Experimental and Theoretical Reaction Cross Sections for the H + HCl System

The dynamics of the gas-phase reaction of H atoms with HCl has been studied experimentally employing the laser photolysis/vacuum-UV laser-induced fluorescence (LP/VUV-LIF) “pump-and-probe” technique and theoretically by means of quasiclassical trajectory (QCT) calculations performed on two versions of the new potential energy surface of Bian and Werner [Bian, W.; Werner, H.-J. J. Chem. Phys. <BO>2000</BO>, 112, 220]. In the experimental studies translationally energetic H atoms with average collision energies of E<INF>col</INF> = 1.4 and 1.7 eV were generated by pulsed laser photolysis of H<INF>2</INF>S and HBr at 222 nm, respectively. Ground-state Cl(²P<INF>3/2</INF>) and spin−orbit excited Cl*(²P<INF>1/2</INF>) atoms produced in the reactive collision of the H atoms with room-temperature HCl were detected under single collision conditions by VUV-LIF. The measurements of the Cl* formation spin−orbit branching ratio φ<INF>Cl*</INF>(1.4 eV) = [Cl*]/[Cl + Cl*] = 0.07 ± 0.01 and φ<INF>Cl*</INF>(1.7 eV) = 0.19 ± 0.02 revealed the increasing importance of the nonadiabatic reaction channel H + HCl → H<INF>2</INF> + Cl* with increasing collision energy. To allow for comparison with the QCT calculations, total absolute reaction cross sections for chlorine atom formation, σ<INF>R</INF>(1.4 eV) = (0.35 ± 0.16) Ų and σ<INF>R</INF>(1.7 eV) = (0.13 ± 0.06) Ų, have been measured using a photolytic calibration method. In addition, further QCT calculations have been carried out for the H + DCl isotope reaction which can be compared with the results of previous reaction dynamics experiments of Barclay et al. [Barclay, V. J.; Collings, B. A.; Polanyi, J. C.; Wang, J. H. J. Phys. Chem. <BO>1991</BO>, 95, 2921].