Rotational level spacings in HD from vibrational saturation spectroscopy

The $R(1)$, $R(3)$, and $P(3)$ rovibrational transitions in the (2-0) overtone band of HD molecule are measured Doppler-free saturation using technique NICE-OHMS spectroscopy. For line, hitherto not observed saturation, we report a frequency $203\phantom{\rule{0.16em}{0ex}}821\phantom{\rule{0.16em}{0ex}}936\phantom{\rule{0.16em}{0ex}}805\phantom{\rule{0.16em}{0ex}}(60)$ kHz. dispersive line shapes three spectra show strong correlations, allowing for extraction accurate information on rotational level spacings. This leads to spacings ${\mathrm{\ensuremath{\Delta}}}_{(J=3)\ensuremath{-}(J=1)}=13\phantom{\rule{0.16em}{0ex}}283\phantom{\rule{0.16em}{0ex}}245\phantom{\rule{0.16em}{0ex}}098\phantom{\rule{0.16em}{0ex}}(30)$ kHz $v=0$ ground state ${\mathrm{\ensuremath{\Delta}}}_{(J=4)\ensuremath{-}(J=2)}=16\phantom{\rule{0.16em}{0ex}}882\phantom{\rule{0.16em}{0ex}}368\phantom{\rule{0.16em}{0ex}}179\phantom{\rule{0.16em}{0ex}}(20)$ $v=2$ excited vibration HD. These results that experimental values consistently larger than those obtained with advanced ab initio theoretical calculations at $1.5\ensuremath{\sigma}$, where uncertainty is determined by theory. same holds vibrational systematic deviations 1.7--1.9$\ensuremath{\sigma}$ found five lines accurately band.