The laser spectroscopy of cool, cold, and very cold molecular ions

Molecular ions have long presented a challenge to spectroscopists. Because of the extreme difficulty in generating molecular ions in high concentrations, sensitivity limitations have restricted work in this field. The technique of laser-induced fluorescence (l.i.f.) has provided us with a means to partially overcome this limitation. L.i.f. spectra of small molecular ions, e.g. N 2 +, CO+ and CO 2 +, show Doppler-limited resolution. Besides their spectroscopic information content, such spectra also serve as quantum-state selective ion detectors, by which dynamical processes involving such ions can be followed. L.i.f. spectra have also been recorded for larger organic ions, e.g. olefinic, acetylenic and benzenoid cations. The large molecular ions’ spectral congestion, caused by the large number of levels populated at room or elevated temperatures, can severely limit the resolution and interpretability of the spectra. As a means of surmounting this problem, we have studied many of the cations at reduced temperatures. The techniques employed include liquid N 2 and free jet expansion cooling in the gas phase, and isolation in solid Ne matrices. The combination of these different spectra lead to a wealth of information about electronic structure, vibrational frequencies, Jahn-Teller effects, etc., in organic molecular ions.