Entrance Channel Effects in the Reaction of Aligned Ca(1P) with HCl

In the (harpooning) reaction Ca(1P) + HCl f CaCl(A2Π,B2Σ+) + H we study the effect of the long-range interactions between the multipole moments of HCl and the quadrupole moment of the Ca(1P) atom, using semiclassical dynamics. The relative translational motion of the reagents is described by classical trajectories, while the rotation of HCl and the evolution of the Ca(1P) state are treated quantum mechanically. We pay special attention to the influence of the alignment of the Ca(1P) state, investigated experimentally by Rettner and Zare [J. Chem. Phys. 1982, 77, 2416]. We find orbital following when this excited state is polarized parallel (Σ) to the initial velocity vector, and we obtain adiabatic and nonadiabatic transitions into the Σ substate when the Ca(1P) atom is initially polarized perpendicular (Π) to the beam. Simultaneously, there is a strong tendency of HCl to become orientationally localized, with H toward Ca. It is the anisotropy of the adiabatic potential energy surfaces and the polarization of the corresponding adiabatic states which relate these two phenomena. The initial polarization of the Ca(1P) atom is clearly reflected in the orientational distribution of the HCl molecule when it reaches the harpooning radius. From a simple model for the chemical reaction we infer that these long-range effects have an important influence on the A2Π/B2Σ+ branching ratio of the CaCl product, but we cannot quantitatively compute the (experimentally observed) effect of the Ca(1P) alignment on this branching ratio.