Doppler-free laser spectroscopy of buffer-gas-cooled molecular radicals

We demonstrate Doppler-free saturated absorption spectroscopy of cold molecular radicals formed by laser ablation inside a cryogenic buffer gas cell. By lowering the temperature, congested regions of the spectrum can be simplified, and by using different temperatures for different regions of the spectrum a wide range of rotational states can be studied optimally. We use the technique to study the optical spectrum of YbF radicals with a resolution of 30 MHz, measuring the magnetic hyperfine parameters of the electronic ground state. The method is suitable for high-resolution spectroscopy of a great variety of molecules at controlled temperature and pressure, and is particularly well suited to those that are difficult to produce in the gas phase.