Multiplex CARS study of infrared-multiphoton-excited OCS

The vibrational energy distribution following the ?2 overtone excitation of OCS by a pulsed CO2 laser can be studied by coherent anti-Stokes Raman spectroscopy (CARS) of the ?1 mode. Since the anharmonicity of the pumped mode is small, and since the relaxation to other modes is slow, OCS allows one to study the interaction of an isolated harmonic mode with an intense laser field. Single-shot spectra are obtained by using a broadband dye laser and recording the CARS signal on a multichannel analyzer. The phase matching of the beams is done in a so-called folded boxcars arrangement. The resulting spectra have a high signal-to-noise ratio, and good spatial and spectral resolution, and make it possible to carry out a state-specific study of the dynamics of highly vibrationally excited molecules. Hot bands from the ?2 mode are observed up to v = 4. The data show that there is a very rapid equilibration within the ?2 mode, with temperatures up to 2000 K, and a slow energy transfer to the other two modes. In addition the spectra provide information on the anharmonicity constants x12, and x13, and V-V energy transfer rates.