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

The Doppler broadened R (0) and R (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 R (0) and R (1) transition, respectively. This is the first sub-MHz frequency determination of the R (0) transition frequency and represents a three order of magnitude accuracy improvement compared to literature. The R (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 11 -3 12 line of H 2 16 O interfering with the R (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 -13 ). These measurement sets provide stringent reference values for validating future advances in the theoretical description of the hydrogen (and water) molecule.