Spectroscopy of the low-frequency vibrational modes of [formula omitted] isotopologues.

The low-frequency stretching and bending vibrations of the isotopologues CH 2 D + , CD 2 H + and CD 3 + have been recorded at low temperature and low resolution. For this, a cryogenic 22-pole trapping machine coupled to an IR beamline of the FELIX free electron laser facility has been used. To record the overview spectra, the laser induced reactions CD m H n + + H 2 → h ν CD m - 1 H n + 1 + + HD have been applied for these species. As this scheme is not applicable to CH 3 + , the latter has been tagged with He and subsequently dissociated by the IR beam. For the resulting CH 3 + - He spectrum, broad features are observed below 1000  cm - 1 possibly related to vibrational motions involving the He atom. The extracted vibrational band positions for all species are compared to results from high-level quantum-chemical calculations. [ABSTRACT FROM AUTHOR]