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We report absolute optical frequencies of electronic transitions from the$X\, ^2\Pi_{3/2},v''=0,J''=3/2$ rovibronic ground state to the 12 lowest levelsof the $A\, ^2\Sigma^+,v'=0$ vibronic state in $^{16}$OH, as well as to the 16lowest levels of the same vibronic state in $^{16}$OD. The absolute frequenciesof these transitions have been determined with a relative uncertainty of a fewparts in $10^{11}$, representing a $\sim$1000-fold improvement over previousmeasurements. To reach this level of precision, an optical frequency comb hasbeen used to transfer the stability of a narrow-linewidth I$_2$-stabilizedreference laser onto the 308-nm spectroscopy laser. The comb is also used tocompare the optical frequency of the spectroscopy laser to an atomic clockreference, providing absolute accuracy. Measurements have been carried out onOH/OD molecules in a highly-collimated molecular beam, reducing possiblepressure shifts and minimizing Doppler broadening. Systematic shifts due toretroreflection quality, the Zeeman effect, and the ac Stark effect have beenconsidered during the analysis of the measured spectra; particularly in thecase of the OD isotopologue, these effects can result in shifts of the fittedline positions of as much as 300 kHz. The transition frequencies extracted inthe analysis were also used to determine spectroscopic constants for the $A\,^2\Sigma^+,v'=0$ vibronic state. The constants fitted in this work differsignificantly from those reported in previous works that measured the $A - X$transitions, resulting in typical deviations of the predicted opticaltransition frequencies of $\sim$150 MHz, but they generally agree quite wellwith the constants determined using hyperfine-resolved measurements ofsplittings within the $A$ state.

Precision spectra of $A\, ^2\Sigma^+,v'=0 \leftarrow X\, ^2\Pi_{3/2},v''=0,J''=3/2$ transitions in $^{16}$OH and $^{16}$OD