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 H 2 18 O 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 ν 1 +ν 3 band and have been chosen in the wavelength range from 1.38 to 1.41μm 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 H 2 17 O spectrum, has been performed with an overall uncertainty of ∼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.