We report on the improved determination of the center frequency of a hydrogen deuteride vibration-rotation transition. As a result of a refined analysis of cavity ring-down absorption spectra, it was possible to quantify the influence of the hyperfine structure on the line center frequency of the weak 2-0 $R\left(1\right)$ line, as retrieved in the Doppler-limited regime. It turns out that ignoring the hyperfine structure leads to an underestimation of the center frequency of about 350 kHz. The updated determination gives an absolute frequency of the $R\left(1\right)$ centroid of 217 105 181.936 MHz with a global uncertainty of 76 kHz.

• Received 13 December 2020
• Accepted 2 February 2021

DOI:https://doi.org/10.1103/PhysRevA.103.022828