Vibrational Overtone Spectra in Polymeric Thin Films
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Vibrational Overtone Spectra in Polymeric Thin Films ANDREW SKUMANICI1 and CHRISTOPIIER MOYLAN I1M Almaden Research Center, K67/802, 650 llarry Rd. San Jose, CA 95120. A BSIRACT With the enhanced sensitivity of Photothermal Deflection Spectroscopy (PDS), vibrational overtone spectra have been obtained in organic thin films for the first time, covering the range from the near IR to the visible (0.4 - 2 eV). The absorption spectra of polycarbonate, for instance, exhibits C-I1 stretch modes for An> I where n is the vibrational quantum level. Previous measurements were constrained to either long fibers, which pose the problem of various scattering losses not related to absorption, or to a restricted wavelength region in the visible accessible by dye laser sources. The unparalleled sensitivity of PDS allows precise determination of frequency, lineshape and intensity of the various modes, even for films -10Im thick over a broad energy range. The overtone spectra can be used as a probe of various basic molecular properties. In a manner similar to NMR spectroscopy, it is possible to study specific atomic bonds. I. Introduction Research on polymeric optical materials has progressed quickly in the last few years. Many new polymers with unusual optical and electrical properties have been developed. Polymer optical fibers are particularly attractive because of their low cost, easy handling and good flexibility at large diameters. Two intrinsic loss factors are C-li overtone absorption and Rayleigh scattering (the latter due mainly to microscopic density fluctuations). From both a practical and fundamental perspective, it is important to measure and quantify the overtone absorption properties in polymers. Vibrational overtones result from the anharmonicity of the vibrations in a particular mode, 12i.e. a deviation from a quadratic potential which would forbid the An> I transitions by selection rules. 2 However, even for highly anharmonic systems, these transitions arc extremely weak and cannot be detected by standard spectroscopic techniques. To improve detection by increasing the optical interaction, long fibers can be used, but losses result from various scattering sources which mask the absorption. Dye lasers can be used, but only the high overtone regime is accessible. In this paper, we report on the observation and analysis of overtone and combination bands in thin solid films in the important range of one to five quanta of C-Il stretching vibrational energy.3 2. Samples Various thin polymer films were studied, including polycarbonate, polyethylene, and films of anthracene and bi-phenyl. The range of thickness spanned -10ILm to I mm. The films were measured by both PDS and a UV-Vis transmission spectrometer, as previously described.4. The PDS sensitivity is similar 2 to that of thermal lensing spectroscopy, which has been used to detect overtone transitions in liquids. 3. Results 1
Figure I shows the absorption spectrum for polycarbonate from 4000 cm-u (0.5 eV) to 14,000 cm' (1.7 eV) with an inset showing the full range.
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