Rotational level spacings in HD from vibrational saturation spectroscopy

Abstract

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