The remarkable influence of an 'insignificant' quantity: How recoil orbital angular momentum determines product j distributions and (v;j) correlation in H plus LH reactions

摘要

Reactions for which the reactant (r)-to-product (p) mass ratio (mu(r)/mu(p)) is high, the well-known H+LH -> HH+L processes, convert most of available energy to product rotation, while that disposed as recoil is often regarded as negligible. In angular momentum (AM) terms, however, this recoil orbital AM (l(p)) is shown to be a critical component of the overall AM balance. For products of light mu(p), the maximum value of l(p) is energy limited and as a result the formation of products in low rotational (j(p)) states is severely restricted. Here energy constraints on recoil orbital AM and the consequent restrictions on j(p)-state populations are quantified using novel diagrammatic methods that illustrate how constraints on l(p) determine the j(p) states that are allowed or forbidden by the need to conserve energy and AM for each state-to-state transition. The method accurately predicts j(BaI)j (v=0,1,2) peaks from crossed-beam Ba+HI experiments, providing a quantitative and physically transparent rationale for the observed BaI rotational distributions. Extension to a wider range of reactions having mu(r)/mu(p)>1 shows that at least some j(p) are formally forbidden for each given reactant relative velocity or, more accurately, l(r). The fraction of inaccessible product states for a given initial velocity rises rapidly with mu(r)/mu(p) (>96% in Ba+HI). The method is also used to demonstrate that recoil orbital AM will be strongly aligned parallel to product rotational AM for high mu(r)/mu(p), although this correlation is generally lost in the low j(p) region as the parallel vector requirement is relaxed.