OVERTONE ABSORPTION BY THERMO-OPTICAL SPECTROSCOPY

Abstract

When a laser beam passes through liquids commonly regarded as transparent, a dramatic ``blooming” of the beam can be $seen.^{1}$ Apparently some of the energy of the light is coupled into the liquid and converted to heat. A radial temperature gradient develops and, with a negative temperature coefficient of the index of refraction of the liquid, a divergent lens appears. We have examined the question of how energy is coupled into the liquids. Using isotopic substitution as a tool, we have obtained evidence that one possibly general mechanism is optical transitions into vibrational $overtones.^{2}$ The most likely candidates for overtone optical absorption should be large quanta fundamentals (low mass, large force constants) for which the molecular potential energy is least harmonic. Thus, modes involving hydrogen stretching in weak bonds are suspect. Deuteration at those sites which contain a large amplitude of the active normal mode should serve to quench the blooming effect, for now higher overtones are involved in the spectral region of interest. The mechanism of energy absorption as well as the active normal mode may thus be identified. Thermo-optical spectroscopy has been carried out for methanol ($CH_{3}OH, CH_{3}OD, CD_{3}OD)$ and benzene $(C_{6}H_{6}, C_{6}D_{6})$ using a CW tunable dye laser as well as individual lines from HeNe, argon ion, and krypton ion lasers. In methanol, blooming is seen only in the nondeuterated molecule, indicating that the mode involving the O-H stretching vibration is overtone active. Similarly, while benzene reveals a distinct absorption peak in this spectral region, perdeuterobenzene exhibits only a flat baseline absorption. Overtone absorption by the $e_{lu}$ C-H stretching node appears to be responsible. This kind of thermo-optical spectroscopy holds considerable promise for exploring details of molecular potential energy surface along coordinates below the dissociation limit of bonds involving hydrogen.