  We report on an ultrastable molecular clock based on the interrogation of the
3{\nu} rotational-vibrational combination band at 1563nm of carbon monoxide
12C16O. The laser stabilization scheme is based on frequency modulation (FM)
saturation spectroscopy in a high-finesse ultra-low expansion (ULE) glass
optical cavity using CO as the molecular reference for longterm stabilization
of the cavity resonance. The NICEOHMS FM modulation method is employed to
maintain a high signal-to-noise ratio and avoid conversion of laser frequency
noise into amplitude noise by the high finesse cavity. We report an Allan
Deviation of 1.8x10-12 at 1 second that improves to 3.5x10-14 after 1000
seconds of averaging.
