Slow molecule detection or Ramsey fringes in two-photon spectroscopy: which is better for high resolution spectroscopy and metrology ?

The CO2 laser locked onto a saturated absorption resonance of OsO4 provides a secondary frequency standard in the 10 mm region, with an accuracy of 50 Hz to 1 kHz. For averaging times less than 100 s its stability performance is better than the Hydrogen maser. This paper deals with the present attempt to increase this performance by using a two-photon molecular resonance as a reference. We begin with some preliminary and promising results on a two-photon line of SF6 leading to characteristics similar to those obtained with a saturation line of OsO4 . Then two alternative methods to increase the resolution are presented : optical detection of slow molecules and a new development of the well-known Ramsey fringes. Metrological features are analyzed for both methods.