We report a direct measurement of the 1-0 Rð0Þ vibrational transition frequency in ground-state hydrogen deuteride (HD) using infrared-ultraviolet double resonance spectroscopy in a molecular beam. Ground-state molecules are vibrationally excited using a frequency comb referenced continuous-wave infrared laser, and the excited molecules are detected via state-selective ionization with a pulsed ultraviolet laser. We determine an absolute transition frequency of 111 448 815 477(13) kHz. The 0.12 parts-perbillion (ppb) uncertainty is limited primarily by the residual first-order Doppler shift.

We report absolute optical frequencies of electronic transitions from the X 2 3/2 , v = 0, J = 3/2 rovibronic ground state to the 12 lowest levels of the A 2 + , v = 0 vibronic state in 16 OH, as well as to the 16 lowest levels of the same vibronic state in 16 OD. The absolute frequencies of these transitions have been determined with a relative uncertainty of a few parts in 10 11 , representing a ∼1000-fold improvement over previous measurements. To reach this level of precision, an optical frequency comb has been used to transfer the stability of a narrow-linewidth I 2 -stabilized reference laser onto the 308-nm spectroscopy laser. The comb is also used to compare the optical frequency of the spectroscopy laser to an atomic clock reference, providing absolute accuracy. Measurements have been carried out on OH and OD molecules in a highly collimated molecular beam, reducing possible pressure shifts and minimizing Doppler broadening. Systematic shifts due to retroreflection quality, the Zeeman effect, and the ac Stark effect have been considered during the analysis of the measured spectra; particularly in the case of the OD isotopologue, these effects can result in shifts of the fitted line positions of as much as 300 kHz. The transition frequencies extracted in the analysis were also used to determine spectroscopic constants for the A 2 + , v = 0 vibronic state. The constants fitted in this work differ significantly from those reported in previous works that measured the A-X transitions, resulting in typical deviations of the predicted optical transition frequencies of ∼150 MHz, but they generally agree quite well with the constants determined using hyperfine-resolved measurements of splittings within the A state.

We report absolute transition frequencies for the allowed transitions from the X2Π3/2, [Formula: see text] = 0, J″ = 3/2 rovibronic ground state of 32S1H to the A2Σ+, [Formula: see text] = 0 vibronic state. The frequencies have been determined with an uncertainty of less than 1 MHz, representing a more than 1000-fold improvement over previous measurements. Spectral traces are measured by scanning a frequency comb stabilized continuous-wave spectroscopy laser over the molecular transitions and detecting laser-induced fluorescence from SH molecules in a highly collimated molecular beam. To determine the absolute transition frequencies, the traces are fit with a quantum-mechanical model that accounts for saturation effects and shifts due to quantum interference. The model also provides estimates of the predissociation rate of the excited-state levels. Weighted averages of the hyperfine-resolved transition frequencies are computed in order to enable comparisons to measurements where the hyperfine structure is not resolved. These hyperfine-averaged frequencies indicate that the absolute transition frequencies determined in previous measurements were about ∼2.1 GHz (0.07 cm−1) too high. Finally, the measured transition frequencies are fit using an effective Hamiltonian model, resulting in more precise estimates of the spectroscopic constants.

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