Double resonance techniques for high resolution spectroscopy of unstable molecules

Free radicals often exhibit complex optical spectra. Dopplerfree laser spectroscopy is the appropriate tool in many cases, but its limitations may be reached already when the fine and hyperfine structure of a heavy diatomic is to be analyzed. Double resonance techniques help to simplify dense spectra and to assign lines. If microwave radiation is used in addition to a single frequency laser, small energy splittings in individual electronic states can be investigated independently. The recently developed microwave optical polarization spectroscopy (MOPS)1 has proved to offer the necessary high resolution and the sensitivity needed to study chemically unstable species at low concentrations. If a beam of the species can be produced, ground state level splittings and shifts in external fields can be measured with 10-kHz accuracy by using the molecular beam laser-microwave double resonance technique.2 The power of these methods has been demonstrated in applications to alkaline earth monohalide radicals. Local perturbations in excited states were revealed and details of the electronic structure could be derived from hyperfine structure and Stark effect measurements.