Lamb-dip spectroscopy of HD at cryogenic temperatures down to 12 K

Precision measurements of molecular hydrogen and its isotopologues provide critical tests of molecular theory, including quantum electrodynamics (QED) and fundamental constants. However, advancing the experimental accuracy of hydrogen overtone transitions remains challenging due to unexplained asymmetries in spectral line profiles. We present the Lamb dip measurement at a temperature as low as 12 K, where the spectral features exhibit enhanced resolution. By applying a lineshape centroid extrapolation method – which is independent of the lineshape model – we determine the transition frequency of the R(0) line in the (2-0) overtone of HD to be 214 905 335 170 (27)stat(124)sys kHz. While the low-temperature Lamb-dip measurements challenge conventional models of spectral line shapes, our results are in excellent agreement with the latest theoretical results.