Absolute rotational assignment of the origin band of theÃ2Πu↔[Xtilde]2Πgtransition of the diacetylene cation

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Abstract
The rotational analysis of the origin bands of the à 2Π u -[Xtilde] 2Π g transition of diacetylene and dideuterodiacetylene cations is considered. Although an analysis for diacetylene cation was made in 1956 by Callomon, some ambiguity remained concerning the absolute numbering and hence led to uncertainty in the derived constants. This situation has now been resolved by recording the emission spectra of this transition of the two isotopic cationic species rotationally cooled to ≈ 10 K in a supersonic free jet, as well as their rotationally resolved laser excitation spectra. By comparison of the Q-branch locations in the emission spectra, which only become discernible at such low temperatures, with those predicted using the two possible sets of constants, the rotational numbering is established. Consequently, the correct spectro-scopic constants of diacetylene and dideutero-diacetylene cations in their [Xtilde] 2Π g and à 2Π u electronic states are presented.