Electronic Spectra of Diacetylene Cations (HC₄H⁺) Tagged with Ar and N₂

Electronic spectra of mass-selected HC₄H⁺-Ar (n = 1-3) and HC₄H⁺-(N₂) (n = 1-2) complexes are measured over the 290-530 nm range using resonance-enhanced photodissociation spectroscopy in a tandem mass spectrometer. Vibronic transitions in the visible region are compared with previous experimental and theoretical results for the òΠ_u ← X̃²Π_g band system of HC₄H⁺. Hole burning experiments confirm that transitions over the 290-340 nm range involve the diacetylene cation (HC₄H⁺). On the basis of previous experiments and comparison with spectra of isoelectronic molecules the peaks are assigned to the 2²Π_u ← X̃²Π_g band system, with the origin transition for HC₄H⁺-Ar occurring at 29723 cm^-1. The main progression has a spacing of 906 cm^-1 and is assigned to the symmetric C-C stretch vibrational mode (ν₃). The assignment of additional bands is complicated by spectral congestion, the possible presence of energetically close-lying electronic states, vibronic coupling effects, and by the fact that HC₄H⁺ possibly becomes nonlinear in the 2²Π_u state.