Marked Influence of the Nature of the Chemical Bond on CP-Violating Signature in Molecular Ions HBr⁺ and HI⁺

Heavy polar molecules offer a great sensitivity to the electron electric dipole moment (EDM). To guide emerging searches for EDMs with molecular ions, we estimate the EDM-induced energy corrections for hydrogen halide ions HBr⁺ and HI⁺ in their respective ground X ²Π_3/2 states. We find that the energy corrections due to EDM for the two ions differ by an unexpectedly large factor of 15. We demonstrate that a major part of this enhancement is due to a dissimilarity in the nature of the chemical bond for the two ions: the bond that is nearly of ionic character in HBr⁺ exhibits predominantly a covalent nature in HI⁺. We conclude that because of this enhancement the HI⁺ ion may be a potentially competitive candidate for the EDM search.