Rest frequencies for alkaline earth hydroxide radicals (X²Σ⁺)

Rest frequencies for the pure rotational transitions of the alkaline earth hydroxide radicals MgOH, CaOH, SrOH, and BaOH are presented. These data were obtained using millimeter/submillimeter direct absorption spectroscopy in the range 100-400 GHz. The hydroxide molecules, which all have ²Σ⁺ ground electronic states, were created by the reaction of hydrogen peroxide and metal vapor, produced in a Broida-type oven. Transitions arising in the ground (000) state as well as excited vibrational modes (01¹0), (02⁰0), (02²0), and (100), were recorded. Fine-structure splittings were resolved in every transition. The data were analyzed using a ²Σ Hamiltonian, and rotational, fine-structure, and l-type doubling constants were determined for the radicals. These measurements will enable the identification of the alkaline earth hydroxide species in the interstellar medium.