CaF and CaCl in their X 2E ground states

Microwave/optical double resonance studies have been described for SrF by Domaille, Steimle and Harris [38], for CaCl by Domaille, Steimle and Harris [39], and for CaF by 

Six rotational transitions in the v = 0 level and three in the v = 1 level were observed, the microwave frequency spanning the range 29 to 60 GHz. Although 19F hyperfine splittings were expected, the particular transitions necessary to determine the interaction constants were not observable. The experimental results were therefore fitted to the usual effective Hamiltonian, 

The most interesting aspect of these constants is the much larger spin rotation constant observed for the upper state, which undoubtedly arises because of second-order mixing with other nearby electronic states. 

An important development in microwave/optical double resonance, called microwave/optical polarisation spectroscopy, was described by Ernst and Torring [42], The principles of this technique are illustrated in figure 11.22. A linearly polarised probe beam from a tunable laser is sent through the gas sample and a nearly crossed linear polariser, before its final detection. Polarised microwave radiation resonant with a rotational transition in the gas sample is introduced via a microwave horn as shown, and resonant absorption results in a partial change in polarisation of 

The first row of transition elements, with their lowest energy electron configurations, are as follows. 

---