Frequency metrology on theEFΣg+1←XΣg+1(0,0)transition inH2, HD, andD2

We present a frequency metrology study on the lowest rotational levels of the hydrogen $EF{}^1\Sigma _{\mathit{g}}^{+}\leftarrow X{}^1\Sigma _{\mathit{g}}^{+}(0,0)$ two-photon transition near $202\mathrm{nm}$. For this purpose, the fourth harmonic of an injection-seeded titanium:sapphire pulsed oscillator is employed in a Doppler-free REMPI-detection scheme on a molecular beam of hydrogen. A frequency comb laser is used to perform the absolute frequency calibration on the continuous-wave (CW) laser that injection-seeds the oscillator. Chirp-induced frequency differences between the output of the pulsed oscillator and the seeding light are monitored on-line, while possible systematic shifts related to the AC-Stark and Doppler effects are addressed in detail. The transition frequencies of the $Q\left(0\right)$ to $Q\left(2\right)$ lines in ${\mathrm{H}}_2$ and ${\mathrm{D}}_2$, and the $Q\left(0\right)$ and $Q\left(1\right)$ lines in HD are determined with an absolute accuracy at the ${10}^{-9}$ level.