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J . Phys. Chem. 1991, 95, 9636-9638
Second Harmonic Generation Coherent Interferometry Disclosing the Orientation of the Submerged Hemicyanine Layer in a Langmuir-Blodgett Multilayer Structure 0. Sato, R. Baba, K. Hashimoto, and A. Fujishima* Department of Synthetic Chemistry, Faculty of Engineering, The Unicersitj, of Tokyo, Hongo, Bunkyo- ku, Tokyo 113. Japan (Received: September 16, 1991)
Second harmonic generation coherent interferometry (SHG-CI) was first applied to the structural study of multilayered LB films. Two kinds of LB multilayers were prepared by installing a hemicyanine monolayer among the layers of stearic acid by way of either Y-type or Z-type deposition process, and those films were investigated by the SHG-CI method in terms of the molecular orientation of the submerged hemicyanine layer. It was shown that the Z-type structure of the hemicyanine layer was readily achieved and remained stable within the LB multilayers. This was the first example to disclose the direction of the molecular axis of a specific layer sandwiched among nonlinearly inactive foreign layers.
Introduction Since Langmuir-Blodgett (LB) films are promising materials for molecular devices and nonlinear optical devices, understanding of the microstructures of various types of LB films is very important. The relation between their molecular structures and their functions such as conductivity and optical nonlinearity, however, has not been made clear in detail yet because of the difficulty in their microscopic characterization. During the past few decades several kinds of probing methods, such as FT-IR1 and Raman scattering,2 have been developed and applied to study LB films and Langmuir films. Optical second harmonic generation measurement3 on the various kinds of surfaces and interfaces has also been focused as one of these probing techniques and it has received much more attention because of its surface specificity and its simplicity in operation. This optically nonlinear method has been used in, for example, estimating the tilt angle of the constituent molecules in Langmuir films,4 observing the phase transition of Langmuir films during their preparation5 and so In the conventional surface second harmonic generation (SHG) measurement those properties such as the intensity and the polarization of the SH light generated from surfaces have mainly been considered. Kemnitz et aL6 and Berkovic et al.,’ on the other hand, monitored the absolute phase of the SH radiation from the monolayers of phenol or organic dyes by using a quartz reference plate traveling along the path of the beams, and they discussed the orientation of the component molecules and the effect of the substrate on the generated SH signal. In our previous paper,* a new type of S H G interferometry was first introduced, and the time phase of the SH waves from the test molecules deposited on a fused silica plate was monitored with reference to that generated from the back surface of the sample substrate which was covered with well-defined molecules as a phase standard of the S H wave. The relative phase of the SH wave radiated from sample molecules to that of the reference can be deduced from the fringe pattern obtained by tilting the substrate against a pump light. By introducing this SHG coherent interferometry (SHGCI). it was shown that the inversion of the molecular configuration in the test LB film brought about the temporal phase shift of r radians in the generated S H waves, accompanying the inversion of the interference fringe. It was also suggested that an infor( I ) Rabe. J . P.: Rabolt, J. F.; Brown, C. A,; Swalen, J . D.J . Chem. Phys. 1986. 84, 4096. ( 2 ) Takenaka. T.; Fukuzaki, H. J . Ramon. Specrrosc. 1979, 8, 151. ( 3 ) Shen, Y. R. Nature 1989, 337, 519. (4) Shirota, K.; Kajikawa, K.;Takezoe. H.: Fukuda, A . Jpn. J . Appl. Phys. 1990, 29, 750. ( 5 ) Rasing, Th.; Shen. Y. R.: Kim, M. W.: Grubb, S. Phys. Rec. Lett. 1985. 55. 2903 ( 6 ) Kemnitz, K.; Bhattacharyya. K.; Hicks, J . M.; Pinto, G . R.; Eisenthal, K. B.: Heinz. T . F. Chem. Phys. Lett. 1986, 131. 2 8 5 . ( 7 ) Berkovic, G . ; Shen. Y . R.; Marowsky, G.; Steinhoff, R. J . Opt. Sac A m B 1989., 6.. ~~. 205. (8) Sato, 0.; Baba, R.; Hashimoto. K.; Fujishima, A . J . Electroanal. Cheni. 1991. 306, 291. ~
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mation as to whether the test molecules directed its molecular axis upward or downward on the substrate glass could be obtained by the SH fringe pattern. Referring a multilayered LB film, it is essential to confirm the actual configuration of molecules in the LB film, since it has been reported9%l0 that the X- and the Z-type structures of LB multilayers are generally unstable and that they sometimes overturn into a stable Y-type structure, and thus the resulted molecular configurations often appear different from what was assumed by their deposition procedure. It is also important to characterize a given molecule located at inner layers of a multilayer structure, because a macroscopic function of a multilayered LB film mostly depends on the inner structure, and every layer except the outermost is buried among other layers in the structure. Hence in this report two types of multilayered LB samples were prepared, both of which were installed with a single layer of a hemicyanine molecule,” Le., 4-[4-(dimethylamino)styryl]- 1-docosylpyridinium bromide, among nonlinearly inactive layers of stearic acid by way of either Y-type (for sample A) or Z-type (for sample B) deposition processes. Then the SHG-CI method was utilized to make clear the orientation of the sandwiched hemicyanine molecules within these LB multilayer structures.
Experimental Section Experimental details for the SHG measurements were described in our previous paper.8 Two kinds of multilayered LB samples as described above and one reference sample were made by the following procedure (Figure 1). For sample A, in the first step, three layers of stearic acid were deposited as Y-type on a fused silica plate to make the substrate surface hydrophobic (step A I ) . A monolayer of 1:3.7 mixture of hemicyanine and arachidic acid (hereafter a hemicyanine film, for short) was transferred in the downward process (step A2). Then a stearic acid layer was further deposited in the upward process (step A3). A single face of the thus prepared sample was stripped off by means of plasma etching and wiped up with acetone, and finally a hemicyanine film was transferred on this bare surface by the upward stroke of the substrate (step A4), while the multicoated face of this sample was protected in the last step by laying a plate glass on it. For sample B, three layers of stearic acid were first deposited (step B1) in the same manner as in step A l , and after that a single face of the sample was stripped off in a way similar to that in step A4. Then, the sample substrate was soaked into a Langmuir trough (step B2), and a monolayer of a hemicyanine film was spread and compressed on the aqueous subphase, and in the upward process the film was transferred onto both faces of the substrate (step B3). Subsequently, this sample was dipped into another clean subphase (9) Kato, T . Chem. Let(. 1988, 1993. ( I O ) Era, M.; Fukuda, M.; Tsutsui, T.; Saito, S. Jpn. J . Appl. Phys. 1987. 26, 1809. ( I I ) Girling, I . R.; Cade, N . A.; Kolinsky, P. V . ; Earls. J D.; Cross, G H . ; Peterson, 1. R. Thin Solid Films 1985, 132, 101
0 199 1 American Chemical Society
The Journal of Physical Chemistry, Vol. 95, No. 24, 1991
Letters
reference
SAMPLE A
step A?
A2
A3
A4
a
b
c
standvd I
B2
B3
E4
r6 test
mdecules SAWLE B step B1
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molecules
sanpleA
I
65
=&-:EEE wnple
Figure 1. Deposition p r d u r e s of samples A and B. Those amphiphilic molecules employed in this study are (a) hemicyanine, (b) arachidic acid,
and (c) stearic acid.
B
++
Figure 3. Schematic illustration of the layered structures for both samples A and B, as well as the reference sample employed: (a) hemicyanine, (b) arachidic acid, and (c) stearic acid.
0
20 40 60 incident angle / deg Figure 2. SHG interferograms of sample A (dashed line), sample B (dash-dot line), and the reference sample (solid line), as a function of the incident angle for plane-polarized pump light.
again with its reference surface of a hemicyanine film covered with a glass slide (step B4), and finally, the stearic acid was spread to form a monolayer, followed by its deposition on the multilayered side of the sample in the upward process (step B5). A reference LB sampe of well-established orientation of the molecules was prepared with a hemicyanine film deposited directly on both sides of a hydrophilic substrate of a fused silica slide, which made this sample symmetrically coated with a hemicyanine film at each surface. The film deposition was carried out at the surface pressure of 25 dyn/cm on an aqueous subphase containing 0.2 m M CdCI2 in every step of the preparation of each sample. From the observed transfer ratio it was confirmed that a monolayer of each film was indeed deposited a t every single stroke of the deposition process. Since the SH intensity from the fused silica substrate and the stearic acid monolayer was found to be so small, the detected S H signal was exclusively attributed to the hemicyanine molecules.
Results and Discussion Figure 2 depicts those SH-fringe patterns obtained for the two kinds of LB samples A and B along with the reference one prepared above. The complete inversion of the fringe patterns observed between samples A and B implies that the molecular axes of the hemicyanine films were different in the pointing direction between those samples, since both of the two samples employed a common film of hemicyanine with the same orientation as the phase standard on one face of each sample. Comparing these patterns with that of the reference sample, the angles which gave the maximum and the minimum of the SH-fringe pattern of sample B appeared in good accordance with those for the reference LB sample. On the other hand, the fringe pattern of sample A was completely inverted against that of the reference in the present study. From these it can be deduced that the alignments of the molecular structure of the hemicyanine in sample A and sample B were just what schematically illustrated in Figure 3. That is,
the hydrophobic side of the hemicyanine layer in sample A is supposed to have faced toward the initially deposited stearic acid films, while in sample B the hemicyanine layer was deposited with its hydrophilic side toward the underlying stearic acid. Hence it was demonstrated that the structure of the submerged hemicyanine layer varied in type depending on the deposition processes. However, this is not necessarily the case, because it was often reportedgJOthat the overturn of molecules may occur in a layer prepared by Z-type deposition. According to the mechanism of the overturn of a molecular layer that was demonstrated by Katog in the case of the compulsory Z-type deposition of 10,12-pentacosadiynoic acid, the molecules overturn three times in total for every two cycles of the compulsory Z-type deposition. If the hemicyanine molecules in the Z-type sample of B followed this mechanism, the overturn of the deposited hemicyanine molecules could occur in the dipping process of the substrate into the aqueous subphase through its clean surface, Le., in step B4. If this was the case, it could have been expected that the SH-fringe patterns which were obtained before and after the deposition of stearic acid on the layer of hemicyanine, i.e., after steps B3 or B5, respectively, were completely inverted with each other. However, this contrast of the fringe patterns for these two samples was not observed, which indicated that the overturn of the hemicyanine layer did not occur in the present multilayers. Since the deposited layers of stearic acid emitted negligible S H light, the Z-type configuration of the layers as illustrated in Figure 3 was empirically assumed by the observed transfer ratio. Hayden et a l l 2 reported that the noncentrosymmetric multilayer of the same hemicyanine film in Z-type structure was accomplished under some conditions, judging from the fact that the S H G signals monotonically increased as the number of the layers in spite of the deviation from the expected quadratic dependence. Considering their experimental results together with our observation in the present study, it can possibly be said that the hemicyanine molecules employed are rather easily to form Z-type structures in LB films. Although the exact reason of this specific feature of a hemicyanine film was not clarified yet, one possible explanation is that the two methyl groups attached to the nitrogen atom of amino group moderately act as a hydrophobic site to form both Y- and Z-type structures. However, the hydrophobic character of those methyl groups seemed relatively weak, since the S H G intensity of sample B appeared weaker as shown in Figure 2, while an intense S H light comparable to that of sample A was observed for the sample B without its outermost layer of (12) Hayden, L. 61, 351.
M.;Kowel, S.T.; Srinivasan, M.P. Upr. Comntun. 1987,
J . Phys. Chem. 1991, 95, 9638-9647
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the stearic acid. This was probably because the weaker alignment of the methyl groups of the hemicyanine were easily disturbed by the interaction between the deposited hemicyanine film and the water or the stearic acid during the deposition processes of the stearic acid overlayer in steps B4 and B5. Therefore, a well-constructed 2-type multilayer is supposed to be produced with the hemicyanine molecule which is modified with a more hydrophobic moiety than methyl groups. Those measurements such as X-ray diffraction method13 and Stark effect spectral0 are known to be employed to characterize the structure of LB films. However, they can be used only to distinguish Y-type structure from those of X- or Z-type, and it seems rather impossible to determine the pointing direction of the specific molecules in a submerged layer by this method. Contact angle measurement has also been utilized to determine whether the outermost layer of a given multilayered LB sample is hy( I 3) Matsuda, A.; Sugi, M.; Fukui, T.: Iizima, S.;Miyahara, M.: Otsubo, Y . J . Appl. Phys. 1977, 48, 771.
drophilic or hydrophobic, and undoubtedly this method cannot be utilized to analyze the inner structure of a multilayer. On the other hand, the SHG-CI method is applicable to the layer of optically nonlinear heteromolecules which is sandwiched among other nonlinearly inactive molecules as demonstrated in this report, because this method is quite sensitive to the polar structure of a given interface. The present study is based on the phase inversion of SH waves and this property is characteristic to an even-ordered nonlinear optical phenomenon such as SHG. Therefore, the phase inversion in SH radiation induced by a certain molecular configurational change at a given interface should be emphasized as another advantage of the S H G method besides its surface specificity and simplicity. In conclusion, the SHG-CI method was applied to analyze the molecular configuration of hemicyanine layer installed in multilayered LB films by the Y - and the Z-type deposition. It was demonstrated for the first time to clarify which direction the axis of the hemicyanine molecule pointed in a given layer sandwiched among other nonlinearly inactive layers.
FEATURE ARTICLE Scaling Laws for Inelastlc Collision Processes In Diatomic Molecules J. I. Steinfeld,* Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
P. Ruttenberg, Atomic Collisions Data Center, Joint Institute for Laboratory Astrophysics, University of Colorado and National Bureau of Standards, Boulder, Colorado 80309
G. Millot, G . Fanjoux, and B. Lavorel Laboratoire de Spectronomie Moldculaire et Instrumentation Laser, Universite de Bourgogne, U.R.A. CNRS No. 777, 6 Bd Gabriel, 21000 Dijon, France (Received: May 16, 1991; In Final Form: August 23, 1991)
A variety of fitting and scaling laws have been developed for the purpose of modeling rotational energy transfer (RET) in diatomic molecules. These include exponential energy gap (EGL), statistical power gap (SPG), and dynamically based angular-momentum scaling laws (e.g., the energy-corrected sudden approximation, ECS). These scaling laws are tested against state-to-state energy-transfer data for diatomic halogens, and stimulated Raman Q-branch band shapes in nitrogen. For state-testate RET in halogens, an ECS scaling law, modified to account for restrictions on angular-momentum transfer, is found to be superior to the EGL. When all available data on Raman band shapes in N2, particularly including the collision-induced Raman line shifts, are taken into account, the angular-momentum-based ECS-EP scaling law again provides the best representation of the data. We conclude that dynamically based scaling laws are to be preferred for modeling rotational energy transfer in diatomic molecules. Several unresolved questions and possible future directions for energy-transfer scaling laws and fitting procedures are discussed, including extension to polyatomic systems, possible contributions to the line width from elastic dephasing processes, and the development of global fitting procedures which will simultaneously account for line shape, line shift, and (when available) state-to-state RET measurements on molecular systems.
I . Introduction The development of highly specific and selective methods for molecular quantum state preparation and detection, such as laser-induced fluorescence, optical double resonance, and molecular beam techniques, has made available a large body of data on state-testate inelastic processes in diatomic and small polyatomic molecules. Indeed, a recent survey' of literature on the diatomic
halogens retrieved over 2000 measurements of rates and/or cross sections for vibrational, rotational, and electronic relaxation processes in these systems alone. ( I ) (a) Steinfeld, J. I . Rate Data for Inelastic Collision Processes in the Diatomic Halogen Molecules. J . Phys. Chem. Rex Dora 1984, 13, 445. (b) Steinfeld, J. I . Supplement to Rate Data for Inelastic Collision Processes in the Diatomic Halogen Molecules. J . Phys. Chem. Re/ Data 1987, /6. 903.
0022-365419 1 12095-9638%02.50/0 0 1991 American Chemical Society
