The focus of our research is the study of highly excited vibrational states of polyatomic molecules to obtain information about molecular structure, vibrational energy levels, and intramolecular dynamics. This information is used to test local mode theories and to study the kinetics of vibrational energy transfer and the photochemistry of the molecules. The vibrational overtone spectra of polyatomic molecules are observed in the near-infrared and visible regions of the electromagnetic spectrum. Two complementary techniques , laser absorption in supersonic expansions and piezoelectric detection of absorptions in cryogenic solutions , are currently employed to study samples at low temperatures.
Supersonic expansions provide a method of simplification of the fundamental and overtone bands.
The low translational and rotational temperatures achieved with the supersonic expansion, allow the detection of vibrational bands with elimination of spectral congestion from internally excited rotational and vibrational states. These overtone transitions are very weak and require sensitive techniques of detection. We use a technique that measures the rate of absorption rather than the magnitude of the absorption of a laser pulse confined within a closed optical cavity. Laser photoacoustic spectroscopy at cryogenic temperatures is another technique employed. Optical absorption spectra of molecules diluted in cryogenic solvents often exhibit much finer detail than the corresponding room temperature spectra due to the reduction of thermal broadening and the elimination of hot bands.