  We use quantum-logic spectroscopy (QLS) and interrogate rotational
transitions of a single CaH+ ion with a highly coherent frequency comb,
achieving a fractional statistical uncertainty for a transition line center of
4 x 10^-13. We also improve the resolution in measurement of the Stark effect
due to the radio-frequency (rf) electric field experienced by a molecular ion
in an rf Paul trap, which we characterize and model. This allows us to
determine the electric dipole moment of CaH+ by systematically displacing the
ion to sample different known rf electric fields and measuring the resultant
shifts in transition frequency.
