Nascent vibrational and rotational distributions of NH(A 3Π,c 1Π) in the dissociative excitation of NH3 by Ar(3P2,0) at thermal energy

The Ar(3P2,0)+NH3 reaction has been studied by optical emission spectroscopy. Experiments have been performed by using a flowing afterglow apparatus and a new low-pressure apparatus. In the latter experiment the Ar(3P2,0) atoms generated by a microwave discharge were expanded into a high vacuum chamber through an orifice. Essentially the same results have been obtained from the two experimental systems. The NH(c 1Π–a 1Δ) emission as well as the NH(A 3Π–X 3∑−) emission was observed. The emission intensity ratio INH(c)/INH(A) was measured to be 0.31±0.03. The vibrational population ratio P(v′=1)/P(v′=0) for the NH(A 3Π) state was determined to be 0.35±0.05, while the effective rotational temperatures for the NH(A 3Π,v′=0) and NH(c 1Π,v′=0) states were estimated to be 2000±300 K and 1050±70 K, respectively. The observed vibrational and rotational distributions of the NH(A 3Π) state and the rotational distribution of the NH(c 1Π) state were much colder than those predicted from a simple statistical theory. The fraction of the available energy converted into vibration 〈fv〉 and rotation 〈fr〉 of NH(A 3Π) were estimated to be 3% and 4%, respectively, suggesting that a large amount of the available energy was transferred into relative translation between NH(A 3Π) and H2.