The missing halide: Millimeter-wave spectroscopy of the MgI radical (X ²Σ⁺)

The pure rotational spectrum of the MgI radical in its ground electronic state (X ²Σ⁺) has been measured using millimeter/submillimeter wave direct absorption techniques in the region of 200–300 GHz. The molecule was created in a DC discharge by the reaction of magnesium vapor, produced in a Broida-type oven, with CH₃I. Between five to twelve transitions were recorded for the v = 0, 1, and 2 vibrational states of ²⁴MgI, as well as the v = 0 state for the isotopologues ²⁵MgI and ²⁶MgI, measured in their natural magnesium abundance. All observed transitions exhibited large spin-rotation splittings. Rotational, centrifugal distortion, and spin-rotation constants were determined for each isotopologue and the excited vibrational states of ²⁴MgI. Equilibrium parameters B_e, α_e, D_e, β_e, γ_e and γ_e^′ were derived for the main isotopologue, as well as the equilibrium bond length, r_e = 2.5730 (1) Å. The large spin-rotation constant of γ ∼ 300 MHz in MgI is thought to arise from second-order spin-orbit coupling originating in the nearby A²Π state, with possible contributions from other excited ²Π states.