Collision induced rotational energy transfer probed by time-resolved coherent anti-Stokes Raman scattering

We show that the technique of femtosecond time-resolved coherent anti-Stokes Raman scattering (CARS) spectroscopy provides a powerful tool for the investigation of collision-induced linewidths and the validation of rotational energy transfer (RET) models. The fs-CARS method is applied to the N-2-N-2 collision system, and a comparison between the commonly used exponential gap (ECS-E), power gap (ECS-P), frequency corrected (EFCS), and the recently proposed angular momentum and energy corrected (AECS) variants of the ECS model is presented. As result we show that the AECS scaling law requires only two free parameters, and is appropriate for the determination of RET rates from the measured fs-CARS signals. The AECS model is also applied to the more complex C2H2-C2H2 collision system. As vibrational energy transfer and dephasing is not negligible in this case, the model has to be modified by introducing a vibrational relaxation factor. With this modification the fs-CARS signals from acetylene can be described successfully. (C) 2003 American Institute of Physics.