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 X2Π3/2 states. We find that the energy corrections due to EDM for the two
ions differ by an unexpectedly large factor of fifteen. 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 covalent nature in HI+.
We conclude that because of this enhancement the HI+ ion may be a
potentially competitive candidate for the EDM search.Comment: This manuscript has been accepted for publication in Physical Review
Letters. The paper is now being prepared for publicatio