Millimetre-Wave and Submillimetre-Wave Spectroscopy of Molecular Ions

In the last few years, the specificities of millimetre-wave spectroscopy coupled with active developments in other fields (ab initio calculation, radioastronomy and infrared spectroscopy) have led to an impressive increase in our knowledge of the spectral characteristics of molecular ions. The advances to be reviewed in this paper are a result of the development of highly sensitive spectrometers and of efficient methods of production. Some specific examples, including ions with large-amplitude motion (H₃O⁺), weakly polar ions (HCNH⁺), asymmetric-top ions (HCO₂⁺, HON₂⁺) and ionic clusters (Ar\cdot H₃⁺), will be used to illustrate the potentialities of millimetre- and submillimetre-wave spectroscopy.