Accurate Analytic Potential and Born–Oppenheimer Breakdown Functions for MgH and MgD from a Direct-Potential-Fit Data Analysis

New high-resolution visible Fourier transform emission spectra of the A 2Π → X 2Σ+ and B′ 2Σ+ → X 2Σ+ systems of 24MgD and of the B′ 2Σ+ → X 2Σ+ systems of 25,26MgD and 25,26MgH have been combined with earlier results for 24MgH in a multi-isotopologue direct-potential-fit analysis to yield improved analytic potential energy and Born–Oppenheimer breakdown functions for the ground X 2Σ+ state of MgH. Vibrational levels of the ground state of 24MgD were observed up to v″ = 15, which is bound by only 30.6 ± 0.10 cm–1. Including deuteride and minor magnesium isotopologue data allowed us also to determine the adiabatic Born–Oppenheimer breakdown effects in this molecule. The fitting procedure used the recently developed Morse/Long-Range (MLR) potential energy function, whose asymptotic behavior incorporates the correct inverse-power form. A spin-splitting radial correction function to take account of the 2Σ spin–rotation interaction was also determined. Our refined value for the ground-state dissociation energy of the dominant isotopologue (24MgH) is De = 11 104.25 ± 0.8 cm –1, in which the uncertainty also accounts for the model dependence of the fitted De values for a range of physically acceptable fits. We were also able to determine the marked difference in the well depths of 24MgH and 24MgD (with the deuteride potential curve being 7.58 ± 0.30 cm–1 deeper than that of the hydride) as well as smaller well-depth differences for the minor 25,26Mg isotopologues. This analytic potential function also predicts that the highest bound level of 24MgD is v″ = 16 and that it is bound by only 2.73 ± 0.10 cm–1.