An Electron EDM Search Using Trapped Molecular Ions

A sample of trapped molecular ions offers unique possibilities to search for a permanent electron electric dipole moment (EDM). Specifically, we plan to perform this search using the unpaired electron spins in the $^3\Delta_1$ state of trapped HfF$^+$ molecular ions. Ions are easy to trap which will provide the long coherence times necessary to measure the small energy differences associated with an electron EDM. Additionally, the internal electric fields in polarized diatomic molecules can exceed $10^{10}$~V/cm, which will amplify any EDM induced energy splittings. We have created HfF$^+$ ions in a supersonic expansion jet by ablating a Hf target with a pulsed Nd:YAG laser in a He + 1\%SF$_6$ environment. The chemical reaction $\rm{Hf}^+ + \rm{SF}_6 \longrightarrow \rm{HfF}^+ + \rm{SF}_5$ is exothermic and proceeds rapidly. The He buffer gas in the expansion cools the molecular translational, vibrational, and rotational degrees of freedom to $\sim 10$~K. We have measured these temperatures via laser induced fluorescence spectroscopy on known neutral Hf atomic lines and newly identified neutral HfF molecular lines, and are currently searching for the unknown HfF$^+$ electronic transitions.