Frequency comb-referenced spectroscopic measurements in the second overtone of HD

The Doppler broadened R(0), P(1) and P(2) lines of the (v’←v’’) = (3←0) vibrational band of HD have been measured at the MHz accuracy level using a comb-referenced continuous-wave cavity ring-down spectrometer. The transition frequencies are found to be 305141769.4 ± 5.0, 308138756.2 ± 4.0 and 313148302.4 ± 9.0 MHz for the P(2), P(1) and R(0) transitions, respectively. This is the first spectroscopic measurement of HD at this level of accuracy for such a high-vibrational excitation. It represents an accuracy improvement by a factor of 50 compared to the literature.