Emission spectra of two C 6H 6+ cations in the gas phase: competition of the radiative, Ã(π −1) → X̃(π −1), and fragmentation decay of the Ã(π −1) excited state of 2,4- and 1,3-hexadiyne radical cations

Abstract

The radiative relaxation of the first excited states, Ã(π −1), of two C 6H 6 +radical cations in the gaseous phase has been detected following excitation by low energy electron impact. The à 2E u → X̃ 2E g emission band system of 2,4-hexadiyne cation, and the O 0 0 band of the corresponding emission of 1,3-hexadiyne cation, which are identified by comparison with their photoelectron spectra are presented and discussed. The decay of the Ã(π −1) excited states of these C 6H 6 ∓ions is exceptional as the fragmentation and radiative pathways are competitive and are both detected experimentally. The lifetime of the à 2E u state of 2,4-hexadiyne radical cation was measured to be 24 ± 2 ns, which considered together with the upper limit of 0.6 for the quantum yield of emission inferred from the cation breakdown diagram of the à 2E u state, indicates that the radiative and non-radiative pathways deplete the à 2E u state at approximately the same rate of 2 × 10 7 s −1. The implications of the observations of the radiative relaxation of these two electronically excited C 6H 6 ∓ radical cations lying above the fragment ion appearance potentials are considered with respect to the isomerization and fragmentation of C 6H 6 ∓isomers.