Radiationless transitions in molecular ions: Monochloroacetylene and dichloroacetylene cations

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Abstract
We apply recent theories of radiationless transitions to the case of photon emission from molecular ions. Mono‐ and dichloroacetylene cations are formed in excited electronic states by 21.22 eV (He i) or 16.85 eV (Ne i) photon excitation of the parent molecules. Multiexponential decay is observed for emission from the ? 2Π(g) state of each ion as previously reported for 30 V electron impact excitation. Emission quantum yields are determined in the photon excitation experiments. Coupling between the ? 2Π(g) state and high vibrational levels of the ? 2Π(u) ground state is invoked to explain the data presented. The ’’strong‐coupling’’ theory of ’’intermediate’’ case radiationless transitions is used to infer values for average coupling energies, densities of states, and level widths. We conclude by treating several possible explanations for the fate of ’’hot’’ ground‐electronic‐state ions: (i) intramolecular vibrational relaxation; (ii) collisional quenching; and (iii) infrared emission.