B 1997, 101, 221. (9) Hayden, B. E.; King, A.; Newton, M. A. J. Phys. Chem. B 1999, 103, 203. (10) Guo, Q.; Cocks, I.; Williams, E. M. J. Chem. Phys. 1997, 106, 2924. (11) Gutie´rrez-Sosa, A.; Martı´nez-Escolano, P.; Raza, H.; Lindsay, R.; Wincott, P. L.; Thornton, G. Surf. Sci. 2001, 471, 163. (12) Onishi, H.; Iwasawa, Y. Chem. Phys. Lett. 1994, 226, 111. (13) Sasahara, A.; Uetsuka, H.; Onishi, H. J. Phys. Chem. B 2001, 105, 1. (14) Ka¨eckell, P.; Terakura, K. Surf. Sci. 2000, 461, 191. (15) Sasahara, A.; Uetsuka, H.; Onishi, H. Surf. Sci. 2001, 481, L437. (16) Uetsuka, H.; Sasahara, A.; Yamakata, A.; Onishi, H. J. Phys. Chem. B 2002, 106, 11549. (17) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin, B. K. N.; Strain, C. M. C.; Farkas, D. O.; Tomasi, E. J.; Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.; Petersson, F. A.; Ayala, G. P. Y.; Cui, H. Q.; Morokuma, K.; Salvador, P.; Dannenberg, I. J.; Malick, J. D. K.; Rabuck, A. D.; Raghavachari, B. K.; Foresam, C. J. B.; Cioslowski, D. J.; Ortiz, K. V.; Baboul, L. A. G.; Stefanov, M. B. B.; Liu, N. G.; Liashenko, O. A.; Piskorz, P.; Komaromi, Q. I.; Gomperts, R. R.; Martin, S. R. L.; Fox, T. D. J.; Keith, U.; Al-Laham, V. M. A.; Peng, W. C. Y.; Nanayakkara, X. A.; Challacombe, Y. M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Andres, J. L.; Gonzalez, C.; Head-Gordon, M.; Replogle, E. S.; Pople, F. J. A. Gaussian 98, reVision A.11; Gaussian, Inc.: Pittsburgh, PA, 2001. (18) Sasahara, A.; Uetsuka, H.; Ishibashi, T.; Onishi, H. J. Phys. Chem. B 2003, 107, 13925. (19) Ishibashi, T.; Onishi, H. Appl. Spectrosc. 2002, 56, 1298. (20) Ishibashi, T.; Onishi, H. Appl. Phys. Lett. 2002, 81, 1338. (21) Curry, B.; Palings, I.; Borek, A. D. Pardoen, J. A.; Lugtenburg, J.; Mathies, R. AdVances in Infrared and Raman Spectroscopy; Wiley: New York, 1985; Volume 12; Chapter 3. (22) Raschke, M. B.; Hayashi, M.; Lin, S. H.; Shen, Y. R. Chem. Phys. Lett. 2002, 359, 367. (23) Sahyun, M. R. V.; Serpone, N. J. Photochem. Photobiol. A 1998, 115, 231. (24) Drikos, G.; Ru¨ppel, H. Photochem. Photobiol. 1984, 40, 85. Drikos, G.; Ru¨ppel, H. Photochem. Photobiol. 1984, 40, 93. Drikos, G.; Ru¨ppel, H. Photochem. Photobiol. 1984, 40, 133. (25) Doukas, A G.; Aton, B.; Callender, R. H. Honig, B. Chem. Phys. Lett. 1978, 56, 248. (26) Hunt, J. H.; Guyot-Sionnest, P.; Shen, Y. R. Chem. Phys. Lett. 1987, 133, 189. (27) Colthup, N. B.; Daly, L. H.; Wiberley, S. E. Introduction to Infrared and Raman Spectroscopy; Academic Press: San Diego, CA, 1990. (28) Miranda, P. B.; Shen, Y. R. J. Phys. Chem. B 1999, 103, 3292. (29) Li, M.; Hebenstreit, W.; Gross, L.; Diebold, U.; Henderson, M. A.; Jennison, D. R.; Schultz, P. A.; Sears, M. P. Surf. Sci. 1999, 437, 173. (30) Onishi, H.; Iwasawa, Y. Phys. ReV. Lett. 1996, 76, 791.
