Quantum Stereodynamics of the F + H2 --> HF + H Reaction by the Stereodirected S-matrix Approach

Reaction stereodynamics can be studied in quantum mechanics using alternative representations of the S matrix. In this paper we employ the equations for the orthogonal transformations (expressed in terms of Wigner 3j symbols) that convert the S matrix from the body fixed (jjXi) representation into the stereodirected one (jmXi). This representation is characterized by the introduction of the steric quantum number m, which in the vector model of quantum mechanics is put into correspondence with given precession cones of attack of the incoming atom on the diatomic molecule for the reactants’ channels, and of cones of escape for the departing atom away from the diatomic molecule for the products’ channels. The angles of aperture of such cones are determined from the un- certainty principle. As the m quantum number increases (semiclassical limit), the grid of discrete values of the precession cones more finely scans the angle between the Jacobi vectors. Using a time-independent hyperspherical coordinate method we have generated the full S matrix including all open reactive and inelastic channels for two potential energy surfaces corresponding to the F + H2 ! HF + H reaction and they have been used to calculate, via jjXi ! jmXi matrix transformations, the attack and exit cu- mulative reaction probabilities. During the calculations, we have distinguished between ortho-H2 and para-H2. Clear stereody- namical effects have being identified, in particular, regarding the reaction entrance channel, that F-atom attacks are preferred at the transition state (bent) geometry, while for the exit channel the H-atom departs in a collinear geometry by the H-end side of HF. Ó 2004 Elsevier B.V. All rights reserved.