Excited state dynamics and bimolecular quenching processes for NH 2(Ã 2A 1)

Abstract

Radiative lifetimes and quenching rate constants have been measured for the à 2A 1 state of NH 2, formed through ArF excimer laser photolysis of NH 3 at 193 nm. The NH 2(à 2A 1 → X̃ 2B 1) emission is discrete, allowing single vibrational and rotational levels to be isolated. An average radiative lifetime of 31 ± 4 μs and an NH 3 quenching rate constant of (6.1 ± 0.2) × 10 −10 cm 3 molecule −1 s −1 is found to be independent of the K″ a rotational level and ν′ 2 vibrational level for K″ a = 1, 4, 5 and 6 and ν′ 2 = 6, 7 and 8. The 050 Σ level ( K′ a = 0) lifetime is slightly longer (46 ± 6 μs) and its quenching rate constant somewhat lower [(5.0 ± 0.2) × 10 −10 cm 3 molecule −1 s −1]. These lifetimes are interpreted as radiative lifetimes of levels of the à 2A 1 state, unperturbed by Renner—Teller coupling with the ground state. The quenching by NH 3 is shown to be predominantely electronic quenching of the à 2A 1 state, with vibrational and rotational relaxation within the excited state being at least ten times slower. A second, previously unobserved, long-lived component is found in the decay of broad-band emission, comprising about 15% of the fluorescence intensity between 620 and 890 nm. Its lifetime is ⩾ 100 μs, coupling between some levels of the à 2A 1 and X̃ 2B 1 states.