State-to-state dynamics of H+HX collisions. II. The H+HX→HX°+H (X=Cl,Br,I) reactive exchange and inelastic collisions at 1.6 eV collision energy

We report measurement of product state distributions for the rotationally and/or vibrationally excited HX formed in collisions of translationally hot H atoms with HX (X=Cl, Br, and I) at 1.6 eV collision energy. The product state distributions are probed after only one collision of the fast H atom, using coherent anti-Stokes Raman scattering spectroscopy. Whether proceeding by inelastic collisions or reactive exchange, the transfer of translational energy to vibrational and rotational energy is quite inefficient in H+HX collisions at 1.6 eV. For all three hydrogen halides only 2–3% of the initial translational energy appears as HX vibration. For H+HCl only 6% of the initial energy is converted to HCl rotational energy, while for H+HBr and H+HI, this percentage is twice as large, 11–12%, but still small. The indistinguishability of the two H atoms involved makes it impossible to distinguish reactive exchange from inelastic energy transfer in these H+HX collisions. However, the difference in rotational energy partitioning for H+HBr and H+HI as compared with H+HCl, suggests that reactive exchange is dominant in the former and inelastic energy transfer dominates in the latter. The total cross sections for the combined energy transfer/reactive exchange do not change much with the identity of X, being 13±3, 11±2, and 11±2 Å2, for H+HCl, H+HBr, and H+HI, respectively.