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Resonantly enhanced two-photon ionization has been used to obtain detailed information about the rotational and electronic relaxation in pulsed supersonic expansions of nitric oxide seeded in helium and argon. Due to the sensitivity of the method we were able to measure rotational distributions up to high quantum numbers J″24.5. Measurements were made for stagnation pressures ( p0) and nozzle diameters (d) in the range 0.44≤p0d≤22 Torr cm for NO/Ar beams, and 0.88≤p0d≤36 Torr cm for NO/He beams. In general non-Boltzmann rotational population distributions were observed. Furthermore, we found that for NO/He beams, the two electronic substates 2Π1/2 and 2Π3/2 of NO were not in local equilibrium. These observations can be understood in terms of a simple model using state-to-state collision cross sections combined with the empirical hydrodynamic equations to describe the isentropic expansions.
Zacharias et al. (Mon,) studied this question.