Frequency metrology on the EF (1)Sigma(+)(g)<- X (1)Sigma(+)(g)(0,0) transition in H2, HD, and D2

We present a frequency metrology study on the lowest rotational levels of the hydrogen EF Σg+1 ←X Σg+1 (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 H2 and D2, and the Q (0) and Q (1) lines in HD are determined with an absolute accuracy at the 10-9 level. © 2006 The American Physical Society.