  Optical control enables new high resolution probes of narrow collisional
(Feshbach) resonances, which are strongly dependent on the relative momentum of
colliding atom pairs, and important for simulating neutron matter with
ultracold atomic gases. We demonstrate a two-field optical vernier, which
expands kHz (mG) magnetic field detunings near a narrow resonance into MHz
optical field detunings, enabling precise control and characterization of the
momentum-dependent scattering amplitude. Two-photon loss spectra are measured
for the narrow resonance in $^6$Li, revealing rich structure in very good
agreement with our theoretical model. However, anomalous frequency shifts
between the measured and predicted two-photon spectra are not yet explained.
