NH(A 3Π) rotational population generated by ArF laser photolysis of NH3

Abstract

The energy distribution of the NH(A) radicals generated by two photons in the ArF laser (193.3 nm) photolysis of NH₃ was studied in detail using high spectral resolution. The different spin components were found to be statistically populated. For the likely photodissociation process NH₃+2hν(ArF)→NH(A, v’=0, N’)+H+H, the rotational population matches the prior distribution for fragmentation into two rather than three fragments indicating that both atomic fragments are not equivalent with respect to the dissociation process. It therefore seems to be likely that first an excited NH₂ intermediate is formed which subsequently is photolyzed to yield NH(A). From energy conservation, the upper limit for the heat of formation of NH(X ³Σ⁻) is deduced to be ΔH⁰_f0 (NH)≤367 kJ mol⁻¹. The quantum yield for the generation of NH(A) was estimated to be φ[NH(A)]≊5×10⁻⁴. About 5.2% of the excited radicals were found to be produced in v’=1. In addition, a process involving the absorption of at least three photons was observed. About 1% of the excited radicals are formed this way with a rotational temperature of 2500 K for v’=0. These radicals are generated with such a short delay that reactions of ions appear to be involved in their formation.