Quantum stereodynamics of Li + HF reactive collisions: the role of reactants polarization on the differential cross sectionw

A complete quantum study for the state-to-state Li + HF(v,j,m) LiF(v0,j0,O0) + H reactive collisions has been performed using a wave packet method, for different initial rotational states and helicity states of the reactants. The state-to-state differential cross section has been simulated, and the polarization of products extracted. It is found that the reactivity is enhanced for nearly collinear collisions, which produces a vibrational excitation of HF, needed to overcome the late barrier. It is also found that LiF(v0 = 0) products are preferentially forward scattered, while vibrationally excited LiF(v0 = 1 and 2) are backward scattered. These results are interpreted with a simple reaction mechanism, based on the late character and bent geometry of the transition state, originating from a covalent/ionic crossing, which consists of two steps: the arrival at the transition state and the dissociation. In the first step, in order to get to the saddle point some HF vibrational excitation is required, which favors head-on collisions and therefore low values of m. In the second step a fast dissociation of H atom takes place, which is explained by the ionic LiF H character of the bent transition state: the FH is repulsive making that H depart rapidly leaving a highly rotating LiF molecule. For the higher energy analyzed, where resonances slightly contribute, the orientation and alignment of product rotational states, referred to as reactants frame (with the z-axis parallel to k), are approximately constant with the scattering angle. The alignment is close to 1, showing that j0 is perpendicular to k, while starting from initial states with well defined rotational orientation, as states with pure m values, the final rotational are also oriented. It is also found that when using products frame (with the z0-axis parallel to k0) the rotational alignment and orientation of products varies a lot with the scattering angle just because the z0 axis changes from being parallel to anti-parallel to k when varying from y = 0 to p.