Geometric structure of diacetylene cation in the X 2Πg and A 2Πu electronic states

The complete rs structure of the diacetylene cation in the X̃ 2Πg and à 2Πu electronic states is reported. To this end the rotationally resolved origin bands in the à 2Πu–X̃ 2Πg laser excitation spectra of H–13C 3/4 C–C 3/4 C–H+ and H–C 3/4 13C–13C 3/4 C–H+ have been measured and analyzed. From the determined rotational constants as well as those known for H–C 3/4 C–C 3/4 C–H+ and D–C 3/4 C–C 3/4 C–D+, all the bond lengths in the two electronic states have been inferred.