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 Δn> 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 ∼10μm 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.