Electronic spectra of chloro- and bromotriacetylene cations in neon matrices

Mass-selected, chlorinated (ClC6Cl+, HC6Cl+) and brominated (BrC6Br+, HC6Br+) derivatives of triacetylene cation produced from polyhalogenated benzenes in a hot-cathode discharge source were trapped in 6 K neon matrices. Absorption is detected with onset at 710.8 nm for BrC6Br+, 655.9 nm for HC6Br+, 649.1 nm for ClC6Cl+ and 624.0 nm for HC6Cl+. Radiative decay from the v'  =  0 level of the excited electronic state is observed on laser excitation of the transition. The systems are assigned to the à 2Πg ←  2Πu electronic transition of ClC6Cl+ and BrC6Br+, and à 2Π ←  2Π of HC6Cl+ and HC6Br+. A weaker system commencing at 495.4 nm for BrC6Br+, 574.5 nm for HC6Br+, 519.9 nm for ClC6Cl+ and 499.5 nm for HC6Cl+ is present. This is due to excitation to the 2Πg/- electronic state of the halogenated triacetylene cations. The assignment is based on calculated vertical excitations energies, vibrational frequencies and similarities in the absorption and fluorescence spectra to the 2Πg, à 2Πg ←  2Πu electronic systems of triacetylene cation.