The Four Isotopomer Reactions of NH(a) and ND(a) with NH3(X) and ND3(X)

The reactions NH(a) + NH3(X̃) → products (1) ND(a) + NH3(X̃) → products (2) NH(a) + ND3(X̃) → products (3) ND(a) + ND3(X̃) → products (4) were studied in a quasi-static reaction cell at room temperature and pressures of 10 and 20mbar with He as the main carrier gas. The electronically excited reactants NH(a) and ND(a) were generated by laser-flash photolysis of HN3 and DN3, respectively, at λ = 308nm and detected by laser-induced fluorescence (LIF). Also the ground state species NH(X) and ND(X) as products were detected by LIF. From the measured concentration-time profiles of NH(a) and ND(a) under pseudo-first order conditions, the following rate constants were obtained: NH(a) + NH3(X̃) → products (1) ND(a) + NH3(X̃) → products (2) NH(a) + ND3(X̃) → products (3) ND(a) + ND3(X̃) → products (4) k 1 = (9.1 ± 0.9) × 1013 cm3mol−1s−1 k 2 = (9.6 ± 1.0) × 1013 cm3mol−1s−1 k 3 = (8.0 ± 1.0) × 1013 cm3mol−1s−1 k 4 = (7.2 ± 0.8) × 1013 cm3mol−1s−1. The major products are the corresponding NH i D2 − i (X̃) radicals (i = 0, 1, 2), whereas quenching processes such as NH(a) + ND3 → NH(X) + ND3 are of minor importance (1%). The isotope exchange NH(a) + ND3 → ND(X) + NHD2 is negligible, and the corresponding channel on the singlet surface NH(a) + ND3(X ~) → ND(a) + NHD2(X̃) contributes with 1% to the overall NH(a) depletion in that reaction. The experimental findings are discussed in terms of a chemical activation mechanism by means of statistical rate theory.