Unraveling Molecular Adsorption with Surface Raman Spectroscopy

Dec 9, 2010 - Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei, Taiwan, Institute of Atomic and Molecu...
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Unraveling Molecular Adsorption with Surface Raman Spectroscopy: trans-Stilbene, trans,trans-Distyrylbenzene, and trans-Azobenzene on Ag/Ge(111) Li-Wei Chou,†,‡ Ya-Rong Lee,‡ Jyh-Chiang Jiang,*,† Jiing-Chyuan Lin,*,‡ and Juen-Kai Wang*,‡,§ Department of Chemical Engineering, National Taiwan UniVersity of Science and Technology, Taipei, Taiwan, Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei, Taiwan, Center for Condensed Matter Sciences, National Taiwan UniVersity, Taipei, Taiwan ReceiVed: September 19, 2010; ReVised Manuscript ReceiVed: NoVember 9, 2010

Deposition of organic molecules on inorganic substrates is an important step in the fabrication of organic electronic devices with designated optoelectronic properties. Their interaction with the surface underneath and the interaction among themselves influence the molecular orientation of the first adsorbed layer and thus energy and charge transport properties of the grown organic films. We show that these interactions of transstilbene, trans,trans-distyrylbenzene, and trans-azobenzene adsorbed on Ag/Ge(111) -(3 × 3)R30° surface are reflected on their Raman signatures. Detailed spectral analysis reveals the characteristic vibrational features corresponding to the molecules residing in the first adlayer and in the layers above, separately. The results further show that for all the three different molecular systems, the integrated intensity of the multilayer Raman signature increases linearly with the molecular coverage. This study explores potential utilization of Raman spectroscopy as a sensitive, noninvasive tool to characterize the intricate molecule-surface interaction and to quantify physisorbed molecular systems. 1. Introduction Integrating thin films onto a substrate surface represents a promising approach to adding distinct surface functionalities relative to the substrate underneath, such as friction, chemical reactivity, electrical conductivity, optical reflectivity, and so forth.1-8 Among the coating materials used, organic materials consisting of π-conjugated molecular structures are particularly of great interest to the community of organic electronics.1-5 On the one hand, the charge transport through a metallic electrode is sensitive to the molecular orientation of adsorption.2,9 On the other hand, the energy and charge transport within organic films requires careful layer-by-layer control of film deposition and is sensitive to the relative orientation between adjacent molecules.10,11 Both considerations are fundamental to the development of efficient organic light-emitting and photovoltaic devices. The energetic nature of adsorbate-surface and intermolecule interaction that regulates the π-conjugated overlayer growth from submonolayer to multilayer therefore receives great attention.1,2,10,11 The former interaction plays a key role in the arrangement of the first adsorbed layer (adlayer), whereas the latter one is expected to influence the molecular packing configuration in multilayer growth. Although scanning probe microscopy provides a direct probe to pinpoint the location and orientation of each adsorbed molecule at the first layer,12 it lacks the ability to decipher the delicate energetic factor of the absorbate-surface interaction and to quantify the multilayer growth, owing to its low energy resolution and its inability to resolve individual entities within disordered organic aggregates. * Towhomcorrespondenceshouldbeaddressed.E-mail:[email protected] (J.-K.W.), [email protected] (J.-C.L.), [email protected] (J.-C.J.). † National Taiwan University of Science and Technology. ‡ Academia Sinica. § National Taiwan University.

Raman spectroscopy offers a number of advantages in the context of monitoring molecular adsorption.13,14 First, as an optical technique, it is a noninvasive analytical method, allowing for identifying the composition and structure of the adsorbate from the obtained vibrational characteristics yet preserving its molecular integrity. Second, its high energy resolution enables the differentiation of minute spectral variation caused by disparate interaction characters within sole molecular adsorption system described above. These two characters are of great contrast to other electron-based spectroscopic techniques, such as electron-energy loss spectroscopy, Auger electron spectroscopy, and photoemission spectroscopy, for the potentially destructive influence of energetic electrons and for their comparably low energy resolution. Third, the orientation of uniform adsorbates with respect to the surface can be deduced from the Raman spectra that are taken at specific polarization schemes, owing to the directional character of Raman polarizability. Finally, the Raman signal strength generated with a nonabsorbing excitation light wave reflects the total amount of deposited species because it is barely affected by the surface in close proximity. This is noticeably distinct from fluorescence that can be greatly depleted by a nearby metallic object through efficient energy transfer.15,16 Such a vantage point is particularly attractive, as the technique can potentially serve as an in situ quantitative diagnostic tool for the amount of adsorbed species. In our previous study,17 we demonstrated the vibrational features of a self-organized submonolayer of physisorbed trans-stilbene (TSB) on Ag/Ge(111) - (3 × 3)R30° (abbreviated as Ag/ Ge(111) - 3) surface with Raman spectroscopy and showed that the CdC stretching mode of the olefinic group experiences frequency blue-shifting as the apparent surface coverage (θ) increases from below one monolayer (ML) to multilayer.17 On the other hand, the peak width is broadened as θ is increased, whereas it is narrowed as θ is more than 1.3 ML, as shown in the Figure 3 of ref 17. Two questions then transpire from this

10.1021/jp108953y  2011 American Chemical Society Published on Web 12/09/2010

Molecular Adsorption with Surface Raman Spectroscopy peculiar observation. What is the origin of the observed spectral profile variation? And, from a more general perspective, can the entangled molecule-surface and intermolecular interactions, prevailing during the organic film growth, be disclosed by a more detailed spectral analysis? The answers to these questions would lead to the emergence of a new tool to examine molecular deposition. In this article, we present the results of surface Raman spectroscopic measurements of trans-stilbene, trans,transdistyrylbenzene (TTDSB), and trans-azobenzene (TAB) adsorbed on the Ag/Ge(111) - 3 surface plus their spectral profile analysis at different deposition conditions. The three molecules are chosen as model systems to represent typical conjugated molecules that physisorb favorably on semiconducting and metallic surfaces. We have interrogated their adsorption on the Ag/Ge(111) - 3 surface with low-temperature scanning tunneling microscopy in ultrahigh vacuum.18-20 At the monolayer adsorption, TSB forms large (2 × 1) ordered domains on the surface where the phenyl groups resides nearly perfectly on two adjacent Ag-trimer sites, because the separation between the two phenyl rings matches well with the surface lattice constant.18 In the case of TTDSB, the distance between its two terminal phenyl rings is about twice the surface lattice constant, allowing the adsorbed molecules self-organize into (3 × 1) ordered domains.19 Lastly, small two-dimensional TAB chain structures, formed on theAg/Ge(111) - 3 surface, also transform into large (2 × 1) ordered domains at high coverages,20 presumably owing to its comparable size as TSB and extra intermolecular hydrogen bonding. The packing models of the commensurate superstructures of these molecules on the surface are presented in Figure S1 in Supporting Information. Although the three molecules are all conjugated molecules with double bonds and phenyl rings, the distinctions in their molecule-surface and intermolecular interactions as well as their sizes cause the variation in their emergent surface domains. With one more repeating styryl group than TSB, TTDSB offers an opportunity to perceive how the elongated molecular shape influences the ultimate molecular arrangement of deposition. In spite of their comparable sizes, the additional hydrogen-bond interaction between adjacent TAB molecules and their stronger interaction with the surface direct the adsorbed molecules into a distinct pattern from that of TSB. The comparison study made with Raman spectroscopy among these three cases thus provides an insight into the dependence of molecular packing in film deposition on molecular shape and the interactions with circumjacent molecules and with the surface, which are difficult, if not impossible, to obtain with other experimental techniques. 2. Experimental Methods The experimental setup and procedure were detailed elsewhere.17,21 Briefly, surface Raman spectroscopy was performed in a home-built ultrahigh vacuum (UHV) chamber ( 20 ML respectively during the fitting process. The reason for conducting such spectral analysis will manifest itself below. 3. Results and Discussion Part a of Figure 1 shows three characteristic Raman spectra of TSB, from 900 to 1700 cm-1, deposited on the Ag/Ge(111) - 3 surface at the coverage conditions of θ ) 0.8, 1.3, and 20 ML. The relevant vibrational features have been assigned in our previous study.17 The CdC stretching mode evolves from 1625 to 1637 cm-1 as the surface coverage increases and reaches its largest full width at half-maximum of 26 cm-1 at θ ) 1.3 ML which is almost twice of those at θ ) 0.8 and 20 ML, suggesting that this perceptible width evolution may come from the progressive variation of the relative contribution of the vibrational signatures at the submonolayer coverage, where the adsorbed molecules directly interact with the Ag/Ge(111) - 3 surface, and at the multilayer coverage, where the deposited molecules predominantly interact among themselves. Namely, the spectral feature of this CdC stretching mode at any coverage condition is composed of that purely from the first adlayer and that from the multilayer. The insert of part b of Figure 1 illustrates the two decomposed Lorentzian peaks at θ ) 1.3 ML, representing the vibrational features of the two molecular species. This spectral profile analysis thus confers the peak positions and their relative contributions. The contributions of these two species are shown in part b of Figure 1. Notice that the integrated Raman intensity of the 1637 cm-1 multilayer peak increases linearly with the surface coverage, while that of the 1625 cm-1 remains unchanged. The observation of the constant intensity of the 1625 cm-1 peak as a function of the surface

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Figure 1. (a) Experimental Raman spectra of trans-stilbene in the coverage conditions of 20, 1.3, and 0.8 ML on Ag/Ge(111) - (3 × 3)R30° surface, where ML represents monolayer. Characteristic peaks were extracted by curve fitting with Lorentzian profiles (solid curves) and marked numerically. (b) Normalized integrated intensities (I/I0) of the Raman peaks of the molecules directly adsorbed on the surface and that on top of the first adlayer as a function of surface coverage (θ). Blue squares and red circles represent respectively the integrated intensities of the deconvoluted peaks (the blue peak at 1637 cm-1 and red peak at 1625 cm-1, as shown in the insert for θ ) 1.3 ML). I0 is the integrated Raman intensity of the red peak at its saturated monolayer coverage. Blue and red lines are linear fitting curves up to θ ) 20 ML. To more clearly reveal the data at low coverage, the data points after 14 ML are not displayed.

coverage is consistent with the fact that this Raman peak reflects the molecules of the first adsorbed layer. This observation thus answers the first question raised above regarding the observed variation of the spectral profile. The distinction between the Raman shifts of the first adsorbed layer and the molecules deposited above is attributed to the attractive interaction between the olefinic group and the silver atoms on the surface.17 Furthermore, one inference drawn from this analysis result is that the first layer of adsorbed molecules remains intact during the continuing dosing process in terms of their vibrational characteristics. On the other hand, the linear dependence of the Raman intensity of the blue peak on the surface coverage indicates that the sticking coefficient at high coverage, creating multilayer, remains the same. As another note, the retrieved high-frequency peak at 1637 cm-1 in the coverage condition of θ ) 1.3 ML matches that obtained at θ ) 20 ML, indicating that the second layer of TSB, which is estimated about 5 Å

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Figure 2. (a) Experimental Raman spectra of trans,trans-distyrylbenzene in the coverage conditions of 72, 1.4, and 0.8 ML on Ag/Ge(111) - (3 × 3)R30° surface, where ML represents monolayer. Characteristic peaks were extracted by curve fitting with Lorentzian profiles (solid curves) and marked numerically. (b) Normalized integrated intensities (I/I0) of the Raman peaks of the molecules directly adsorbed on the surface and that on top of the first adlayer as a function of surface coverage (θ). Blue squares and red circles represent, respectively, the integrated intensities of the deconvoluted peaks (the blue peak at 1631 cm-1 and red peak at 1622 cm-1, as shown in insert for θ ) 1.4 ML). I0 is the integrated Raman intensity of the red peak at the saturated monolayer coverage. Blue and red lines are linear fitting curves up to θ ) 72 ML. To more clearly reveal the data at low coverage, the data points after 14 ML are not displayed.

above the surface by the van der Waals radius, barely experiences any effect from the surface. This result also suggests that the intermolecular interaction between adjacent TSB layers remains almost unchanged and hardly causes any noticeable effect on the Raman spectra. Similar to the case of TSB on the Ag/Ge(111) - 3 surface, the two benzoic C-C stretching modes and the CdC stretch in the olefinic groups of TTDSB on the same surface appear in the spectral range from 1530 to 1670 cm-1, exemplified in the deposition conditions of θ ) 0.8, 1.4, and 72 ML, as shown in part a of Figure 2. The spectral assignment agrees with the previous study in solution by Hrenar and co-workers.23 The Raman peaks of the CdC stretching mode undergo similar frequency shifting and peak-width variation as in the case of TSB on the Ag/Ge(111) - 3 surface. The spectral profile analysis was also applied to the CdC stretching mode of TTDSB at different coverage conditions, yielding two decomposed Lorentzian peaks at θ ) 1.4 ML. Part b of Figure 2 shows the corresponding integrated Raman

Molecular Adsorption with Surface Raman Spectroscopy intensities of the two decomposed peaks centered at 1622 and 1631 cm-1, shown in the insert of part b of Figure 2, as a function of its surface coverage. The frequency splitting of 9 cm-1 is smaller than that in the case of TSB. The smaller frequency splitting could be originated from the fact that the two terminal phenyl rings of TTDSB are twisted by a torsional angles of 26° with respect to the center one.19 When being adsorbed with the central phenyl rings lying parallel to the substrate, the two olefinic CdC bonds tend to locate further away from the surface due to the steric hindrance, leading to weaker interaction with the substrate as compared to the case of adsorbed TSB, where two phenyl rings are almost coplanar18 and olefinic CdC bond would be close to the surface. Notably, the Raman intensity of the 1622 cm-1 peak increases linearly with the surface coverage before the fluorescence onset is reached, reflecting the formation of the first layer of TTDSB on the surface. This behavior is almost invisible in the case of TSB, possibly due to the smaller Raman cross section of TSB than that of TTDSB (below). Finally, can the conclusions reached from the experimental results of TSB and TTDSB on the Ag/Ge(111) - 3 surface be obtained similarly for TAB, in the light that TAB consists of two phenyl rings linked by the azo group instead of the olefinic group? Likewise, part a of Figure 3 shows the Raman spectra of TAB adsorbed on the Ag/Ge(111) - 3 surface in the deposition conditions of θ ) 0.7, 2.7, and 20 ML. In comparison, the NdN stretching mode (1427 cm-1) of the submonolayer TAB upon direct adsorption on the surface has a red shift of 14 cm-1 with respect to the multilayer one in part a of Figure 3, suggesting that the azo group interacts more strongly with the surface. The known nonbonding character of the azo group in the (highest occupied molecular orbital) HOMO of TAB is very likely to interact more strongly with the surface to form the hybridization states as compared to the HOMO of TSB, which is mainly of π-bonding characteristics over the olefinic group. The intense electronic disturbance on the NdN double bond can be expected and, thus, effectively weaken the resonance of the conjugated character. In addition to the NdN stretching mode located at 1430 cm-1, the two adjacent peaks at 1470 and 1485 cm-1 are assigned to the main benzoic C-C vibrations coupled with the C-N inplane bending and the NdN stretching modes, respectively (Supporting Information).24,25 The broadened NdN stretching mode at θ ) 2.7 ML is decomposed into two peaks (1427 and 1441 cm-1, the insert of part b of Figure 3). As the coverage increases, the dependences of the Raman intensities of both decomposed peaks on the surface coverage, shown in part b of Figure 3, behave similarly as the CdC stretching modes of TSB and TTDSB. As a final note, the extracted linear proportional coefficients of the multilayer signatures with respect to the surface coverage are 1.1 (0.02), 1.3 (0.06), and 1.0 (0.03) for TSB, TTDSB, and TAB respectively, where the numbers in the parentheses are the corresponding fitting errors. This result indicates that the Raman cross-section of the first layer TAB remains invariable upon adsorption, whereas those of TSB and TTDSB decrease significantly. The invariable Raman crosssection of TAB upon adsorption might result from the fact that the Raman polarizability of the azo moiety in TAB responds differently to the surface adsorption from that of the olefinic groups in TSB and TTDSB. Further investigation is needed to disclose its origin. As a consequence, the maximal peak widths of the CdC and NdN stretching mode are achieved at different surface coverage conditions, since the requisite for the maximal width is that the Raman signals from the first adlayer and from the multilayer are comparable.

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Figure 3. (a) Experimental Raman spectra of trans-azobenzene in the coverage conditions of 20, 2.7, and 0.7 ML on Ag/Ge(111) - (3 × 3)R30° surface, where ML represents monolayer. Characteristic peaks were extracted by curve fitting with Lorentzian profiles (solid curves) and marked numerically. (b) Normalized integrated intensities (I/I0) of the Raman peaks of the molecules directly adsorbed on the surface and that on top of the first adlayer as a function of surface coverage (θ). Blue squares and red circles represent the integrated intensities of the deconvoluted peaks (the blue peak at 1441 cm-1 and red peak at 1427 cm-1, as shown in the insert for θ ) 2.7 ML). I0 is the integrated Raman intensity of the red peak at the saturated monolayer coverage. Blue and red lines are linear fitting curves up to θ ) 20 ML. To more clearly reveal the data at low coverage, the data points after 14 ML are not displayed.

On the basis of the three surface Raman spectroscopic results presented above, four common experimental characteristics can be concluded. First, the integrated Raman intensity of the molecules adsorbed directly on the surface increases linearly with the surface coverage until reaching fluorescence onset, if its Raman cross section is large enough for the observation. The combination of this observation and the fact that its Raman peak frequency and width remain constant indicates that the molecular configuration of the adsorbed molecules remains intact as the first adlayer is formed. Second, after the onset of the fluorescence signal, the Raman characteristics of the first-layer peak (intensity, peak frequency, and peak width) remain unchanged as the coverage is increased, further corroborating that the additional deposited molecules above do not alter the molecular configuration of the first adlayer and therefore its interaction with the surface. Third, the integrated Raman intensity corresponding to the molecules located above the first

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adlayer increases linearly with the coverage, whereas its Raman spectral profile stays identical. This result shows that, similar to the first adlayer, the multilayer adsorption does not modify the nature of the molecules and the sticking coefficients in different coverage conditions remain constant. Fourth, the Raman peak corresponding to the multilayer emerges at the fluorescence onset and the peak position remains unchanged at the coverages whereas the Raman signal of the adlayer reaches a plateau. This observation further supports that the secondlayer adsorption begins as the first-layer adsorption is nearly completed under the present adsorption condition and the molecules adsorbed directly on top of the first adlayer do not sense the interaction from the surface, at least in view of Raman spectroscopy, although they are only separated from the surface by the first adlayer. All of these experimental inferences strongly suggest that Raman spectroscopy can potentially serve as a noninvasive probe to determine the amount of molecules adsorbed on the surface, namely the surface coverage, as compared with the traditional temperature-programmed desorption method - which is an invasive characterization method or other common electron-based techniques, like the Auger spectroscopy, high-resolution electron energy loss spectroscopy, and so forth, which may disturb the intrinsic adsorption behavior of physisorbed molecules during the detection. Moreover, the high spectral resolution of Raman spectroscopy enables disentangling the two mingled energetic interactions - moleculesurface and intermolecular interactions - in scores of molecular deposition conditions. 4. Conclusions Systematic layer-by-layer Raman characterization of transstilbene, trans,trans-distyrylbenzene, and trans-azobenzene on Ag/Ge(111) - (3 × 3)R30° surface were conducted to reveal the effects of molecular-surface and intermolecular interaction on their adsorption. The Raman peaks corresponding to their CdC and NdN stretching modes were found to consistently undergo significant blue shifting and width variation as the deposition condition was varied from submonolayer to multilayer. The frequency and the integrated intensity of the decomposed low-frequency peak, reflecting the molecules in the first adlayer, remain constant as multilayer molecules are deposited onto the surface. For trans,trans-distyrylbenzene, the two severely twisted terminal phenyl rings (relative to the center one) tend to cause adsorbed molecules sitting further away from the surface, leading to a weakened molecule-surface interaction and thus inducing a smaller frequency shift in the CdC stretch than that of trans-stilbene. A larger shift of the NdN stretch of trans-azobenzene at the first adlayer is conversely caused by stronger interaction of its NdN bond with the surface silver atoms. The integrated intensity of the decomposed highfrequency peak, signifying the molecules above the first adlayer, increases linearly with the surface coverage, whereas its

Chou et al. frequency is unchanged. The relatively invariable intermolecular interaction entails the multilayer molecules to behave identically in terms of Raman spectroscopy. These observations demonstrate that Raman spectroscopy is a powerful tool to characterize the weak molecule-surface interaction and to quantify the amount of deposited molecules in physisorption-type systems. Acknowledgment. The authors thank financial support from the National Science Council and Academia Sinica in Taiwan. Supporting Information Available: Additional information mentioned within the text is provided. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Madueno, R.; Raisanen, M. T.; Silien, C.; Buck, M. Nature 2008, 454, 618. (2) Eremtchenko, M.; Schaefer, J. A.; Tautz, F. S. Nature 2003, 425, 602. (3) Comstock, M. J.; Levy, N.; Kirakosian, A.; Cho, J.; Lauterwasser, F.; Harvey, J. H.; Strubbe, D. A.; Frechet, J. M. J.; Trauner, D.; Louie, S. G. Phys. ReV. Lett. 2007, 99, 38301. (4) Berggren, M.; Nilsson, D.; Robinson, N. D. Nat. Mater. 2007, 6, 3. (5) Malliaras, G.; Friend, R. Phys. Today 2005, 58, 53. (6) Ulgut, B.; Abruna, H. D. Chem. ReV. 2008, 108, 2721. (7) Misra, R.; Li, J.; Cannon, G. C.; Morgan, S. E. Biomacromolecules 2006, 7, 1463. (8) Lin, M.-H.; Chen, H.-Y.; Gwo, S. J. Am. Chem. Soc. 2010, 132, 11259. (9) Zang, L.; Che, Y.; Moore, J. S. Acc. Chem. Res. 2008, 41, 1596. (10) Bredas, J.-L.; Beljonne, D.; Coropceanu, V.; Cornil, J. Chem. ReV. 2004, 104, 4971. (11) Coropceanu, V.; Cornil, J.; da Silva Filho, D. A.; Olivier, Y.; Silbey, R.; Bredas, J.-L. Chem. ReV. 2007, 107, 926. (12) Chen, C. J. Introduction to Scanning Tunneling Microscopy, 2nd ed.; Oxford University Press: New York, 2007. (13) Yates, J. T., Jr.; Madey, T. E. Vibrational spectroscopy of molecules on surfaces; Plenum Press: New York, 1987. (14) Suetaka, W. Surface Infrared and Raman Spectroscopy: Methods and Applications; Springer: New York, 1995. (15) Chance, R. R.; Prock, A.; Silbey, R. AdV. Chem. Phys. 1978, 37, 65. (16) Becker, H.; Burns, S. E.; Friend, R. H. Phys. ReV. B 1997, 56, 1893. (17) Chou, L.-W.; Lee, Y.-R.; Wei, C.-M.; Jiang, J.-C.; Lin, J.-C.; Wang, J.-K. J. Phys. Chem. C 2009, 113, 208. (18) Tsai, C. S.; Su, C.; Wang, J. K.; Lin, J. C. Langmuir 2003, 19, 822. (19) Wu, H. C.; Tsai, C. S.; Chou, L. W.; Lee, Y. R.; Jiang, J. C.; Su, C.; Lin, J. C. Langmuir 2007, 23, 12521. (20) Wu, H. C.; Chou, L. W.; Lee, Y. R.; Su, C.; Lin, J. C. Surf. Sci. 2009, 603, 2935. (21) Wu, H. C.; Chou, L. W.; Wang, L. C.; Lee, Y. R.; Wei, C. M.; Jiang, J. C.; Su, C.; Lin, J. C. J. Phys. Chem. C 2008, 112, 14464. (22) Campbell, T. W.; McDonald, R. N. J. Org. Chem. 1959, 24, 1246. (23) Hrenar, T.; Mitric, R.; Meic, Z.; Meier, H.; Stalmach, U. J. Mol. Struct. 2003, 661-662, 33. (24) Meic, Z.; Baranovic, G.; Smrecki, V.; Novak, P.; Keresztury, G.; Holly, S. J. Mol. Struct. 1997, 408-409, 399. (25) Biswas, N.; Umapathy, S. J. Phys. Chem. A 1997, 101, 5555.

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