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

Electronic spectra of mass-selected HC4H+-Arn (n = 1-3) and HC4H+-(N2)n (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 Ã2Πu ← X̃2Πg 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 22Πu ← X̃2Πg 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 22Πu state.