VIBRATIONAL STATES OF ORGANIC MOLECULES IN THE VISIBLE REGION BY THERMAL LENSING SPECTROSCOPY AND THE LOCAL MODE MODEL

$^{1}$ M. E. Long, R. L. Swofford, and A. C. Albrecht, Science 191, 183 (1976). $^{2}$ J. P. Gordon, R.C.C. Leite, R. S. Moore S.P.S. Porto, and J. R. Whinnery, J. Appl. Phys. 36, 3 (1965). $^{3}$ R. L. Swofford, M. E. Lone, and A. C. Albrecht, J. Chem. Phys. 65, 179 (1976). $^{4}$ R. L. Swofford, M. S. Burberry, J. A. Morrell, and A. C. Albrecht, (submitted for publication) This research has been supported by a grant from the National Science Foundation and by the Materials Science Center, Cornell University.Author Institution: Department of Chemistry, Cornell UniversityA sensitive laser thermo-optical spectrometer has been used to study the weak visible absorption in various organic $liquids.^{1}$ The spectrometer is based on the thermal lens which forms in a liquid placed in the laser $beam.^{2}$ The liquid is heated by the absorbed power, creating in the liquid a radial temperature gradient. This temperature profile acts through the temperature coefficient of the index of refraction to create a diverging lens the strength of the lens being easily related to the absorption of the liquid. Using a cw tunable dye laser, one can obtain the absorption spectrum of the liquid across the tuning range of the dye. The weak visible absorption present in numerous organic liquids has been characterized as overtone absorption of the hydrogen stretching $vibrations.^{3}$ These highly energetic molecular states may be easily described in terms of the local mode model, which treats the molecule as a set of uncoupled anharmonic C-H oscillators. A direct test of the local mode model of benzene is provided by comparative examination of the $v = 6$ spectra of benzene, benzene-$d_{5}$, and benzene-$f_{5}.^{4} $ Should there be significant coupling among the C-H oscillators, one might anticipate conspicuous spectral changes upon, pentadeuteration. On the other hand, the local mode model predicts an unchanged absorption band diminished in intensity. It will be shown how the predictions of the local mode model are borne out experimentally. Additional results will be presented on a number of organic liquids