Sub-Doppler mid-infrared spectroscopy of molecular ions

► An instrument for sub-Doppler mid-IR spectroscopy of molecular ions is described. ► NICE-OHMS is performed with an optical parametric oscillator (OPO). ► Fitting to Lamb dips provides ∼70 kHz precision in line center determination. ► Velocity modulation with phase sensitive detection gives ion-neutral discrimination. ► A sensitivity of 8.5×10-10cm-1Hz-1/2 is achieved.