Crossed molecular beam study of the endoergic reaction Hg+I2→HgI+I from threshold to 2.6 eV (c.m.)

The endoergic reaction Hg+I2→HgI+I−1.15 eV has been studied by the crossed molecular beam technique over a range of average relative translational energy, ?tr, from 0.9 to 2.6 eV (c.m.). A supersonic seeded beam of Hg in H2, of variable Hg velocity, is crossed by a thermal beam of I2. In-plane angular distributions of the HgI product are measured as a function of ?tr, with an electron bombardment (EB) ionizer, mass filter detector. Center-of-mass (c.m.) product angular and recoil velocity distributions are deduced from the laboratory angular distributions by an iterative fitting procedure. The HgI angular distributions are bimodal, characteristic of a long-lived complex mechanism. The products’ recoil energy distribution is peaked at abnormally high E′tr. The average value, ?′tr, increases monotonically with ?tr. Reaction proceeds essentially at the thermodynamic threshold (i.e., with no intrinsic activation barrier), and the post-threshold excitation function is found to be of the form σR(Etr) ∝ (E−E0)s/Etr, where 1