Analysis of NH Spectrum

Extract

The A³Π – X³Σ⁻ transition of NH is a common feature of cometary spectra. Since the NH molecule is likely to be formed by photodissociation of molecules such as ammonia or hydrazine, identifying the final states of the photolysis would shed light on the identity of the parent. Stief and DeCarlo noted that the photolysis of ammonia at 123.6 nm results in emission at 324.0 nm in the c¹Π – a¹Δ system. They suggested that in the absence of collisions in the coma, the NH(a¹Δ) should accumulate if ammonia is the source of NH radicals. The absence of the singlet system in this view would suggest another parent molecule than ammonia as the source of the NH radical. However, the pumping rate of the UV transitions in a comet is also very small. This note will show that the transition rate for the a¹Δ – X³Σ⁻ transition is sufficiently fast to deplete any a¹Δ concentration formed in the original photolysis process.