Millimeter-wave spectroscopy has been exploited to study molecular ions produced in a d.c. glow discharge with magnetic confinement. A species of astrophysical interest like protonated formaldehyde
is being studied in order to determine its molecular structure. The analysis of a Coriolis-type interaction in the rotational spectra of the formyl ion
isotopomers allows an accurate determination of the equilibrium structure of this ion.