Reaction and deactivation of HCl(v=1,2) by Cl, Br, and H atoms

Total decay rates for HCl(v=2, 1) were measured in the range 294–439 K for Cl, 295–390 K for Br, and at 296 K for H. HCl(v=2) was produced directly by pulsed laser excitation of the overtone. The fraction of HCl(v=2) relaxed to HCl(v=1) was determined. For HCl(v=2) + Cl, relaxation gave entirely HCl(v=1) within an experimental uncertainty of ±10%, the total relaxation rate was large, k/v̄?5 Å2, and rates varied only slightly with temperature. For Br+HCl(v=2) reaction to HBr(v=0) + Cl is exoergic by about kT. Relaxation to HCl(v=1) is the dominant process. Reaction contributes roughly 17% and 34% to the loss of HCl(v=2) at 295 and 390 K, respectively. In contrast to the result for Br and for O(reported previously), H+HCl(v=2) gives 65% H2+Cl and only 35% HCl(v=1)+H. For HCl(v=1)+H, Cl, O the vibrational excitation energy is greater than the activation energy for reaction. The relaxation rates are between 1/2 and 1/3 of the A values for the measured thermal exchange rates A exp(−ΔEact/RT). For HCl(v=1)+Br, well below threshold, the rate is some 20 times less than for HCl(v=1)+Cl. The rate for HCl(v=2)+Br, just above reaction threshold is not dramatically larger. The ratios of vibrational relaxation rates for HCl(v=2) vs HCl(v=1) are 5.0±1.3, 4.2±0.4, and 5.0±1.3 for O, Cl, and Br, respectively. Any first order linear perturbation treatment gives rates proportional to v; the data scale more closely as v2.