Nonradiative transitions and unimolecular dissociation of excited electronic states of ClCN⁺, BrCN⁺, and ICN⁺

Radiative and nonradiative relaxation processes of electronic excited states of ClCN⁺, BrCN⁺, and ICN⁺ formed by He I and Ne I lamp excitation have been studied by photoelectron-photoion and photoion-fluorescence photon (PIFCO) coincidence techniques. The B̃²II states of these ions are found to fluoresce with an intrinsic biexponential decay. The results are interpreted in terms of the small molecule, resonance limit model of radiationless transitions. Average values of the interelectronic state mixing coefficients and coupling matrix elements are determined. Morse function sections of the potential energy surfaces of the X̃²II, òS⁺, and B̃²II states of the representative ion ClCN⁺ are calculated and are shown to be propitious for B̃ state relaxation via interelectronic interactions. Dissociative ionization processes of the XCN⁺ species are also studied, in particular fragmentation into the three channels X⁺ + CN, CX⁺ + N, and CN⁺ + X. Electronically excited CN radicals are detected in He I excited dissociative ionization of BrCN and ICN.