Production of C₂H₄Cl⁺ by dissociative photoionization of weak molecular complexes in C₂H₄+HCl mixtures

The photoionization efficiency (PIE) spectrum from 600 to 1200 Å for the production of the ion C₂H₄Cl⁺ by dissociative photoionization of the products of room-temperature jet expansions of a 1:4 mixture of C₂H₄ and HCl was measured at several nozzle pressures. The results were resolved into the PIE yield curve for the heterodimer process C₂H₄·HCl+hv→C₂H₄Cl⁺+H+e. This reaction is necessarily characterized by a large change in geometry between neutral complex and ionic product. The observed spectrum exhibits an unusual and conspicuous peak at 15.2 eV that is characterized by a sharp cutoff to the high energy side. This feature points to the onset of strongly nonstatistical channels for the production of C₂H₄Cl⁺ at this energy such that product formation proceeds through very few states. The observed onset of C₂H₄Cl⁺ at 11.92±0.24 eV is 17±6 kcal mol^-1 above the true threshold. An important conclusion is that at all energies above the onset the yield of dissociative ionization of the heterodimer to the cation C₂H₄Cl⁺ is determined by dynamical factors.