Collision free lifetimes of excited NO2 under very high resolution

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Abstract
The output beam from a single mode argon laser, tunable around λ=4880 and 5145 Å, is crossed perpendicularly with a well collimated NO2 beam. The lifetimes of selectively populated hyperfine structure levels in electronically excited states have been measured under collision free in electronically excited states have been measured under collision free conditions at NO2 pressures below 10−4 torr, using pulse modulation of the laser beam and single photon counting techniques. The experimental results prove that several directly excited states have short lifetimes (1–3 μsec) which are determined mainly by fast internal conversions to neighboring excited states, but only partly by fluorescence to the ground state. There are at least two such vibronic states, which are populated by nonradiative transitions from the primarily excited state and which show fluorescence decay with lifetimes around 30 and 100 μsec. Comparison with other measurements indicates that the primarily excited state has 2B1 character; the fluorescence, however, is mainly emitted from 2B2 states.