J . Phys. Chem. 1990, 94, 939-943 after repeated sample cycling to high temperature supports the emphasis of results obtained for virgin curium hydride samples. Discussion The dihdyride dissociation enthalpies derived here for americium and curium are summarized in Table I. Our value for AmH,, (1 77 f 12 kJ mol-') is intermediate between those determined by Roddy6 (1 90 f 6) and by Olson and Mulford18 (169 f 5) and is in excellent agreement with the value selected by Flotow et aL5 (179 f 13). A dissociation enthalpy for curium dihydride has been determined here for the first time: AdH[CmH2+] = 187 f 14 kJ mol-'. Based upon both the measured dihydride dissociation pressures (free energies) and the derived enthalpies, the following order of dihydride stabilities has been established: Ln (most stable) > Cm > Am > Pu The expected approach to trivalent lanthanide-like behavior for americium and curium (see, for example, ref 2) is thus confirmed, but we have observed that there are significant differences in their thermodynamic stabilities. This is in contrast to results of earlier work2' in which we found the structural behavior of these transplutonium hydrides to be comparable to those of the lanthanide hydrides. Thus, the dissociation thermodynamics of these materials are a more sensitive probe than phase and structural properties in determining trends and differences in the behaviors of the
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lanthanides and actinides and in investigating the changing role of the 5f electrons in the actinide series. The physicochemical properties of the actinide hydrides and their relationships to electronic structures and to the corresponding lanthanide hydride systems have been discussed by Ward.2 A dynamic Knudsen effusion mass spectrometric technique has been developed for studying the thermal stabilities of lanthanide/actinide hydride samples on the scale of 10-20 mg. Results with 8-12-mg samples of curium-248 hydride have suggested that -50 pmol of metal is the current practical limit of this approach. To study 1-mg (-4-pmol) hydride specimens of the even more scarce and radioactive elements berkelium and californium, it will be necessary to further modify this technique to achieve even higher sensitivities.
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Acknowledgment. We are grateful to Dr. 0. L. Keller, Jr., for helpful discussions during the preparation of this manuscript. The authors are indebted for the use of americium-243 and curium-248 in this work to the Office of Basic Energy Sciences, Division of Chemical Sciences, US. Department of Energy, through the transplutonium element production facilities a t the Oak Ridge National Laboratory. This research was sponsored by the Division of Chemical Sciences, US. Department of Energy, under Contract DEAC05-840R21400 with Martin Marietta Energy Systems, Inc. Registry No. AmH2, 13774-24-8; CmH,, 29556-43-2.
Applications of Fourier Transform Raman Spectroscopy to Studies of Thin Polymer Films C. G.Zimba, V. M. Hallmark, S. Turrell,+J. D. Swalen, and J. F. Rabolt* IBM Research, Almaden Research Center, San Jose, California 95120-6099 (Received: April 10, 1989)
Raman spectroscopicstudies of submicron-thick films have been accomplished through the use of integrated optical techniques. By using the film as an asymmetric slab waveguide for the laser excitation, and collecting the scattering emanating from the guided streak, we have obtained Raman spectra of organic films, polymer laminates, and molecular composites. The utility of the waveguide Raman spectroscopy technique has been limited by high levels of fluorescence when visible wavelength excitation is used. With the advent of FT-Raman spectroscopy, in which a near-infrared laser is used, Raman spectra of highly fluorescent and intensely colored materials could be easily obtained. In this study, waveguide Raman spectroscopic measurements using near-infrared excitation and a Michelson interferometer have been demonstrated. The use of a fiber optic bundle to collect the scattering and convert the image from a line to a circle has resulted in a 15-fold improvement over conventional lens collection. With the improved sensitivity, FT-Raman spectra of films containing small molecule chromophores imbedded in a polymer matrix have been obtained. In addition, extension of this method to polymers of low refractive index, by using a sublayer of very low refractive index material, such as MgF,, has been outlined.
Introduction Interest in the structure of thin films continues to grow because of their expanding importance in optics, microelectronics, and coatings. However, before correlations between structure and properties can be understood, highly sensitive nondestructive characterization techniques must be realized. Toward this end, spectroscopic techniques have made impressive progress. Infrared measurements of thin films down to thicknesses of less than 25 A have been reported for a number of self-assembled' and Langmuir-Blodgett films2 Polarization studies have elucidated the nature of intermolecular interactions and the extent of molecular orientation relative to the substrate ~ u r f a c e . ~Knowledge gained through these experiments has led to a new era of molecular
architecture designed to improve such macroscopic properties as lubrication and wetting." Raman spectroscopic investigations of thin films, on the other hand, have lagged behind due to the increased difficulty in studying films whose thicknesses are less than 5 pm. The diminishing scattering volume at these thicknesses provides a challenge even to the seasoned experimentalist. During the past decade, major advances have been made through the combination of Raman spectroscopy and integrated optical techr~iques.~This technique, referred to as waveguide Raman spectroscopy (WRS), has proven
'Present address: Laboratoire de Spectrcchimie Infrarouge et Raman, CNRS, Universite des Sciences et Techniques de Lille, 59655 Villeneuved'Ascq Cedex, France.
Phys. 1983, 78. 946. (4) Novotny, V.; Swalen, J. D.; Rabe, J. Langmuir, in press. (5) Rabolt, J. F.: Santo, R.: Swalen, J. D. Appl. Spectrosc. 1979, 33, 549.
(1) Nuzzo, R. G.; Fusco, F. A.; Allara, D. L. J . Am. Chem. Soc. 1987,109, 2358. (2) Allara, D. L.; Swalen, J. D. J. Phys. Chem. 1982, 86,2700.
(3) Rabolt, J. F.; Burns, F. C.; Schlotter, N. E.; Swalen, J. D. J. Chem.
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to be useful in the study of submicron-thick organic polymer and host-guest interactions in thin filmslo using visible laser excitation. This latter example has taken on increased importance with the recent report of nonlinear optical (NLO) studies of thin films containing oriented highly conjugated chromophores." Unfortunately, excitation at visible wavelengths in such films can lead to sample degradation or, at best, fluorescence which can easily overwhelm any weak Raman scattering. Hence, WRS has met with only sporadic success in studies of thin films containing chromophores. Recently, laser excitation at a near-infrared wavelength (1.064 pm) has been used in conjunction with an FT-IR to analyze Raman scattering from long-chain organic molecules containing chromophores.I2 This new FT-Raman technique shows considerable promise in avoiding fluorescence as well as offering a unique opportunity to characterize materials containing chromophores.13 Through the use of FT-Raman spectroscopy, the spectrum of both the conjugated chromophore and the saturated moieties of the molecule can be obtained simultaneously, since the excitation wavelength ( I .064 pm) is considerably removed from any resonance within the molecule which could selectively enhance only certain molecular vibrations. It is the purpose of this work to report the study of thin films containing chromophores using FT-Raman spectroscopy and integrated optical techniques. The adaptation of WRS to infrared wavelengths leads to some interesting constraints on the film thickness and the refractive index differences between the film and substrate. Suggestions for waveguide construction to relax these constraints are also discussed.
Experimental Section Asymmetric slab waveguides were prepared from (1) a saturated solution of polystyrene in chlorobenzene, (2) 16% (w/w) cellulose acetate in dimethylformamide, (3) 3% (w/w) naphthalene and 16% (w/w) cellulose acetate in dimethylformamide, and (4) I % (w/w) 2-nitro-5-(N-methyl-N-octadecylamino)benzoic acid and 16% (w/w) cellulose acetate in dimethylformamide. The films were all spin cast at 2500 rpm for 30 s and then dried overnight under vacuum at 80 "C. Due to the volatility of naphthalene, these films were not heated. The waveguide apparatus used for this work was originally designed for use in the visible spectral region on a scanning Raman spe~trometer.~ The focused laser beam was coupled into and out of the film by means of LaSF5 right-angle prisms (Karl Lambrecht). The laser beam was directed to the input prism by two mirrors whose movement was synchronized so that the focus of the laser beam stayed at the apex of the prism as the angle of incidence was varied, allowing the angle to be easily adjusted for maximum coupling of the laser beam into the waveguide. Typically, an initial coupling angle was determined by visual inspection of the guided streak using a HeNe alignment laser. A slight adjustment to higher angle, using an infrared viewer (FJW Industries), resulted in guiding the Nd:YAG laser beam in the film. The FT-Raman spectrometer used in these measurements has been previously described.I2-l4 Briefly, it is based on a Bomem DA3.02 FT-IR equipped with a visible quartz beam splitter and a thermoelectrically cooled (-40 "C) InGaAs detector (Epitaxx, NEP = 1 X W/Hz1l2). The use of an InGaAs detector represents a 6-fold improvement in sensitivity in comparison to (6) Rabolt, J . F.; Santo, R.; Swalen, J. D. Appl. Spectrosc. 1980, 34, 517. (7) Miller. D. R.; Han, 0. H.; Bohn, P. W. Appl. Specrrosc. 1987, 41, 245. (8) Rabolt. J . F.; Schlotter, N. E.; Swalen. J. D. J. Phys. Chem. 1981, 85, 4141.
(9) Miller, D. R.; Han, 0. H.; Bohn, P. W. Appl. Specrrosc. 1987,41, 249. ( I O ) Schlotter, N. E.; Rabolt. J. F. Appl. Spectrosc. 1984, 38, 208. ( 1 1 ) Tredgold. R. H.; Young, M . C. J.; Jones, R.; Hodge, P.; Kolinsky, P.; Jones, R. J . Elecrron. Lett. 1988, 24, 308. (12) Zimba, C. G.; Hallmark, V. W.; Swalen, J . D.; Rabolt, J. F. Appl. Spectrosc. 1987, 41, 121. ( 1 3 ) Hallmark, V. M.; Zimba, C. G.; Swalen, J. D.; Rabolt. J . F. Spectroscopy 1987, 2, 40. (14) Zimba, C. G.; Hallmark, V. M.; Rabolt, J . F.: Swalen, J. D. Thin Solid Films 1988, 160, 3 1 I .
Zimba et al. our earlier waveguide FT-Raman work,I4 where a thermoelectrically cooled germanium detector was used. A 2 ft X 2 ft Newport breadboard, placed adjacent to the emission input port of the interferometer, served as a platform for the Raman sample compartment, allowing the optics to be easily interchanged to accommodate a wide variety of sample geometries and accessories. A C W Nd:YAG laser (QEI) operating at 1.064 pm was used as the excitation source. Extraneous laser emission lines were removed by a prism monochromator (Applied Photophysics). In contrast to the much lower power levels of 50-500 mW typical of conventional solid and liquid samples, laser powers of approximately 1 W were used for the waveguide samples, due to the low efficiency (520%)of coupling the light into the waveguide film. The Rayleigh scattering was removed from the Raman scattering by one or more multilayer dielectric filters which allowed spectra to be measured down to 125 cm-I. All the waveguide spectra were recorded at 4-cm-I resolution with approximately 1 W of laser power incident to the waveguide apparatus, in 5.3-h elapsed measurement time. The refractive indexes at several different wavelengths were measured by use of a prism-coupled waveguide apparatus of either in-house fabrication" or a commercial vendor (Metricon, Pennington, NJ). Calculation of the optical field distribution within the waveguide structure was done using an APL program previously d e ~ c r i b e d . ~ ~ , ' ~
Integrated Optics Considerations To examine thin polymer films, an asymmetric slab waveguide was used, consisting of a thin polymer film, with refractive index n,, sandwiched between a substrate of refractive index n3 and a superstrate of refractive index n,, such that n2 > n,, n3. The electric field in the polymer film is assumed to be sinusoidal while the field in the substrate and superstrate is assumed to have a decaying exponential form E , = A , exp(k,,z) E , = A2
COS
(kz2
+ 4)
(z = 0 ) (z = d)
E , = A, exp[-kz3(z
-
d)]
The choice of axes is such that the positive z direction is down. By matching both the electric and magnetic fields at the two interfaces, z = 0 and z = d, an eigenvalue condition can be written that must be satisfied for the polymer film to act as a waveguide: tan-'
PI, + t a d
032
+ m?r = k,,d,
where tan-] P12and tan-' 0 3 2 are half the negative phase shifts which occur upon reflection at the film-superstrate and filmsubstrate interfaces, respectively, while kz,dz is the phase shift which occurs as the light traverses the polymer film in the z direction. The integer, m, labels the mode and gives the number of antinodes in the optical field distribution. The propagation constants, Pi,, can be defined for transverse electric (TE) and transverse magnetic (TM) polarized light as
where c is the dielectric tensor and kZi = koni cos Oi, k, = konj cos Oj, k, = 2a/X, and Oi and Oj are the angles of incidence for the ith and j t h layer of the waveguide. The above eigenvalue expression can be used to calculate the modes, the coupling angles at which the film acts as a waveguide, and the optical field distribution within the waveguide. A more detailed development of these expressions can be found elsewhere." In the work described ( 1 5 ) Swalen, J. D.; Santo, R.; Tacke, M.;Fischer, J. IBM J . Res. Deo. 1977, 21, 168. (16) Swalen, J. D.: Tacke, M.; Santo, R.; Fischer, J. Opt. Commun. 1976, 18, 387. (17) Rabolt, J. F.; Swalen, J. D. In Spectroscopy ojSurfaces; Clarke, R. J. H., Hester, R. E., Eds.; Wiley: New York, 1988; pp 1-36.
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Figure 1. Optical field intensity as a function of film depth in a l-pmthick film of polystyrene on a quartz substrate at three different wavelengths: (A) 488.0 nm, (B)632.8 nm, and (C) 1.064 pm. For the three wavelengths, the refractive indexes of polystyrene are 1.6008, 1.5845, and 1.5691, respectively, while the refractive indexes of quartz are 1.46301, 1.45702, and 1.44963, respectively.
here, the substrate and superstrate were quartz and air, respectively, and the incident laser beam was TE polarized; Le., the incident beam had its electric field vector polarized parallel to the film plane and perpendicular to the plane of incidence.
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Figure 2. Optical field intensity as a function of film depth in a 0.75pm-thick film of poly(viny1 alcohol) on a quartz substrate at three different wavelengths: (A) 488.0 nm, (B) 632.8 nm,and (C) 1.064 pm. For the three wavelengths, the refractive indexes are 1.5251, 1.5145, and 1.5072, respectively.
I -1
Alignment Laser
N d : Y A G Laser
Results and Discussion Comparison of WRS in the Visible and the Near-Infrared Regions. Initially, it was anticipated that it would be more difficult to support a waveguide in, and consequently to observe Raman scattering from, a thin polymer film as the excitation wavelength was changed from 488 nm to 1.064 pm. Beyond constraints on visually inspecting the quality of the invisible guided streak, the principal difficulty in waveguiding near-infrared light is that the differences in the refractive indexes of the organic film and the substrate are appreciably smaller than at visible wavelengths. For example, polystyrene and quartz have refractive indexes of 1.6008 and 1.46301 at 488 nm, respectively, and 1.5691 and 1.44963 at 1.064 pm, respe~tive1y.l~ Since the quality of the waveguide is greatly influenced by the difference in the refractive indexes of the film and substrate, the 15% change in the refractive index difference at 488 nm and 1.064 pm substantially changes the characteristics of the waveguide. Figure 1 illustrates the changes in the optical field strength, which is proportional to IEI2,as plotted against film depth, at three different excitation wavelengths. With 488-nm light, three modes ( m = 0, 1, 2) are supported in a I-pm-thick film, while only one mode ( m = 0) is present with 1.064-pm light. Additionally, the integrated optical field intensity at 1.064 pm is slightly weaker (-8%) within the polystyrene film and more broadly distributed, having more intense evanescent tails in the quartz substrate (8X) and the air superstrate (7X), relative to the m = 0 mode present with 488-nm light. Since the difference in refractive index between polystyrene and quartz is still quite large in the near-infrared region, the optical field intensity of the m = 0 mode is largely confined within the polymer film and the change in wavelength and refractive index should pose little difficulty in the measurement of the FT-Raman spectrum of the film. However, for polymer films of much lower refractive index or thickness, where only one mode is supported with 488.0-nm light, the optical field distribution of the m = 0 mode, if the mode exists at all, is much more diffuse with 1.064-pm excitation and leaks considerably into the substrate, making Raman measurements much more difficult. An example of this diffuse mode distribution is shown in Figure 2 for a 0.75-pm-thick film of poly(viny1 alcohol) which has refractive indexes of 1.5224, 1.5145, and 1.5072 at 488.0 nm, 632.8 nm, and 1.064 pm, respectively. With this thin film waveguide, the integrated optical field intensity with 1.064-pm excitation is 26% less in the polymer film, relative to the distribution with 488.0-nm light. Furthermore,
\ Thin Film Waveguide
FTI R
P
u
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Col IectorIConverter
Figure 3. Block diagram of the apparatus used to measure the waveguide FT-Raman spectra with (a, top) lens collection and (b, bottom) fiber
optic collection of the Raman scattering. with 1.064-pm light, 30% of the integrated optical field is within the substrate, compared to only 7% at 488.0 nm. Geometric Considerations in Adaptation of WRS to FT-Raman Spectroscopy. Initially, the scattering from a streak formed by guiding a Nd:YAG laser beam in a thin polymer film was collected at 90° and focused into an FT-IR, using an f / l air-spaced doublet lens, as shown in Figure 3a. While demonstrating that WRS was feasible in the near-infrared region, these results were surprising since the spectrum of a polystyrene film was much poorer than the spectrum of bulk polystyrene, in marked contrast to results5 obtained with visible excitation and a scanning spectrometer where a 100-fold improvement was observed for the film relative to the bulk. Consideration of the imaging of the scattering arising from the waveguided laser beam provides some insight into this difference in performance. In the case of the scanning
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Figure 4. FT-Raman spectra of a 1-pm-thick film of polystyrene on a quartz substrate obtained by using (a) grazing incidence excitation and lens collection, (b) waveguide excitation and lens collection, and (c) waveguide excitation and fiber optic collection. The spectra were recorded with approximately 1 W of laser power and 4.5-h measurement
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Figure 5. Waveguide FT-Raman spectra of (a) a cellulose acetate film and (b) a composite film of cellulose acetate with 20% (w/w) naphthalene (bands marked with an asterisk). Both films were 4.5-pm-thick and on quartz substrates. The spectra were recorded with approximately 1 W of laser power and 4.5-h measurement time.
time. E
spectrometer, the scattering from the waveguide film is better matched to a slit entrance aperture than the scattering from the bulk sample. The waveguide film gives a linear image, which easily passes through the slit, while the bulk sample gives a circular image, some of which overfills the slit. In contrast, the FT-Raman measurementst4employ a Michelson interferometer with a circular entrance aperture. The image of the bulk sample is easily accommodated while a large part of the linear image of the waveguide film falls outside the entrance aperture in a situation similar to fitting a long rectangle into a small circular hole. Thus, much of the Raman scattering from the thin-film waveguide never enters the interferometer, resulting in spectra of very poor signal-to-noise ratio, as shown in Figure 4b. It is, however, considerably better than the spectrum obtained by illuminating the film at grazing incidence (Figure 4a). If the linear image were reconfigured so that it more closely resembled the circular aperture of the interferometer, more of the scattered light from the film could be collected. One such way of doing this is to use a fiber optic bundle which is linear on one end and circular on the other, thus acting as an image converter. As shown in Figure 3b, the linear end of the fiber optic bundle was placed in close proximity to the surface of the thin-film waveguide. The light emitting from the circular end was focused into the spectrometer by means of any/ 1 air-spaced doublet lens. The fiber optic bundle (Dolan-Jenner Industries, Woburn, MA) used in this work was composed of 2-mil-diameter fibers (flint glass core, soda lime cladding) and has a 0.307 X 0.010 in.2 rectangular end and a circular end of approximately 1/16-in. diameter. Adjustment of the ends of the fiber bundle was achieved using standard fiber positioners (Newport, FPR-2). The Raman spectrum obtained with the same polystyrene thin film waveguide by using fiber optic collection, shown in Figure 4c, has a signal-to-noise ratio that is approximately 15 times higher than that of the spectrum obtained by using conventional lens collection. Composite Films. Having demonstrated the importance of fiber optic collection with regard to waveguide FT-Raman spectroscopy, these studies were extended to thin films in which a “guest” molecule was embedded in a “host” polymer matrix. A cellulose acetate film was chosen as the host matrix, and as shown in Figure 5a, its FT-Raman spectrum obtained on a quartz substrate. Although the thickness was 4.5 pm, the spectrum has a signalto-noise ratio that is considerably poorer than that obtained with the I-Mm polystyrene film shown in Figure 4c. Certainly, part of this diminished quality is due to the smaller Raman scattering cross section of the cellulose acetate relative to polystyrene which contains a phenyl side chain. However. consideration of the optical field intensity distribution within the waveguide structure reveals a dramatic difference between a film of cellulose acetate, which has a relatively low refractive index ( n = 1.4707), and polystyrene ( n = 1.5691). A plot of the optical
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@ Figure 6. Optical field intensity as a function of film depth, with 1.064-pm excitation, for 1.5-pm films of (A) polystyrene ( n = 1.5691), (B) poly(viny1 alcohol) ( n = 1.5072), and (C) cellulose acetate (n = 1.4707) on quartz ( n = 1.44963) substrates.
field intensity versus thickness for three polymer films at the Nd:YAG excitation wavelength, 1.064 pm, is shown in Figure 6. As the refractive index of the polymer approaches that of the substrate, the optical field distribution becomes more diffuse and considerably more intensity is present in the substrate. With more of the optical field intensity in the substrate, and less in the film, the intensity of the Raman scattering (being proportional to the optical field intensity) is much lower and a spectrum of lower signal-to-noise ratio results. In spite of the decreased signal-to-noise level, the Raman bands of an embedded molecule, naphthalene, are readily observed in Figure 5b. The lack of crystal field splitting in the bands arising from naphthalene at 390 and 510 cm-’ is consistent with previous waveguide and solution studies, indicating that the intermolecular forces present in crystals of naphthalene are absent. This suggests that the naphthalene molecules are probably isolated within the polymer matrix, even at this high concentration (20% (w/w)) of naphthalene in cellulose acetate. Realistically, the naphthalene-cellulose acetate film did not suffer from the presence of fluorescence and contained an additive (naphthalene) which was a very strong Raman scatterer. As such, this film does not represent the typical situation where fluorescence is a problem and the additive is a much weaker Raman scatterer. A more representative sample was chosen, consisting of a 4.5-pm film of cellulose acetate embedded with 4% 2-nitro-5-(Nmethyl-N-octadecy1amino)benzoicacid. The film is deep yellow, having an absorption maximum of 0.3 optical density at 41 5 nm, and, as shown in Figure 7c, exhibits a high level of fluorescence when a visible laser is used to obtain the Raman spectrum. When
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Figure 7. (a) Waveguide FT-Raman spectrum of a cellulose acetate film. (b) Waveguide FT-Raman spectrum of a composite film of cellulose acetate with 4% 2-nitro-5-(N-methyl-N-octadecylamino)benzoic acid. (d) FT-Raman spectrum of 2-nitro-5-(N-methyl-N-octadecylamino)benzoic acid powder. (c) Waveguide Raman spectrum of a composite film of cellulose acetate with 4% 2-nitro-5-(N-methyl-N-octadecylamino)benzoicacid with 488.0-nm excitation. The films were 4.5-pmthick and on quartz substrates. The waveguide spectra were recorded with approximately 1 W of laser power and 4.5-h measurement time while the powder spectrum was recorded with 400 mW of laser power in 35 min.
a near-infrared laser is used, as in the FT-Raman technique, the fluorescence is completely removed and a band at 1350 cm-l is readily observed (Figure 7b). The origin of the broadening of the 1350-cm-' band relative to the spectrum of crystalline 2nitro-5-(N-methyl-N-octadecylamino)benzoic acid may reside in slightly different chemical environments within the polymer matrix (Figure 7b). The vibrational bands arising from the various chemical environments would be slightly displaced in frequency and, when superimposed, would generate a broadened band similar to that observed. It should be noted that this spectrum represents the first nonresonant Raman spectrum of a dye chromophore in a polymer matrix. Future studies with various concentrations of dye should elucidate the exact nature of the polymer-dye interaction. Waveguide FT- Raman Spectroscopy of Low Refractive Index Films. One of the major difficulties in waveguide Raman spectroscopy, independent of the excitation wavelength, is the quality of the guided streak in the film used to obtain the Raman spectrum. The intensity of the Raman scattering is directly related to the integrated intensity of the optical field within the film. When the difference in the index of refraction between the polymer film and the substrate is large, as in the case of polystyrene on a quartz substrate, the optical field is well-confined within the film and Raman spectra can easily be observed. In films of low index, such as cellulose acetate and poly(methy1 methacrylate), the field is more diffuse and leaks into the substrate considerably, as previously discussed. This difficulty becomes particularly acute when a Nd:YAG laser is used in the near-infrared region, since the differences in the refractive indexes of the substrate and film are considerably diminished. Compared to the situation with visible excitation, containment of the 1.064-pm excitation within the thin-film waveguide is much more difficult. For this reason, obtaining an FT-Raman spectrum of cellulose acetate films of 1-pm thickness was not possible. The approach taken in this study to ensure an optical field of sufficient intensity within the film in order to measure Raman spectra was to make the film thicker.
Y
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Figure 8. Electric field intensity as a function of film depth, with 1.064-pm excitation, for 1.5-pm films of (A) polystyrene ( n = 1.5691), (B) poly(viny1 alcohol) (n = 1.5072), and (C) cellulose acetate (n = 1.4707) on quartz substrates with a I-pm-thick coating of MgF2 (n =
1.3794). Another possible approach is to increase the difference in the indexes of refraction between the film and the substrate by using a sublayer of lower refractive index as shown in Figure 8. Here, the effect of a 1-pm sublayer of MgF2 ( n = 1.3379 at 1.064 pm) on the optical field distribution is shown for 1-pm films of the same three polymers as shown in Figure 6. In each case, the optical field intensity is more confined within the polymer film. With this improved distribution of the optical field, Raman spectra should be more easily acquired. It should also be noted that this sublayer approach may be equally applicable to waveguide Raman spectroscopy when visible excitation is used and may ultimately allow thinner films to be studied.
Conclusions This study has demonstrated that waveguide Raman spectroscopic measurements can be made in the near-infrared region using a Michelson interferometer. The use of a fiber optic bundle to collect the scattering and convert the image from a line to a circle has resulted in a 15-fold improvement over conventional lens collection. With the improved sensitivity, FT-Raman spectra of films containing small molecules imbedded in a polymer matrix have been obtained. Extension of this work to polymers of low refractive index, by using a sublayer of very low refractive index material such as MgF2, has been outlined and will be the subject of a future report. Additionally, the successful application of fiber optics to collect the Raman scattering in this work has led to further investigation regarding the utility of fiber optic probes to simultaneously deliver the laser beam and collect the Raman scattering with a wide variety of samples. Acknowledgment. J.F.R. acknowledges support for part of this work from the Chemistry Division of the Office of Naval Research and expresses his appreciation to Dr. James Scherer (USF) for helpful discussions on imaging with fiber optics. We acknowledge the financial support and technical motivation provided by Dr. Lorraine Siperko of the IBM Endicott Laboratories. We also express our appreciation to Mark Jurich for the refractive index measurements at 1.064 pm. Registry No. MgF2, 7783-40-6;polystyrene, 9003-53-6; poly(viny1 alcohol), 9002-89-5; cellulose acetate, 9004-35-7; 2-nitro-5-(N-methylN-octadecy1amino)benzoic acid, 118523-83-4.
