Hyperfine-resolved optical spectroscopy of the AΠ ← XΣ transition in MgF.

We report on hyperfine-resolved laser spectroscopy of the AΠ ← XΣ transition of magnesium monofluoride (MgF), relevant for laser cooling. We recorded 25 rotational transitions with an absolute accuracy of better than 20 MHz, assigned 56 hyperfine lines, and determined precise rotational, fine, and hyperfine structure parameters for the AΠ state. The radiative lifetime of the AΠ state was determined to be 7.2(3) ns, in good agreement with ab initio calculations. The transition isotope shift between bosonic isotopologues of the molecule is recorded and compared to predicted values within the Born-Oppenheimer approximation. We measured the Stark effect of selected rotational lines of the AΠ ← XΣ transition by applying electric fields of up to 10.6 kV cm and determined the permanent electric dipole moments of MgF in its ground XΣ and first excited AΠ states to be μ = 2.88(20) D and μ = 3.20(22) D, respectively. Based on these measurements, we caution for potential losses from the optical cycling transition due to electric field induced parity mixing in the excited state. In order to scatter 10 photons, the electric field must be controlled to below 1 V cm.