MILLIMETER-WAVE SPECTROSCOPY IN ELECTRIC-DISCHARGES - RARE MOLECULES SHOW THEMSELVES ONLY IF YOU LOOK IN THE OTHER DIRECTION

The important role of free radicals and molecular ions in the chemistry of the interstellar medium has been for years a strong motivation for the study of the millimeter-wave spectra of these reactive species. These species are also of great interest for quantum chemists, since they often exhibit unusual molecular structures. In searching for a new reactive species, the spectroscopist needs mainly three types of tools: (i) a very sensitive spectrometer, since the reactive species are usually produced in very low concentrations; (ii) efficient methods of production of the molecules; (iii) ab initio calculations, which are required to predict the frequency range to be scanned and/or as an aid to the identification of the molecule observed. In this paper, the spectrometer and the electric discharge used to produce the molecules in Lille, France, will be described. The performance of the equipment, the importance of chance in the mysterious chemistry which takes place in an electric discharge, and the great help provided by ab initio calculations in the identification process are illustrated by the presentation ofsome results concerning new, fascinating molecules such as ArH3+, Si2H2, and C2H3+. (C) 1994 Academic Press, Inc.