Kinetics of the phase relaxation of N2 vibrations in a supersonic jet

Direct measurements have been carried out on the decay kinetics of the optical polarization on a vibrational-rotational transition of N2 molecules cooling in a supersonic jet. The measurements were made by time-dependent picosecond coherent active Raman spectroscopy. Quantum beats were detected for the first time in certain vibrational-rotational components of the inhomogeneously broadened spectrum. These beats were used to determine the distribution of the rotational temperature in the jet. The cross section for phase-relaxing collisions increases as the temperature is reduced to 20 K.