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

Abstract

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 σ_R(E_tr) ∝ (E−E₀)^s/E_tr, where 1<sE₀=1.15 eV. The reaction cross section σ_R(?_tr) rises to a broad maximum at about 2 eV with a moderate decline at higher energies. A statistical–dynamical model of the reaction is developed which involves an orientation–averaged barrier (∼0.7 eV) to the formation of a long‐lived complex, presumably IHgI, followed by decay of the complex, either to products or to reactants. The shape of the cross section function in the threshold region, the branching ratio for the decay of the complex, and the product translational energy distributions are strongly correlated with the existence of this barrier to complex formation. The decline in σ_R(?_tr) at high energy is due to collision‐induced dissociation of I₂, and/or dissociation of product HgI molecules formed with internal excitation above 0.39 eV (the dissociation limit of HgI), both processes with a threshold of 1.54 eV. The statistical–dynamical calculations account for the main qualitative features of the reaction with a minimum number of assumptions. However, the present results are not entirely unequivocal because of a possible systematic experimental bias associated with decreased EB detection efficiency of internally excited HgI molecules.