Frequency metrology in the near-infrared spectrum of H217O and H218O molecules: testing a new inversion method for retrieval of energy levels

An extensive Doppler-free spectroscopic investigation of the near-infrared absorption spectrum of the (H2O)-O-18 molecule was performed, for the first time, with absolute frequency calibration by using a GPS-disciplined fiber-based optical frequency comb. The investigated line pairs belong to the nu(1) + nu(3) band and have been chosen in the wavelength range from 1.38 to 1.41 mu m with a lambda scheme, so as to share the excited energy level and allow an accurate determination of the rotational energy separations of the fundamental vibrational state. The measurement of the sub-Doppler line-center frequencies, also extended to the (H2O)-O-17 spectrum, has been performed with an overall uncertainty of similar to 30 kHz, i.e. about three orders of magnitude lower than the HITRAN data set. The retrieved energy separations agree, by less than 80 kHz, with recent findings provided by the so-called MARVEL procedure for spectral data inversion, thus yielding a very stringent test of its accuracy.