Emission spectra of the radical cations of 1,3-dichlorobenzene, 1,4-dichlorobenzene and 1,3,5-trichlorobenzene in the gas phase

Abstract

Emission spectra of the radical cations of 1,3-dichlorobenzene, 1,4-dichlorobenzene- h 4 (and - d 4), and 1,3,5-trichlorobenzene, excited in the gas phase by controlled electron impact, are presented. The band systems, which lie in the 500–750 nm wavelength region, are attributed to the B̃(π −1) → X̃(π −) electronic transition of the cations on the basis of photoelectron spectroscopic data. The NeI excited photoelectron spectra and the ionisation energies of chloro-, o-, m-, p-dichloro- and 1,3,5-trichlorobenzene have been obtained. The information acquired from the emission and photoelectron spectra is discussed and compiled to deduce the symmetry of the B̃ states. Emission, with quantum yield > 10 −5, could not be detected with electronically excited radical cations of chloro-, o-dichloro-, 1,2,4- and 1,2,3-trichloro- and tetrachloro-benzenes. This is attributed to the nature of the B̃ states, which arise by σ −1 ionisation processes. The lifetimes of the zeroth and some vibrationally excited levels of the B̃(π −1) states were also measured and found to be 22 ± 2 ns for 1,3,5-trichlorobenzene cation and < 6 ns for 1,3- and 1,4-dichlorobenzene cations. The lifetimes of the latter two electronically excited cations are estimated to be two orders of magnitude shorter than 6 ns from the measurement of the relative emission intensities of the B̃ → B̃ band systems of the three title cations.