Millimeter-wave rotational spectroscopy of MgOD and CaOD (X ²Σ⁺)

Pure rotational spectra of CaOD and MgOD have been recorded in the range 200-390 GHz using millimeter/sub-mm direct absorption spectroscopy. Transitions arising from the (000), (010), (020), and (100) modes have been measured for the ²Σ⁺ ground electronic states of these free radicals. The data were analyzed successfully using a linear ²Σ⁺ model for CaOD; for MgOD, only the (000) and (010) states could be fit with this Hamiltonian. Moreover, the (010) data required the addition of a substantial p_Π term to account for contamination of excited ²Π electronic states. For both species, the α₂ vibration-rotation term was found to be negative, in contrast to MgOH and CaOH, suggesting a less anharmonic contribution to the bending potential in CaOD and MgOD. These measurements also indicate a shorter O-H bond in MgOH than the other alkaline earth hydroxide radicals, which likely results because this species is quasilinear.