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 <i>A</i> ²Π → <i>X</i> ²Σ⁺ and <i>B</i>′ ²Σ⁺ → <i>X</i> ²Σ⁺ systems of ²⁴MgD and of the <i>B</i>′ ²Σ⁺ → <i>X</i> 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 <i>X</i> ²Σ⁺ state of MgH. Vibrational levels of the ground state of ²⁴MgD were observed up to <i>v</i>″ = 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 D_e = 11 104.25 ± 0.8 cm ^–1, in which the uncertainty also accounts for the model dependence of the fitted D_e 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 <i>v</i>″ = 16 and that it is bound by only 2.73 ± 0.10 cm<sup>–1