Quantum scattering studies of spin–orbit effects in the Cl(2P)+HCl→ClH+Cl(2P) reaction

Abstract

We present quantum scattering calculations for the Cl+HCl→ClH+Cl reaction in which we include the three electronic states that correlate asymptotically to the ground state of Cl(2P)+HCl(X1Σ+). The potential surfacesand couplings are taken from the recent work of Maierle etal., J. Chem. Soc.,FaradayTrans., 1997, 93, 709. They are based on extensive abinitio calcu-lations for geometries in the vicinity of the lowest energy saddle-point, and on an electrostatic expansion (plus empirical dispersion and repulsion) for long range geometries including the van der Waals wells. Spin–orbit coupling has been included using a spin–orbit coupling parameter, λ, that is assumed to be independent of nuclear geometry, and Coriolis interactions are incorporated accurately. The scattering calculations use a hyperspherical coordinate coupled channel method in full dimensionality. A J-shifting approximation is employed to convert cumulative reaction probabilities for total angular momentum quantum number J=1/2 into state selected and thermal rate coefficients. Two issues have been studied: (i) the influence of the magnitude of λ on the fine-structure resolved cumulative probabilities and rate coefficients (we consider λ values that vary from 0 to ±100% of the true Cl value), and (ii) the transition state resonance spectrum, and its variation with λ and with other parameters in the calculations. A surprising result is the existence of a range of λ where the cumulative probability for the 2P1/2 state of Cl is larger than that for the 2P3/2 state, even though 2P1/2 is disfavoured by statistical factors and only reacts via nonadiabatic coupling. This result, which is not connected with resonance formation, may arise from coherent mixing of the Ωj=1/2 components of the 2P3/2 and 2P1/2 states in the van der Waals regions. The 2P1/2 state dominates for values of λ between the statistical and adiabatic limits when mixing converts 2P1/2 into a state that is, for linear geometries, predominantly 2Σ1/2 near the barrier. We find two significant resonances for total energies below 0.7 eV. They are associated with two quanta of asymmetric stretch excitation of the transition state and with zero or one quanta of bend excitation. These resonances are most prominent (i.e., narrowest) in the adiabatic limit of large ∣λ∣. For ∣λ∣≈0 the resonances are largely washed out due to strong mixing between attractive fine-structure states that support the resonances and repulsive ones that produce decay.