The Rates of the Elementary Reactions of NH(a1Δ) with NH3(X̃) and HN3(X̃). The Temperature Dependences

Abstract

AbstractThe reactions      NH(a) + HN3(X̃) → prod.       (1)and                     NH(a) + NH3(X̃) → prod.       (2)were studied in a quasi-static reaction cell in the temperature range 293 ≤T/K ≤ 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 λ = 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 coefficientsk1(T) andk2(T) were obtained.For the rate coefficientk1a positive temperature dependence was observed:k1(T) = (8.1 ± 0.5) × 1013exp[(−0.76 ± 0.05)/RT] cm3/mol swith a small activation energy ofEA= 0.76 kJ/mol.For reaction (2) the rate coefficientk2(T) = (8.6 ± 0.6) × 1013(T/298)−(0.6 ± 0.1)cm3/mol swith 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 channelNH(a) + NH3→ NH(X) + NH3       (2q)has the same temperature dependence as the rate of the overall reaction (2). The contributionk2q/k2= 0.008 over the temperature range 293 ≤T/K ≤ 501.