FT-Raman Spectroscopy Of Thin Films By Integrated And Fiber Optics Techniques

Abstract

Structural studies of thin films containing chromophores have not been possible using Raman spectroscopy with visible excitation due to the presence of fluorescence which results when visible photons are absorbed by the chromophores. To circumvent this problem FT-Raman instrumentation has been developed so that spectra can be obtained with excitation at 1.064 microns, much too low in energy for fluorescence to occur. Integrated optical techniques which have been successfully used to investigate thin films at visible wavelengths has been adapted to the FT-Raman instrumentation with certain modifications. The primary change occurred in the collection optics where the imaging of the laser streak in the film upon the circular entrances aperture of an FTIR yielded a poor S/N spectrum of a thin polymer film. The geometric mismatch of streak image and circular aperture was rectified by the use of a fiber optic image converter. This consisted of a fiber optic bundle which had a linear array of fibers on one end for collection of light from the streak and a circular array of fibers on the other end so as to maximize the amount of light that entered the FTIR. A significant improvement in spectral quality was observed. Feasibility studies of guest/host interactions in thin composite films containing chromophores have been demonstrated and the results for a dye/cellulose acetate film will, in particular, be discussed.