Collisional quenching and depolarization of NO2 Ã 2B2 state fluorescence as studied by Zeeman quantum beat spectroscopy

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Abstract
We have studied the effect of collisions on the time‐resolved fluorescence arising from a single optically selected fine structure level (F1303) of the à 2B2 state of NO2 prepared via absorption near 5933 Å in the presence of a weak magnetic field. The fluorescence modulation induced by the magnetic field (Zeeman quantum beats) is used to monitor the time dependence of the alignment. The alignment decays exponentially and follows simple Stern–Volmer kinetics. The incoherent fluorescence intensity, used to determine population information, fits well a bi‐exponential form at nonzero pressure. This is interpreted by a simple kinetic scheme involving cascade to additional fluorescing states. We find that only about 15% of the population removed from the initial level by collision produces subsequent emission that is detected (630–830 nm). We are unable to identify the exact nature of these emitting states. However, it is clear that pure rotationally inelastic collisions do not dominate the energy transfer process. Collisional population and alignment relaxation rate constants for the optically prepared level have been measured for excited state collisions with He, Ne, Ar, Kr, Xe, N2, CO, and NO2 at 295 K and low density (15 particle/cm3). In particular, we find that the rate for population removal of the initially excited level by collision is faster than the rate for elastic disalignment (depolarization within the same rotational level).