The millimeter-wave spectrum of CaH (X²Σ⁺)

The millimeter-wave spectrum of the CaH and CaD radicals (X²Σ⁺) has been observed in the laboratory via direct absorption spectroscopy. The species were generated by the reaction of calcium vapor, produced in a Broida-type oven, with hydrogen atoms created in a microwave discharge. The six hyperfine components of the N = 0 → 1 rotational transition of CaH near 254 GHz were resolved and their frequencies determined to an accuracy of ± 100 kHz. The main hyperfine lines originating in the N = 0 → 1, 1 → 2, and 2 → 3 rotational transitions of CaD were also detected. Spectroscopic constants were determined for both radicals from a nonlinear least squares fit to the data, using a ²Σ Hamiltonian. These measurements constitute the first laboratory observation of CaH at high spectral resolution, and are essential to the detection of this radical in interstellar space.