Emission and laser excitation spectra of rotationally cooled cations: Ã 2Πu↔X̃ 2Πg band systems of X(C≡C)2 X+, X=Cl, Br

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Abstract
The à 2ΠΩ,u↔X̃ 2ΠΩ,g, Ω=3/2, 1/2, electronic transitions of rotationally and vibrationally cooled dichlorodiacetylene and dibromodiacetylene cations in the gas phase have been studied by emission and laser excitation spectroscopies. The emission spectra were obtained by electron impact excitation of the molecules seeded in a supersonic helium free jet and the excitation spectra by laser-excited fluorescence of cations cooled to liquid nitrogen temperature. The narrowing of the vibronic bands in the spectra allow the vibrational frequencies for these cations in the X̃ 2ΠΩ,g and à 2ΠΩ,u states to be inferred to within 1–2 cm−1. Complementary use of both sets of data enable the spin-orbit components to be unambiguously identified and many spectral features to be interpreted. The vibrational frequency trends are also discussed.