Parity-pair-mixing effects in nonlinear spectroscopy of HDO

• Published in: Optics express 2022-12, Vol.30 (26), p.46040-46059
• Main Authors: Diouf, Meissa L, Tóbiás, Roland, Cozijn, Frank M J, Salumbides, Edcel J, Fábri, Csaba, Puzzarini, Cristina, Császár, Attila G, Ubachs, Wim
• Format: Article
• Language: English
• ISSN: 1094-4087; 1094-4087
• DOI: 10.1364/OE.474525

A non-linear spectroscopic study of the HDO molecule is performed in the wavelength range of 1.36-1.42 m using noise-immune cavity-enhanced optical-heterodyne molecular spectroscopy (NICE-OHMS). More than 100 rovibrational Lamb dips are recorded, with an experimental precision of 2-20 kHz, related to the first overtone of the O-H stretch fundamental of HD O and HD O. Significant perturbations, including distortions, shifts, and splittings, have been observed for a number of Lamb dips. These spectral perturbations are traced back to an AC-Stark effect, arising due to the strong laser field applied in all saturation-spectroscopy experiments. The AC-Stark effect mixes parity pairs, that is pairs of rovibrational states whose assignment differs solely in the K quantum number, where K is part of the standard asymmetric-top rotational label. Parity-pair mixing seems to be especially large for parity pairs with K  ≥ 3, whereby their energy splittings become as small as a few MHz, resulting in multi-component asymmetric Lamb-dip profiles of gradually increasing complexity. These complex profiles often include crossover resonances. This effect is well known in saturation spectroscopy, but has not been reported in combination with parity-pair mixing. Parity-pair mixing is not seen in H2 16O and H2 18O, because their parity pairs correspond to ortho and para nuclear-spin isomers, whose interaction is prohibited. Despite the frequency shifts observed for HD O and HD O, the absolute accuracy of the detected transitions still exceeds that achievable by Doppler-limited techniques.