Frequency comb referenced spectroscopy of A-X 0-0 transitions in SH

We report absolute transition frequencies for the allowed transitions from the X²Π_3/2, v^″ = 0, J″ = 3/2 rovibronic ground state of ³²S¹H to the A²Σ⁺, v^' = 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.