  Determination of enantiomeric excess is important and remains challenges. We
theoretically propose a new spectroscopic method for this issue based on the
chirality-dependent AC Stark effects in cyclic three-level models under the
three-photon resonance condition. The enantiomeric excess of the chiral mixture
is determined by comparing the amplitudes of the two chosen AC Stark peaks in
the Fourier transform spectrum of the induced polarizations, which are
(approximately) proportional to the molecule numbers of the two enantiomers,
respectively. Comparing with current spectroscopic methods based on the
interference between the electric- and (usually weak) magnetic-dipole
transition moments and/or with the need for enantio-pure samples, our method
only involves electric-dipole transitions and does not require the enantio-pure
samples. Therefore, it will give strong chiral signals and can be applied to
the determinations of enantiomeric excess for chiral molecules whose
enantio-pure samples are still challenging to achieve.
