NH in Electronically Excited States of the Singlet and Triplet Manifold

The N2 fragment is energetically constrained to the X state H atom formation due to HN3 + hv H + N3 is of minor importance [1]. However, it should be mentioned that in this NH(a) source other reactive species (H atoms) are reported to be formed in a one-photon process in the laser photolysis of HN3 with quantum yields of 0 = 0.15 (k= 93 nm) and 0 = 0.24 (A. = 248 nm) [39]. At shorter wavelengths (Kr, Xe resonance lines) the formation of NH(c) was observed [40]. The spin-forbidden formation of NH(A H) was due to secondary reactions of N2 with HN3 (see p. 22) [40 to 42]. A detailed analysis of the resulting primary photodissociation products showed for the electronic states NH(a) 0.998 NH(X) 8 x 10 NH(b) 1 xlO-3 and NH(A) 5xiO [6]. 

Also the rotational distribution of NH(a) was observed [8, 13, 16]. The photodissociation dynamics 

The higher singlet states are produced either directly from photolysis or from excitation of NH(a). In this respect the NH(c TT) is of special importance, since this state is used in the NH(a) LIF detection method (see below and p. 27). 

The next state in the singlet ladder is the b state. NH in this state can be obtained in the NH3 VUV photolysis [54 to 57] and ND(b) from ND3 VUV photolysis, respectively [56]. The vacuum ultraviolet radiation is produced in Ar discharge separated from the photolysis volume by an LiF window. The formation of NH(b was also observed in the ArF laser photolysis of HN3 [58]. The NH(b formation was observed in a discharge flow reactor in the reaction of H atoms with N3 radicals [59]. 

The lowest triplet state above the electronic ground state, A n, is used for the LIF detection of NH(X), and is thus of interest in that system (see p. 25). Since the A-X transition is allowed, NH(A H) can be formed by exciting the electronic ground state with light at = 336 nm [68]. Besides the two-step mechanism (formation of NH(X) followed by excitation), it can be obtained directly by NH3 photolysis. There are, however, also ways to produce NH in the A state directly from stable precursor molecules. 

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