Slow chain reactions of Br2 and Cl2 with HI: Multiple state analysis and vibrational relaxation of HBr(v = 2) and HCl(v = 1–4)

Two chain reactions of the general form X+HI→k1HX(v⩽vmax) +I, I+X2→k2IX+X(X =Br, Cl) are studied by realtime detection of infrared chemiluminescence from the vibrationally excited HX(v) products. Both are characterized by k2≪k1. These reactions are initiated by pulsed UV laser photolysis of the diatomic halogens at 355 nm in a flow cell apparatus at 295 K. Observations are made on Δv = −1 fluorescence from individual vibrational levels, and the results are treated with a complete mathematical analysis for the production of multiple vibrational states and their subsequent individual decays. Effects of vibrational cascading on the measurements of k2 are discussed. The chain propagation rate coefficients determined by this technique for k1(X=Br, Cl) and k2(X=Br, Cl) are 8.9(±1.3)×10−12, 1.4(±0.3)×10−10, 3.4(±0.8)×10−13, and 8.5(±1.1)×10−17 cm3 molecule−1 s−1, respectively. The deactivation rate coefficients for HBr(v = 2) and HCl(v = 1,2,3,4) by HI are 1.7(±0.2)×10−12, 1.43 (±0.05)×10−13, 6.3(±0.5)×10−13, 7.0(±2.4)×10−13, and 3.2(±0.7) ×10−12 cm3 molecule−1 s−1, respectively. Vibrational deactivation rate cofficients for HCl(v = 1,2,3,4) by Cl2 are 5.2(±0.3)×10−15, 2.2(±0.3)×10−14, 4.3(±4.2)×10−14, and 2.8(±1.5)×10−13 cm3 molecule−1 s−1, respectively. The vibrational deactivation efficiencies of HCl(v=1−4) by HI and Cl2 scale approximately as vn, where n = 2.1(±0.2) and 2.8(±0.2), respectively.