J. Phys. Chem. C 2008, 112, 11791–11795
11791
Doubly Resonant Sum Frequency Generation Spectroscopy of Adsorbates at an Electrochemical Interface Benedetto Bozzini,*,† Lucia D’Urzo,† Claudio Mele,† Bertrand Busson,‡ Christophe Humbert,‡ and Abderrahmane Tadjeddine§ Dipartimento di Ingegneria dell’InnoVazione, UniVersita` del Salento, V. Monteroni, I-73100 Lecce, Italy, LCP/CLIO, UniVersite´ Paris-Sud, 91405 Orsay Cedex, France, and UDIL/CNRS, BP34 91898 Orsay Cedex, France ReceiVed: NoVember 5, 2007; ReVised Manuscript ReceiVed: May 5, 2008
In this paper we report the first spectroelectrochemical in situ doubly resonant sum frequency generation (DR-SFG) spectroscopy experiment carried out with a free-electron laser tunable IR source and IR and VIS Optical Parametric Oscillators. We studied the potential-dependent adsorption and reaction of the quinoline dye 4-{2-[1-(2-cyanoethyl)-1,2,3,4-tetrahydroquinolin-6-yl]diazenyl} benzonitrile (CTDB) onto a Cu(100) electrode. SFG band assignments were based on density functional theory calculations. Strong SFG enhancement was obtained in the IR range 1100-1300 cm-1, corresponding to ArsCtN stretching, with the VIS beam set at 441.6 nm and in the IR range 2000-2300 cm-1, corresponding to nitrile stretching, with the VIS beam set at 532 nm and the pH adjusted in order to match the bathocromic shift of the adsorbed CTDB chromophore with the input VIS laser frequency. Doubly resonant conditions were achieved via resonance of the VIS input with the electronic structure of the adsorbate orbitals and of the adsorption bond. Introduction In situ sum frequency generation (SFG) spectroscopy exhibits some unique features with respect to other spectroelectrochemical approaches to the study of the electrochemical interface, both in the double-layer charging region and in the presence of faradaic reactions. In addition to the utmost interface sensitivity of second-order nonlinear optical spectroscopies, enabling to sample a single electrodic state, SFG/difference frequency generation (DFG) conveys simultaneously vibrational and electronic information.1 In addition, the possibility of singlestate work with well-defined electrode surfaces allows surfacecoverage quantization. Furthermore, the three-wave mixing process opens up the possibility of vibrational relaxation dynamic studies.2 Doubly resonant SFG, well-known for homogeneous systems, but so far not proved at an electrodic interface, in this field is expected not only to give rise to an enhancement of the signal-to-noise ratio with superior detection capability for low coverage or coadsorption studies but also to allow probing electronic transitions that are not inherent of the molecule, but depend on the adsorption mode. Materials and Methods The electrolyte consisted of Ar-deaerated aqueous solutions of composition: 4-{2-[1-(2-cyanoethyl)-1,2,3,4-tetrahydroquinolin-6-yl]diazenyl}benzonitrile (CTDB) 1 mM, NaClO4 0.1 M, pH 0.5, 3, and 7 by H2SO4. UV-vis spectra of the solutions employed were measured with a dual beam spectrophotometer. The electrode was a Cu(100) disk from Mateck of 8 mm diameter and 3 mm thickness cut with a precision better than 0.2°, electropolished in H3PO4 by applying 2 V between the * To whom correspondence should be addressed. E-mail: benedetto.bozzini@ unile.it. † Universita ` del Salento. ‡ LCP/CLIO, Universite ´ Paris-Sud. § UDIL/CNRS.
single crystal and a Pt foil. The counter electrode was a Pt wire of area ca. 2 cm2. The reference electrode was an Ag/AgCl, KCl 3 M, separated from the working electrolytes by a porous ceramic insert; all voltages are reported on the Ag/AgCl scale. SFG spectra were recorded at the potentials in the range 0.05-1.2 V. The CLIO-based SFG setup used here is analogous to the system described in ref 1. The IR generated by CLIO was scanned in the range 1110-1280 cm-1. Visible radiation was produced by a picosecond Nd:YLF-pumped visible OPO system, capable of delivering the following wavelengths: 441.6, 523.5, 568.2, and 678.0 nm. In the IR-OPO-based experiments, the IR was scanned in the range 2040-2300 cm-1, and the visible radiation was generated by a doubled YAG laser and kept fixed at 532 nm. The IR and VIS laser beams are spatially and temporally overlapped at the surface of the electrodes to produce SFG. Both beams are p-polarized, with angles of incidence 55 and 65° for the visible and IR beams, respectively. The energy resolution of the system is around 10 cm-1. The electrochemical cell was of the type described in refs 1 and 3, and it consisted of a PTFE body, closed by a BaF2 prism.3 Inside the cell, the Cu crystal is fixed to a central stub and kept in contact with a Pt wire by aspiration; the electrode is gently pushed against the prism to carry out SFG measurements in a thin layer configuration. The simulation of geometrical, vibrational, and electronic properties of CTDB and its reaction products has been carried out by the Gaussian 03 package.4 Optimized geometries as well as IR and Raman spectra have been simulated in the framework of the density functional theory (DFT) with the B3LYP hybrid functional and basis set 6-311++g(d). The electronic spectra have been studied via the time-dependent DFT (TD-DFT). Results and Discussion Visible Spectrophotometry. The visible absorption of CDTB was studied by spectrophotometry; relevant spectra are reported in Figure 1, exhibiting a pH-dependent bathochromic shift.
10.1021/jp710608q CCC: $40.75 2008 American Chemical Society Published on Web 07/12/2008
11792 J. Phys. Chem. C, Vol. 112, No. 31, 2008
Figure 1. Visible spectrum of CTDB solutions at pH 7, 3, and 0.5.
Bozzini et al.
Figure 3. SFG spectra of a neutral CTDB solution, in contact with a Cu(100) electrode, recorded at 441.6 nm at the indicated potentials.
Figure 2. SFG spectra of a neutral CTDB solution, in contact with a Cu(100) electrode, recorded at -0.5 V at the indicated visible wavelength.
CLIO-SFG Experiments. SFG experiments were carried out by scanning the CLIO free-electron laser (FEL) IR output, keeping the visible OPO wavelength and the electrode potential fixed in the electrolyte at pH 7. With this electrolyte, featureless spectra are recorded for potentials less cathodic than -0.5 V, regardless of the visible input wavelength employed. The spectra collected at -0.5 V are shown in Figure 2. No discernible vibrational bands are obtained with all visible input wavelengths, except 441.6 nm, where a remarkable resonance was found, exhibiting a double peak structure. A second experiment was run with the same system, by keeping the VIS input fixed at 441.6 nm and changing the electrode potential in the range of 0.05-1.2 V (Figure 3). The same double peak is found, showing no appreciable potential dependence of position, amplitude and width. Previous in situ surface-enhanced Raman scattering (SERS) experiments5 have proved that at -0.5 V CDTB undergoes cleavage of the NdN bond, giving rise to the formation of 3-cyano aniline (3CA) and 6-amino-1-(2-cyan-ethyl)-2,3,4trihydro quinoline (ACETHQ). The molecular geometries optimized by DFT of CDTB, 3CA, and ACETHQ are shown in Figure 4. At higher cathodic polarizations, denitrilation was found, as witnessed by the appearance of a Raman signal attributable to the stretching of adsorbed CN-. On the basis of our DFT calculations and SERS observations of5 the double peak highlighted in Figures 2 and 3 - can be assigned to the Ph-CN stretching band of 3CA. In fact, the relevant moieties of unreacted CTDB and ACETHQ lie flat on
Figure 4. Optimised geometries, as simulated by DFT/B3LYP-6311++g(d), of (a) CTDB, (b) 3CA, and (c) ACETHQ.
the electrode and do not yield an SFG response, owing to surface selection rules. Furthermore, DFT computations show that 3CA exhibits two both IR- and Raman-active bands for Ph-CN stretching at 1206 and 1231 cm-1. This type of enhancement, previously described,6 can still be modeled with the classical expressions for the second-order susceptibility, developed in ref 1. This expression implies, in particular, that the metal response, including interband transitions, is modeled by a sum of Lorentzians. This approach (i) has been proposed on the basis of analogy arguments exposed in7 and summarized in ref 1, (ii) is based on rigorous modeling of the complex dielectric constant of metals exhibiting interband transitions,8,9 (iii) can be derived analytically for χ(2) models of bulk10 and surface11 SHG, and (iv) can be extended to the case of SFG along the lines previously illustrated.12 A formal derivation of the relevant expression is beyond the scope of the present paper. Under the hypothesis that no other species form in addition to 3CA and ACETHQ that have been detected in our SERS experiments, the enhancement found in Figures 2 and 3 cannot originate from a visible resonance in the molecule, since 3CA does not bear a chromophore (DFT computations show that the lowest electronic excited-state lies at 276 nm). Consequently, we tentatively assign such enhancement to a transition between two states of the chemisorbed bond state of the type proved to impact SFG and SERS.6,13
DR-SFG Spectroscopy of Adsorbates
J. Phys. Chem. C, Vol. 112, No. 31, 2008 11793
To rationalize our data, we use the following equation, obtained elaborating on previous works1,14,15
| |
(2) 2 ISFG ≈ χSFG (2) χSFG ≈
AVIS,MET + ωVIS - ωVIS,MET + iΓVIS,MET
AIR,MOL AVIS,MOL × + ωVIS - ωVIS,MOL + iΓVIS,MOL ωIR - ωIR,MOL + iΓIR,MOL (2) (2) χNR
where AVIS,MET and AVIS,MOL are the oscillator strenghts relevant to the visible absorption of the electrode and adsorbate, respectively, AIR,MOL,j is the oscillator strength of the jth SFGactive mode of the adsorbate, ωVIS and ωIR are the input frequencies, ωVIS,MET, ωVIS,MOL, and ωIR,MOL,j are the resonant frequencies of the electrode and of the adsorbate in the VIS and IR, respectively, ΓVIS,MET, ΓVIS,MOL, and ΓIR,MOL,j are the damping coefficients corresponding to the resonance of the electrode and of the adsorbate in the VIS and IR, respectively. The term χNR(2) accounts for nonresonant contributions corresponding to both intraband transitions and interband transitions off resonant with the visible laser lines involved in the present research. For the reasons stated above, the inresonance contribution of the metal interband transition, corresponding to the single visible frequency used in a given experiment, is approximated with the first term of the abovereported formula. In the relevant experiments χNR(2) can be neglected for the reasons stated in the following paragraph. At photon energies below the interband transition threshold, the linear and the nonlinear optical behavior is mainly controlled by the free electrons (6s for Cu), that to a very good level of approximation obey the Drude theory and contribute to the second-order susceptivity by a nonresonant term, that is essentialy real, as demonstrated in several papers and summarized in ref 1. Above the interband transition threshold, the boundelectron contribution plays a key role in the optical properties of Cu, not only because each atom contributes ten d electrons, compared to one 6s-electron, but also because this contribution can be accounted for by a sum of Lorentz oscillators corresponding to the singularities of the density of joint states near the points of high symmetry of the Brillouin zone.8 Since SFG experiments were performed by tuning the IR laser frequency at fixed visible laser frequency within a visible energy range of 1 eV, i.e., much smaller than the width of the first transition band,8 only one Lorentz oscillator is resonant and hence controls the optical behavior of the investigated system. The frequency-dependent contribution of Fresnel factors has not been included in the approximated expression for ISFG given above because, on the basis of literature discussions3,16–18 and computations performed in this work along the lines of previous work19 and with optical constants derived from other previous works,20,21 not reported for brevity, it can be show that the Fresnel factors are not able to account for the observed enhancement, since they would justify enhancement only at wavelengths higher than 523 nm. The results of nonlinear least squares fitting with this equation show that: (i) the peak positions do not shift appreciably with potential (1202.68 ( 1.82 and 1224.92 ( 1.43 cm-1); (ii) the integrated intensities of the two peak are essentially potential independent as well (11.02 ( 1.73 and 4.48 ( 1.55 au); (iii) the oscillator damping constants of the two peaks are 14.4 ( 2.9 cm-1 and 20.6 ( 1.1 cm-1 and for two peak components at higher and lower wavenumbers, respectively. It is thus possible
Figure 5. SFG spectra of a neutral CTDB solution, in contact with a Cu(100) electrode, recorded at 532 nm at the indicated potentials.
to conclude that: (i) no Stark shift can be measured for the two peaks, denoting no electron backdonation from the adsorption bond to the electrode; (ii) the slight changes in integrated intensity might relate to some limited changes of surface coverage or orientation of the adsorbate; (iii) the applied potential, in the investigated range, does not bring about changes in the Cu surface conditions able to affect the heterogeneous broadening. OPO-SFG Experiments. Neutral Electrolytes. In Figure 5 we report a sequence of potential-evolved SFG spectra measured with the Cu electrode in contact with the neutral electrolyte. No discernible peaks can be noticed for potentials less cathodic than -0.5 V, while a positive band is found in the nitrile stretching range of wavenumbers for more cathodic polarizations. As discussed in previously, -0.5 V corresponds to the potential required for cleavage of the NdN bond of CTDB. In this experiment no appreciable enhancement of the SFG band is found. This can be explained as follows: (i) the fixed ωVIS,INPUT ) 532 nm is off the visible absorption peak of CTDB at pH 7; (ii) 3CA and ACETHQ have no chromophores in the VIS able to yield DR-SFG with the available VIS input. Nevertheless, on the basis of SERS work previously reported,5 we have reasons to believe that (i) unreacted CTDB is adsorbed at the Cu electrode at potentials less cathodic than the NdN cleavage threshold and (ii) for cathodic potentials enough for CTDB reaction the three species are coadsorbed. As far as band assignment is concerned, since at pH 7 we are not in the presence of DR, the sign of the resonance can help discriminate between the possibilities allowed by the surface selection rules, along the lines previously discussed.22 (i) If CTDB is adsorbed flat, the aliphatic nitrile would exhibit a vertical component, with the C atom facing the Cu surface. (ii) If 3CA is adsorbed in a vertical or tilted configuration, adsorption would go on preferentially through the nitrile rather than the amine moiety; in this case the N atom would face the Cu eletrode surface. (iii) If ACETHQ is adsorbed flat on the electrode, the nitrile dipole would be approximately in the same tilted orientation as with CTDB. In fact, in view of many experimental and theoretical results,1,23–25 the sign of the nitrile singly resonant SFG band depends on the orientation of its dipole with respect to the Cu electrode. In particular, the positive SFG bands shown in Figure 5 are due to the fact that the CtN dipole is oriented with the N atom facing the Cu surface. Since (i) adsorption of CTDB and ACETHQ via a nitrile can be hardly reconciled with the molecular geometry, (ii) adsorption of 3CA via the nitrile is on the contrary favored by dipole considerations and has been demonstrated for cognate systems,
11794 J. Phys. Chem. C, Vol. 112, No. 31, 2008
Figure 6. SFG spectra of a CTDB solution, of pH 3, in contact with a Cu(100) electrode, recorded at 532 nm at the indicated potentials.
such as 4-cyanopyridine,1 (iii) we have positive SFG information about the presence of 3CA at the electrode, moreover (iv) the potential at which CN bands became visible corresponded to the formation potential of 3CA: we propose to assign this comparatively weak band to CtN stretching of 3CA. Negative peaks related to the dangling aliphatic nitrile of CTDB and ACETHQ may, nonetheless, be present, but not be visible owing to exceedingly low signal-to-noise ratio. Electrolytes at pH 3. In Figure 6 we report a sequence of potential-evolved SFG spectra measured with the Cu electrode in contact with the electrolyte at pH 3. As in the case of the neutral solution, no SFG peaks are found at potentials higher than -0.5 V, while a negative CtN stretching band is observed for more cathodic potentials. It is worth noting that the spectra measured at the highest cathodic potentials also contain a negative band in the adsorbed CN- stretching range, coherently with our SERS observations; this negative band at 2170 cm-1 is not found at pH 7 because nitrile is more reactive in a protonating environment. The sign of the SFG bands as well as their intensity show that no enhancement of the SFG signal is present. Apart from the band orientation, the same discussion applies as in the previous section. The destructive interference between the contributions to χ(2)SPO corresponding to the adsorbate vibrational modes and the response to the visible of the metal substrate denotes an inversion of the orientation of the nitrile dipole. Furthermore, even though the VIS absorption band of CTDB is shifted by the pH change of the solution,26,27 in the particular case of CTDB such shift boils down to a bathochromic effect with progressive acidification, is not enough to bring the visible absorption of the adsorbate in resonance with the VIS laser input. The presence of a negative nitrile peak in singly IR resonant conditions, suggests that the nitrile orientation with respect to the Cu substrate is inverted by shifting the pH from 7 to 3. Protonation of the amine moiety of 3CA with formation of a quaternary ammonium functionality can explain the change of sign of the SFG peak. In fact, the quaternary ammonium terminal becomes the preferred adsorption site with respect to the nitrile functionality. Regarding peak assignment, we stick to the same conclusions drawn above, since the two experiments were run under identical conditions with the exception of the pH. Electrolytes at pH 0.5. In Figure 7 we report a sequence of potential-evolved SFG spectra measured with the Cu electrode in contact with the electrolyte at pH 0.5. The SFG spectra are strongly enhanced, with signals typically higher than 2 orders of magnitude with respect to those measured at the other pH
Bozzini et al.
Figure 7. SFG spectra of a CTDB solution, of pH 0.5, in contact with a Cu(100) electrode, recorded at 532 nm at the indicated potentials.
Figure 8. The chief species, related to CTDB, present in the pH and potential ranges investigated.
values investigated, under otherwise identical conditions. It is worth noting that the nonresonant SFG signal remains weak, indicating that the contribution of the interband transition of the metal substrate to the SFG response is overwhelmed by the molecular visible resonance. Well defined, positive nitrile stretching peaks are measured in the whole investigated potential range. Furthermore, at potentials higher than -0.5 V, an abrupt jump in the peak position (from ca. 2240 to ca. 2230 cm-1) and intensity can be noticed. This remarkable SFG enhancement can be explained on the basis of double resonance due to the fact that the bathochromic effect of acidification has brought the chromophore electronic transition of CTDB in resonance with the visible laser line. This is coherent with the shift in visible spectra reported in Figure 1. The chief species present in the different pH and potential ranges investigated, resulting from cleavage of the NdN bond and protonation, are summarized in Figure 8. In the framework of the discussions presented in the previous sections and with the available information about the interfacial chemistry of CTDB, we are able to explain all the SFG phenomenology observed. In particular, we reckon that the bands shown in Figure 7 for potentials less cathodic than the CTDB reaction threshold (-0.5 V) can be assigned to unreacted CTDB. As discussed above, the fact that a positive band is found, even if the relevant nitrile points the C atom toward to electrode for straightforward steric reasons, is coherent with DR enhancement of the SFG signal.
DR-SFG Spectroscopy of Adsorbates When the potential of ca. -0.5 V, required for the cleavage of the NdN bond is reached, the spectra shown in Figure 7 exhibit an abrupt change in peak position by ca. 10 cm-1 and a notable intensity increase. It is beyond the scope of this paper to provide a definitive explanation of this change, and we simply hint at some guidelines for more insightful investigations. (i) We do not expect 3CA to undergo DR enhancement at 532 nm, since we did not find this effect in the aromatic frequency range investigated by CLIO and discussed in the relevant section; even though the SFG selection rules are satisfied, we therefore doubt that a response from 3CA can add to that of CTDB. (ii) ACETHQ does not possess a chromophore absorbing at 532 nm. In principle, owing to the adsorption bond, this adsorbate could develop an electronic transition justifying DRSFG, as conjectured for adsorbed 3CA, but this can be excluded on the basis of the fact that no such enhancement was found neither at pH 7 (Figure 5) nor at pH 3 (Figure 6), where the conditions for DR-SFG would have been the same, owing to the absence of a bathochromic shift for this species. (iii) We are therefore left with the only option that the band still corresponds to CTDB, but adsorbed in a chemically different environment, due to reaction and coadsorption with 3CA and ACETHQ and possible concurrent hydrogen evolution reaction. This conjecture is required because such enhancement cannot be explained on the basis of variations of surface coverage of CTDB, which is expected to decrease owing to electrochemical reactivity, and of ΓVIS,MOL and ΓIR,MOL, which are expected to increase owing to coadsorption.28 Of course, further investigations and a more insightful study of the interfacial electrochemistry of CTDB can clarify many of these aspects that cannot be conclusively addressed within the scope of this paper. Conclusions In this paper we report an in situ FEL-CLIO/SFG, OPO/SFG, and DFT investigation of Cu electrode in contact with aqueous solutions containing CTDB at pH 0.5, 3, and 7. Strong SFG enhancement was obtained in the IR range 1100-1300 cm-1, corresponding to ArsCtN stretching, with the visible beam set at 441.6 nm and in the IR range 2000-2300 cm-1, corresponding to nitrile stretching, with the visible beam set at 532 nm and the pH adjusted in order to match the bathocromic shift of the adsorbed CTDB chromophore with the input visible laser frequency. The two instances of enhancement highlighted correspond to two different mechanisms. The former is due to the resonance of the visible input with the adsorption bond of the reaction product 3-cyan aniline to the Cu electrode; the second one is caused by the resonance of the molecular electronic structure with the visible input. Electrochemical DRSFG, proved feasible in this paper, offers a novel tool for electrochemical surface science, since it allows to probe adsorbate states in situ, it is characterized by a high signal-tonoise ratio and can be applied to atomically flat as well as nanoparticle systems without need of special surface pretreatments.
J. Phys. Chem. C, Vol. 112, No. 31, 2008 11795 References and Notes (1) Tadjeddine, A.; Le Rille, A. In Interfacial Electrochemistry: Theory, Experiment, and Applications; Wieckowski, A. , Ed.; Marcel Dekker: 1999; pp 317-343. (2) Peremans, A.; Tadjeddine, A.; Guyot-Sionnest, P. Chem. Phys. Lett. 1995, 247, 243. (3) Vidal, F. Etude de l’adsorption de CO et de l′e´lectro-oxydation du me´thanol sur monocristaux de platine par spectroscopie SFG. The`se de doctorat, Universite´ Paris XI, UFR Scientifique d’Orsay (F), 2003. (4) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Gill, P. M. W.; Johnson, B. G.; Robb, M. A.; Cheesemann, J.; Keith, T. A.; Petersson, G. A.; Montgomery, J. A.; Raghavachari, K.; Al-Laham, M. A.; Zakrewski, V. G.; Ortiz, J. V.; Foresman, J. B.; Cioslowski, J.; Stefanov, B. B.; Nanayakkara, A.; Challacombe, M.; Peng, C. Y.; Ayala, P. Y.; Chen, W.; Wong, M. W.; Andres, J. L.; Replogle, E. S.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Binkley, J. S.; Defrees, D. J.; Baker, J.; Stewart, J. P.; HeadGordon, M.; Gonzales, C.; Pople, J. A. Gaussian03; Gaussian, Inc.: Pittsburgh, PA, 2003. (5) Bozzini, B.; D’Urzo, L.; Mele, C.; Busson, B.; Humbert, C.; Tadjeddine, A. Submitted for publication. (6) Chou, K. C.; Westerberg, S.; Shen, Y. R.; Ross., P. N.; Somorjai, G. A. Phys. ReV. B 2004, 69, 153413. (7) Le Rille. A. Spectroscopie non line´aire de l’interface e´lectrochimique par ge´ne´ration des fre´quences somme et diffe´rence. The`se pre´sente´e pour obtenir le grade de docteur en sciences de l’Universite´ Paris XI, Orsay, 1997. (8) Ehrenreich, H.; Philipp, H. R. Phys. ReV. 1962, 128, 1622. (9) Etchegoin, P. G.; Le Ru, E. C.; Meyer, M. J. Chem. Phys. 2006, 125, 164705. (10) Jha, S. S.; Warke, C. S. Phys. ReV. 1967, 153, 751. (11) Walters, M. J.; Roy, D. Appl. Spectrosc. 1998, 12, 1554. (12) Shen, Y. R. The Principles of Nonlinear Optics; John Wiley and Sons: New York, 1984; p 5765. (13) Bozzini, B.; Busson, B.; De Gaudenzi, G. P.; D’Urzo, L.; Tadjeddine, A. J. Electroanal. Chem. 2007, 602, 61. (14) Hayashi, M.; Lin, S. H.; Raschke, M. B.; Shen, Y. R. J. Phys. Chem. A 2002, 106, 2271. (15) Raschke, M. B.; Hayashi, M.; Lin, S. H.; Shen, Y. R. Chem. Phys. Lett. 2002, 359, 367. (16) Matranga, C.; Guyot-Sionnest, P. J. Chem. Phys. 2001, 115, 9503. (17) Dreesen, L.; Humbert, C.; Celebi, M.; Lemaire, J. J.; Mani, A. A.; Thiry, P. A.; Peremans, A. Appl. Phys. B: Laser Opt. 2002, 74, 621. (18) Humbert, C. De´veloppement d’une nouvelle spectroscopie optique non line´arie utilisant la ge´ne´ration de fre´quence somme doublement re´sonante. Application a` l′e´tude des couplages vibrationnels et e´lectroniques aux interfaces films minces-me´taux. The`se de doctorat, Faculte´s Universitaires Notre-Dame de la Paix, Namur (B), 2003. (19) Zhuang, X.; Miranda, P. B.; Kim, D.; Shen, Y. R. Phys. ReV. B 1999, 59, 12632. (20) Palik, E. D. Handbook of Optical constants of solids; Academic Press Inc.: Orlando, 1985. (21) Palik, E. D. Handbook of Optical constants of solids II; Academic Press: Orlando, 1991. (22) O. Pluchery. Etude par spectroscopie non line´aire SFG and DFG de l’asdorption de la 4- cyanopyridine sur des e´lectrodes d’or. The`se pre´sente´e pour obtenir le grade de docteur en sciences de l’Universite´ Paris XI, Orsay, 2000. (23) Bozzini, B.; Fanigliulo, A. J. Appl. Electrochem. 2002, 32, 1043. (24) Bozzini, B.; Mele, C.; Fanigliulo, A.; Busson, B.; Vidal, F.; Tadjeddine, A. J. Electroanal. Chem. 2004, 574, 85. (25) Matranga, C.; Wehrenberg, B. L.; Guyot-Sionnest, P. J. Phys. Chem. B 2002, 106, 8172. (26) Jiang, C.; He, W.; Tai, Z.; Ouyang, J. Spectrochim. Acta A 2000, 56, 1399. (27) Rouhollahi, A.; Kiaie, F. M.; Ghasemi, J. Talanta 2005, 66, 653. (28) Bozzini, B.; D’Urzo, L.; Mele, C. J. Electrochem. Soc. 2005, 152, C255.
JP710608Q
