Frequency metrology on the EF 1Σg+←X 1Σg+(0,0) transition in H2, HD, and D2

We present a frequency metrology study on the lowest rotational levels of the hydrogen EF (1)Sigma(+)(g)<- X (1)Sigma(+)(g)(0,0) two-photon transition near 202 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(0) to Q(2) lines in H-2 and D-2, and the Q(0) and Q(1) lines in HD are determined with an absolute accuracy at the 10(-9) level.