The millimeter-wave spectrum of the MgH and MgD radicals

The pure rotational spectrum of the MgH radical (X ²Σ+) in its ground state v = 0 and v = 1 vibrational modes has been observed in the laboratory using millimeter/submillimeter direct absorption spectroscopy. The rotational spectra of two isotopically substituted species, MgD and ²⁶MgH, have been detected as well. These free radicals were produced by the reaction of magnesium, vaporized in a Broida-type oven, with hydrogen atoms generated in a microwave discharge. All six hyperfine components of the N = 0 → 1 transition of MgH in its v = 0 and v = 1 states have been directly measured to an accuracy of ± 50 kHz, and five components have been observed for ²⁶MgH. The N = 0 → 1 and N = 1 → 2 transitions of MgD have also been detected. Rotational, fine structure, and hyperfine constants were determined for all species from a nonlinear least-squares fit to the data using a ²Σ Hamiltonian. Measurement of these frequencies for MgH should greatly aid in establishing its presence in interstellar space.