Determination of the normal A²Π state in MgF with application to direct laser cooling of molecules

We report high resolution electronic spectroscopy of cold magnesium monofluoride (MgF) molecules in the gas phase, which are created by a combination of laser ablation, chemical reaction, and 6 K helium buffer-gas cooling. Thanks to the sufficient population in the low-lying rotational states, the P, Q, and R branches in the electronic transition of the X²Σ⁺ to A²Π state are able to be measured unambiguously by in-cell absorption spectra. For the first time, we show that the A²Π state of MgF is actually a normal state, not an inverted one. The laser cooling relevant transitions X²Σ⁺v=0,1,N=1→A²Π_1/2(v=0,J^'=1/2) are also identified, along with the hyperfine structure of the X²Σ⁺(v = 0, N = 1) state. This study provides an important step for ongoing laser cooling experiments of MgF molecules.