The millimeter-wave spectrum of the CaF radical (X ²∑⁺)

The pure rotational spectrum of the CaF radical has been measured in the laboratory using millimeter/sub-mm direct absorption specstroscopy. Fourteen rotational transitions have been detected originating in the v = 0 mode of the species. Rotational lines of the vibrationally excited v = 1, 2, and 3 states have also been observed. Spin-rotation splittings, as well as hyperfine interactions arising from the nuclear spin of the fluorine atom, were resolved in the CaF spectra. Rotational, fine-structure, and in some cases, hyperfine parameters were determined for the various vibrational modes of the molecule from a nonlinear least-squares fit to the data, using a ²∑ Hamiltonian, and are in agreement with past measurements. The newly measured rest frequencies for CaF, which are accurate to at least ± 100 kHz, will enable astronomical searches to be conducted for the molecule in interstellar and circumstellar gas. Given the recent detection of AlF in IRC + 10216, metal fluoride species may be more abundant than previously thought.