Picosecond Time-Domain Coherent Active Raman Spectroscopy of Free Nitrogen Jet

Abstract

The picosecond time-domain coherent active (anti-Stokes) Raman spectroscopy (CARS) technique has been used for the first time to study the kinetics of ro-vibrational dephasing of nitrogen molecules cooled in a supersonic free jet. We have directly resolved the free polarization decay of nitrogen molecules accompanied by “quantum beats” of different J-components of the transitions in the Q-branch. The measurement of the “beats” has been used for precise temperature characterization of the molecules in the jet, and that of the mean decay has been used to establish the character of the dephasing and to determine the T₂ relaxation time. The calculation, using experimental data on the values of T₂, of the dephasing N₂ collisional cross-section πσ ²_d in the jet has revealed the sharp increase (up to ten times) in πσ ²_d as the temperature decreases from 300 K down to 20 K.