Determination of the Rate Constant for the NH2(X2B1) + NH2(X2B1) Reaction at Low Pressure and 293 K

The rate constant for the reaction NH2((XB1)-B-2) + NH2((XB1)-B-2) -> products was measured in CF4, N-2 and Ar carrier gases at 293 +/- 2 K over a pressure range from 2 to 10 Torr. The NH2 radical was produced by the 193 nm photolysis of NH3 dilute in the carrier gas. Both the loss of NH3 and its subsequent recovery and the production of NH2 and subsequent reaction were monitored simultaneously following the photolysis laser pulse. Both species were detected using quantitative time-resolved high-resolution absorption spectroscopy. The NH3 molecule was monitored in the N1R using a rotation transition of the v(1) + v(3) first combination band near 1500 nm, and the NH2 radical was monitored using the (1)2(21) <- (1)3(31) rotational transition of the (0,7,0)A(2)A(1) <- (0,0,0) (XB1)-B-2 band near 675 nm. The low-pressure rate constant showed a linear dependence on pressure. The slope of the pressure dependence was dominated by a recombination rate constant for NH2 + NH2 given by (8.0 +/- 0.5) x 10(-29), (5.7 +/- 0.7) x 10(-29), and (3.9 +/- 0.4) x 10(-29) cm(6) molecule(-2) s(-1) in CF4, N-2, and Ar bath gases, respectively, where the uncertainties are +/- 2 sigma in the scatter of the measurements. The average of the three independent measurements of the sum of the disproportionation rate constants (the zero pressure rate constant) was (3.4 +/- 6) x 10(-13) cm(3) molecule(-1)s(-1), where the uncertainty is +/- 2 sigma in the scatter of the measurements.