Radiative Lifetimes of NH2(\Atilde2A1): Rotational Quantum Number Dependence and Implications for Cometary Observations

Fluorescence (X²B₁ ← \Atilde²A₁) from the first electronically excited state transition of NH₂ (\Atilde²A₁), extends from 300 to 830 nm and, can be seen as a major visible emission from comets where it is a marker of ammonia NH₃ concentrations via photolysis and solar excitation. Inference of ammonia concentrations from the intensity of the NH₂ emission depends on knowledge of the radiative lifetime. Petrongolo et al. 2003 remeasured radiative lifetimes of various vibrational levels in the first electronically excited state NH₂ (\Atilde²A₁). Their results were a factor of 2.5 or more lower than previous experimental measurements from 1985 and earlier. Collision free, radiative lifetimes from the first electronically excited state of the amidogen free radical, NH₂(\Atilde²A₁) are reported here for a number of rotational states in the (0,8,0) and the (0,9,0) vibrational levels. In particular, the (0,8,0) band overlaps the strong OI emission at 630 nm and is commonly used to determine NH₃ abundance and the ratio of NH₃ to H₂O in comets. The results from this work generally agree the older measurements and not with those of Petrolongo, et al. Further, the radiative lifetimes in a vibrational level are shown for the first time to increase with increasing rotational quantum number and also with the projection of the total electronic angular momentum along the internuclear axis. The average radiative lifetimes of the (0,9,0) Γ, τ₁ = 18.4 ± 0.2 μs and (0,8,0) Φ, τ₂ = 23.4 ± 0.1 μs levels were much longer than those of the (0,9,0) Σ, τ₃ = 10.5 ± 0.2 μs and (0,8,0) Π, τ₄ = 13.2 ± 0.3 μs states suggesting increased mixing of the first electronic excited and the ground states. This study suggests that the variation of radiative lifetime within the emission band should be taken into consideration in calculations of concentrations of NH₂ in comet tails and the inferred NH₃ concentration in the comets themselves. Petrongolo, C.; Fan, H.; Ionescu, I.; Kuffel, D.; Reid, S. A. Journal of Chemical Physics 2003, 119, 2614.