No fluorescence decay from low-lying electronic states excited into single vibronic levels with synchrotron radiation

Fluorescence decay of NO has been studied by single photon counting using the synchrotron radiation from the Orsay Electron Storage Ring (ACO) as a source of excitation. Emissions from A²Σ⁺(υ = 0,1,2,3), B²Π(υ = 5), C²Π(υ = 0, 1) and D²Σ⁺(υ = 0, 1, 2, 3) levels have been observed in function of NO gas pressure in the 0.02–4 torr range. Collision-free lifetimes and self-quenching rate constants have been derived from these measurements for all these levels and compared to previous data. Particular attention has been paid to the C²Π(υ = 0) level decay properties. By narrow-band (≈ 45 cm^?1) excitation inside the rotational envelope of this level we show that the decay is non-exponential with a short-living component (≈ 3 ns) a long-living one (≈ 20 ns). We develop a number of arguments in order to prove the short-living levels (J > 5 or 7) are weakly predissociated.