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 car ried out by probing vibrational and pure rotational lines of nitrogen in th e Doppler broadened regime. The theoretical analysis of the transient respo nses outlines the role of the geometrical effects. For pure rotational CARS , it is shown that the main contribution to the dephasing of the Raman cohe rence 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 demonst rate that the range of operation of time-domain CARS velocimetry is extende d by probing pure rotational lines. The predictions are validated by experi ments which are performed both in a static gas cell and in a Mach 10 super sonic flow.