Determination of the normal AΠ 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 XΣ to AΠ state are able to be measured unambiguously by in-cell absorption spectra. For the first time, we show that the AΠ state of MgF is actually a normal state, not an inverted one. The laser cooling relevant transitions XΣv=0,1,N=1→AΠ(v=0,J=1/2) are also identified, along with the hyperfine structure of the XΣ(v = 0, N = 1) state. This study provides an important step for ongoing laser cooling experiments of MgF molecules.