Rotationally resolved electronic absorption spectra of triacetylene cation in a supersonic jet

The rotationally resolved cold spectra (Trot≈17 K) of the A 2Πg ← X 2Πu 000 and 401 transitions of triacetylene cation have been recorded in a supersonic jet. A discharge modulation in combination with the external frequency modulation of a cw ring laser is used to enhance detection sensitivity. The advantage of obtaining the spectra of such species under jet conditions is shown by comparison with measurements in a cell of the 401 vibronic transition. The combined analyses of the rotational structure yields rotational and spin–orbit constants for the ν4=1 level of the A 2Πg state.