Comparison of dephasing times for vibrational and rotational coherent anti-Stokes Raman scattering: implications for velocimetry

Time-domain coherent anti-Stokes Raman scattering experiments have been carried out by probing vibrational and pure rotational lines of nitrogen in the Doppler broadened regime. The theoretical analysis of the transient responses outlines the role of the geometrical effects. For pure rotational CARS, it is shown that the main contribution to the dephasing of the Raman coherence results from the change in direction between the pump and anti-Stokes wave vectors whereas the difference between the modulus of these two wave vectors accounts for dephasing in vibrational CARS. Furthermore, we demonstrate that the range of operation of time-domain CARS velocimetry is extended by probing pure rotational lines. The predictions are validated by experiments which are performed both in a static gas cell and in a Mach 10 supersonic flow. Received: 30 March 2000 / Revised version: 9 June 2000 / Published online: 13 September 2000