Terahertz spectroscopy of CaH

Calcium monohydride, CaH, has been a molecule of astrophysical interest ever since its first identification in sunspots. For astrophysical use it is therefore important to provide rest frequencies in the terahertz range. We observed pure rotational transitions for the υ = 0, and υ = 1 levels. This is the first report of pure rotational spectra of the υ = 1 level. We also performed a global fitting of the effective molecular constants for the υ = 0 to υ = 4 levels of the electronic ground state along with infrared transitions from previous studies. • Pure rotational transitions of CaH (X 2 Σ + ) were observed in the 0.25–3.87 THz region. • N values up to 15 and 16 were seen in the υ = 0 and 1 vibrational states, respectively. • Molecular constants were determined for the lowest vibrational states of CaH (X 2 Σ + ). • Accurate transition rest frequencies are provided. • A global analysis including vibrational transitions was also carried out.