Determination of the Rate Constants for the NH2(X(2)B1) + NH2(X(2)B1) and NH2(X(2)B1) + H Recombination Reactions in N2 as a Function of Temperature and Pressure.

Abstract

The recombination rate constants for the reactions NH2 + NH2 → N2H4 (reaction k1b) and NH2 + H → NH3 (reaction k2b) with N2 as a third-body have been measured as a function of temperature and pressure. The temperature range was from 292 to 533 K and the pressure range from a few Torr up to 300-400 Torr, well within the pressure falloff region. The NH2 radical was produced by 193 nm pulsed-laser photolysis of NH3 in a temperature controlled flow chamber. High-resolution time-resolved laser absorption spectroscopy was used to follow the temporal concentration profiles of both NH2 and NH3, simultaneously. The NH2 radical was monitored at 14800.65 cm(-1) using the (1)231 (0,7,0)Ã(2)A1 ← (1)331 (0,0,0)X̃(2)B1 ro-vibronic transition, and NH3 monitored at 3336.39 cm(-1) on the (q)Q3(3)s (1,0,0,0) ← (0,0,0,0) ro-vibrational transition. The necessary collisional broadening parameters for each molecule were measured in separate experiments. The pressure and temperature dependence of k1b can be represented by the Troe parameters: k0, the low-pressure three-body recombination rate constant, k0(T) = (1.14 ± 0.59) × 10(-19)T(-(3.41±0.28)) cm(6) molecule(-2) s(-1), and Fcent, the pressure broadening parameter, Fcent = 0.15 ± 0.12, independent of temperature. The data could not be fit by three-independent parameters, and the high-pressure limiting rate constant k∞(T) = 9.33 × 10(-10)T(-0.414) e(33/T) cm(3) molecule(-1) s(-1) was taken from the high-quality theoretical calculations of Klippenstein et al. (J. Phys. Chem A 2009, 113, 10241). The pressure and temperature dependence of k2b, can be represented by the Troe parameters: k0(T) = (9.95 ± 0.58) × 10(-26)T((-1.76±0.092)) cm(6) molecule(-2) s(-1), Fcent = 0.5 ± 0.2, k∞ = 2.6 × 10(-10) cm(3) molecule(-1) s(-1). Again, the data could not be fit with three independent parameters, and k2b∞ was chosen to be 2.6 × 10(-10) cm(3) molecule(-1) s(-1) and fixed in the analysis.