Determination of the Rate Constants for the NH2(X2B1) + NH2(X2B1) and NH2(X2B1) + H Recombination Reactions in N2 as a Function of Temperature and Pressure

The recombination rate constants for the reactions NH2 + NH2 -> N2H4 (reaction k(1b)) and NH2 + H -> NH3 (reaction k(2b)) with N-2 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)2(31) (0,7,0)(A) over tilde (2)A(1) (1)3(31) (0,0,0)(X) over tilde B-2(1) ro-vibronic transition, and NH3 monitored at 3336.39 cm(-1) on the (q)Q(3)(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 k(1b) can be represented by the Troe parameters: k(0), the low-pressure three-body recombination rate constant, k(0)(T) = (1.14+/ 0.59) x 10(-19)T(-(3.41 +/- 0.28)) cm(6) molecule(-2) s(-1), and F-cent, the pressure broadening parameter, F-cent = 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(infinity)(T) = 9.33 x 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 k(2b), can be represented by the Troe parameters: k(0)(T) = (9.95+/ 0.58) x 10(-26)T((-1.76 +/- 0.092)) cm(6) molecule(-2) s(-1), F-cent = 0.5+/ 0.2, k = 2.6 x 10(-10) cm(3) molecule(-1) s(-1). Again, the data could not be fit with three independent parameters, and k(2b) was chosen to be 2.6 X 10(-10) cm(3) molecule(-1) s(-1) and fixed in the analysis.