Determinations of EF 1Σg+(leftarrow)X1Σg+ transition frequencies in H2, D2, and HD

We have measured the energies of several rotational branches of the (0,0) band of the EF 1Σg+←X1Σg+ transition in molecular hydrogen by use of Doppler-free two-photon laser excitation at 202 nm. The accuracy for all three stable isotopic variations is 8 parts in 109, nominally a fourfold improvement over previously available results, but some of the transition energies differ from previous work by as much as three standard deviations. The improved accuracy derives principally from our ability to measure the optical phase evolution of nanosecond laser pulses, then to predict quantitatively the line shapes of multiphoton transitions excited by them. The results allow comparably accurate determinations of the EF state term energies, which in turn help to calibrate the entire excited-state spectrum of this fundamental molecular system. These improved calibrations are particularly useful to recent and ongoing efforts to determine the dissociation energy and ionization potential with improved accuracy