Indirect information on reactive transition states from conservation of angular momentum

By choosing a kinematically constrained bimolecular reaction of the type H + H′L → HH′+ L, where H and H′ are heavy atoms and L is a light atom, the orbital angular momentum, L, of the reagents appears almost exclusively as HH′ product rotation, J. It follows that the magnitude of J is given by |L|=µvrelb, where µ is the reduced mass of the reagent collision partners, vrel is their relative velocity and b is the impact parameter, i.e. the distance of closest approach if the reagents moved in undeflected straight-line paths. Hence, by measuring both the product rotational distribution and the relative velocity distribution of the reagents, it is possible to deduce the range of impact parameters resulting in specific vibrational levels of the HH′ product. A prototype system is Ba + HI → BaI + H. Experimental results for BaI (v= 0) are presented for this kinematically constrained reaction showing that the range of b is restricted, peaking at or near the energetic cut-off.