Electronic spectrum of the protonated diacetylene cation (H2C4H+).

The B̃1A1←X̃1A1 electronic band system of the protonated diacetylene cation (H2C4H+) is measured over the 230-295 nm range by photodissociating H2C4H+ ions stored in a cryogenic ion trap and by photodissociating H2C4H+ tagged with Ar and N2 in a tandem mass spectrometer. The B̃1A1←X̃1A1 band system has an origin at 34 941 cm-1 for H2C4H+, 34 934 cm-1 for H2C4H+-Ar, and 34 920 cm-1 for H2C4H+-N2. The spectra of H2C4H+, H2C4H+-Ar, and H2C4H+-N2 display similar vibronic structure, which is assigned using ab initio calculations to progressions in two symmetric a1 C-C stretch vibrational modes (ν6 and ν4), with band spacings of 860 and 1481 cm-1, respectively.