Hyperfine structure of rotational spectra: state-of-the-art experimental and theoretical determinations

The determination of (hyper)fine parameters such as quadrupole-coupling, spin-spin coupling, and spin-rotation constants is one of the aims of high-resolution rotational spectroscopy. These parameters are relevant not only from a spectroscopic point of view, but also from a physical and/or chemical viewpoint, as they might provide detailed information on the chemical bond, structure, etc. In addition, the hyperfine structure of rotational spectra is so characteristic that its analysis may help in assigning the spectra of unknown species. Nevertheless, the experimental determination of hyperfine constants can be a challenge not only for actual problems in resolving hyperfine structures themselves, but also due to the lack of reliable estimates or the complexity of the hyperfine structure itself. It is thus important to be able to rely on good predictions for such parameters, which can nowadays be provided by quantum-chemical calculations. In fact, the aim of this presentation is to show how fruitful the interplay between experiment and theory can be in this field. A number of examples will be presented to illustrate this interplay in the investigation of hyperfine structures of rotational spectra. Among others, those include isotopic species of water, bromofluoromethane, ammonia and hydrogen cyanide. From an experimental point of view, we focus on the Lamb-dip technique. This technique allows to improve the resolving power in the millimeter- and submillimeter-wave frequency range by at least one order of magnitude, thus making it possible to perform sub-Doppler measurements as well as to resolve narrow hyperfine structures. In particular, the high resolution that can be achieved by our experimental set up will be demonstrated by a few representative examples. Concerning theory, the theoretical background for the required quantum-chemical calculations will be briefly reviewed, and a particular emphasis on the computational requirements will be given. It will be demonstrated that high-level calculations can provide very reliable values for hyperfine parameters (quadrupole coupling constants, spin-rotation tensors, spin-spin couplings, etc.) and how theoretical predictions are often essential for a detailed analysis of the hyperfine structure of the recorded spectra.