Quasiclassical trajectory study of molecular alignment effects on the dynamics of the reactions of Cl, Br and I whith H2

Resumen

The X + H2 (X = Cl, Br, and I) reactions may be taken as models for endoergic triatomic reactions with heavy-light-light kinematics and collinear saddle point. The dependence of scalar and two-vector properties, angular distributions (k, k′), (k, j′), (k′, j′), and (I′, j′), on (ET, v, j), as well as the effect of considering initial parallel (II), perpendicular (⊥), and random (null) k-j alignment has been studied using the quasiclassical trajectory (QCT) method. The threshold energy for II alignment is always higher than the ones for ⊥ and null alignments, but for high enough ET values σ(II) becomes larger than σ(⊥) and σ(null), and the same occurs for the j-dependence. In the v range of values explored σ(II) is in general equal or larger than σ(⊥) and σ(null). The expression 1/3σ(II) + 2/3σ(⊥) provides a very good estimate to σ(null) if the system is not in the vicinities of the threshold region, and some useful relations to simplify the QCT calculations for II alignment have also been given. For the two-vector properties considered, the results obtained for ⊥ alignment are in general closer to the ones for null alignment than the results obtained for II alignment. The angular correlations that result from the calculation are not a trivial result coming from a kinematic constraint; being particularly remarkable the role played by the rotation of the H2 molecule. These results may be explained taking into account the saddle point properties, the "effective molecular size" of the rovibrationally excited H2 molecule, and the geometrical implications of the alignments.