Analysis of large effective electric fields of weakly polar molecules for electron electric-dipole-moment searches

Combined experimental and theoretical studies on the electric dipole moment of the electron (eEDM) can probe energy scales of a few TeV to PeV. The possible existence of the eEDM gives rise to an experimentally observed energy shift, which is proportional to the effective electric field (E-eff) of a target molecule. Hence, an analysis of the quantities that enhance Eeff is necessary to identify suitable molecules for eEDM searches. In the context of such searches, it is generally believed that a molecule with larger electric polarization also has a larger value of Eeff. However, our Dirac-Fock and relativistic coupled-cluster singles and doubles calculations show that the hydrides of Yb and Hg have larger E-eff than those of fluorides, even though their polarizations are smaller. This is due to significant mixing of valence s and p orbitals of the heavy atom in the molecules. This mixing has been attributed to the energy differences of the valence atomic orbitals and the overlap of the two atomic orbitals based on the orbital interaction theory.