Multiplex coherent anti-Stokes Raman spectroscopy study of infrared-multiphoton-excited OCS

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Abstract
The vibrational energy distribution following ν2 overtone excitation of OCS by a pulsed CO2 laser is studied by monitoring the coherent anti-stokes Raman spectrum of the ν1 mode. Because of the slow energy transfer from the pumped mode to other modes, and because the anharmonicity of the ν2 mode is small, OCS is an ideal system for studying the interaction of an intense infrared laser field with a single, nearly harmonic, oscillator. From the spectra the cross anharmonicities of the ν1 mode are determined to be x12=−6.0 cm−1 and x13=−2.7 cm−1, respectively. The time dependence of the spectra provides information on V–V energy transfer rates. In particular, the measurements put a lower limit of kν2→ν2=1 μs−1 Torr−1 on the vibrational relaxation rate within ν2 mode. At high excitation, the temperature of the ν2 mode rises up to 2000 K, and hot bands are observed up to the n=4 level. This fourth overtone peak is split because of either a Fermi resonance or vibrational angular momentum splitting.