The (2-0) (0) and (1) transition frequencies of HD determined to a 10 relative accuracy by Doppler spectroscopy at 80 K.

The Doppler broadened (0) and (1) lines of the (2-0) vibrational band of HD have been measured at liquid nitrogen temperature and at pressures of 2 Pa, with a comb referenced continuous-wave cavity ring-down spectrometer set-up. Transition frequencies of 214905335185 kHz and 217105181898 kHz were derived from 33 and 83 recordings, with corresponding root mean squared deviation of 53 and 33 kHz for the (0) and (1) transition, respectively. This is the first sub-MHz frequency determination of the (0) transition frequency and represents a three order of magnitude accuracy improvement compared to literature. The (1) transition frequency is in very good agreement with previous determinations in saturation regime reported with similar accuracy. To achieve such accuracy, the transition frequency of the (101)-(000) 2-3 line of HO interfering with the (0) line had to be precisely determined and is reported with a standard error of 100 Hz at 214904329826.49(10) kHz (relative uncertainty of 5 × 10). These measurement sets provide stringent reference values for validating future advances in the theoretical description of the hydrogen (and water) molecule.