Microwave spectroscopy of molecular ions and other transient species in electric discharges

Abstract

We have employed large d.c. glow discharges to produce a variety of unstable species, including molecular ions, in sufficient abundance for detection of their microwave spectra. For simple ions (HCO⁺, HN₂⁺, CO⁺) we have studied several isotopic forms and determined accurate molecular structures and hyperfine parameters, and for HCO⁺ we have measured the linewidth parameter for pressure broadening by several gases. To facilitate observation of additional species we have devoted considerable effort to improving our methods of generating and detecting microwave radiation, of modulating the molecular absorption and of computer processing the resultant spectral data. One of the principal difficulties in searching for new transient species is that of optimizing its production before any microwave spectrum has been seen. To alleviate this problem we have attached instrumentation for high-resolution optical spectroscopy and quadrupole mass spectrometry to the same glow discharge used for the microwave work. The mass spectrometer can sample neutral molecules and positive or negative ions escaping from the discharge; in conjunction with microwave interferometric measurements of electron density it can provide absolute ion densities for particular ions of sufficient precision to determine in advance whether or not the microwave spectrum can be detected.