Knowledge Bank

The JILA electron electric dipole moment (eEDM) experiment will use a low-lying, metastable $3\Delta 1$ state in trapped molecular ions of HfF$+$ or ThF$+$. Prior to this work, the low-lying states of these molecules had been investigated by PFI-ZEKE spectroscopy. However, there were no detailed studies of the electronic structure. The recently developed technique of frequency comb velocity modulation spectroscopy (VMS) provides broad-bandwidth, high-resolution, ion-sensitive spectroscopy, allowing the acquisition of 150 cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$ of continuous spectra in 30 minutes over 1500 simultaneous channels. By supplementing this technique with cw-laser VMS, we have investigated the electronic structure of HfF$+$ in the frequency range of 9950 to 14600 cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$, accurately fitting and assigning 16 rovibronic transitions involving 8 different electronic states including the $X1\Sigma +$ and $a3\Delta 1$ states. In addition, an observed $3\Pi 0+$ state with coupling to both the $X$ and $a$ states has been used in the actual eEDM experiment to coherently transfer population from the rovibronic ground state of HfF$+$ to the eEDM science state. Furthermore, we report on current efforts of applying frequency comb VMS at 700 - 900 nm to the study of ThF$+$, which has a lower energy $3\Delta 1$ state and a greater effective electric field, and will provide increased sensitivity for a measurement of the eEDM.