The rates of the elementary reactions of NH(a1Δ) with NH3((X)over-tilde)and HN3((X)over-tilde).: The temperature dependences

The reactions NH(a) + HN3((X) over tilde) --> prod. (1) and NH(a) + NH3 ((X) over tilde) --> prod. (2) were studied in a quasi-static reaction cell in the temperature range 293 less than or equal to T/K less than or equal to 501 at a pressure of 10 mbar and 20 mbar, respectively, with He as the main carrier gas. The electronically excited reactant NH(a) was generated by laser-flash photolysis of HN3, at lambda = 308 nm and detected by laser-induced fluorescence (LIF). Also the ground state species NH(X) was detected by LIF. From the measured concentration-time profiles of NH(a) under pseudo-first order conditions, the rate coefficients k(1)(T) and k(2)(T) were obtained. For the rate coefficient k(1) a positive temperature dependence was observed: k(1)(T) = (8.1 +/- 0.5) x 10(13) exp[(-0.76 +/- 0.05)/RT]cm(3)/mol s with a small activation energy of E-A = 0.76 kJ/mol. For reaction (2) the rate coefficient k(2) (T) = (8.6 +/- 0.6) x 10(13) (T/298)(-(0.6+/-0.1)) cm(3)/mols with a negative temperature dependence was measured indicating that the intermediate N2H4, which decomposes to 2 NH2, is formed without a barrier. The rate of the quenching channel NH(a) + NH3 --> NH(X) + NH3 (2q) has the same temperature dependence as the rate of the overall reaction (2). The contribution k(2q)/k(2) = 0.008 over the temperature range 293 less than or equal to T/K less than or equal to 501.