Photodissociation of NH3 at 106–200 nm

Abstract

Cross sections and quantum yields of emissions from the photofragments of NH3 were measured in the 106–200 nm region using synchrotron radiation as a light source. The emission systems of NH2 (Ã 2A1→X̃ 2B1), NH(c 1Π→a 1Δ, b 1Σ+), and NH(b 1Σ+→X 3Σ−) were observed from the NH3 photodissociation. The dependences of emission intensities on the NH3 and/or He (buffer gas) pressures were studied. The NH(c→a,b) and NH(b→X) emissions were observed when the NH2(Ã2A1) species was quenched by 10 Torr of He. The NH(b→X) and NH(c→a) emissions start to appear at 182.3±1.5 and 127.9±0.9 nm, respectively. From these emission thresholds the threshold energy for photodissociation of NH3 into NH(X 3Σ−)+H2(X 1Σ+) is determined to be 4.17±0.06 eV, and the upper limit for the heat of formation of NH is 3.81±0.06 eV. For excitation wavelengths longer than 128 nm the NH(b 1Σ+) species is produced by a primary photodissociation process, and at shorter excitation wavelengths the NH(b 1Σ+) may be partly produced by a cascading process following the NH(c→b) transition. Contrary to previous theoretical interpretations, all the vibronic levels of the NH3 (B̃ and C̃) states produce the NH2(Ã→X̃) emission. The quantum yield for the C̃ state is lower than that of the B̃ state. The vibronic levels of the C̃ state are renumbered in accordance with their quantum yields.