Emission spectra of Cl—CC—H+, Br—CC—H+and I—CC—H+radical cations: Ã2Π→X2Π band systems and the decay of the Ã2Π states

Abstract

Emission spectra of the halogenoacetylene radical cations, X—CC—H⁺ with X = Cl, Br, I, excited in the gaseous phase by controlled electron impact, have been observed in the 330–860 nm wavelength region. The band systems are assigned as the òΠ_Ω→X²Π_Ω(Ω=, ½) electronic transitions of the cation by reference to the photoelectron spectra of the halogenoacetylenes. The spinorbit splittings and the vibrational fine structure discernible in the high resolution photoelectron spectra are compared with the emission spectra. The decay curves measured for the emission bands show a pronounced non-exponential behaviour. This decay of the Ã²Π states of the halogenoacetylene cations is characterised by short and long components corresponding to lifetimes in the 10–20 ns and 300–500 ns range respectively. The time-resolved emission spectra recorded show that such decay components are that of the parent radical cations. The possible origins of the non-exponential decays are discussed.