PRECISE DETERMINATIONS OF IONIZATION-POTENTIALS AND EF-STATE ENERGY-LEVELS OF H-2, HD, AND D2

We report measurements of excited states in the three stable isotopic variants of molecular hydrogen: H-2, D2, and HD. The lowest rovibrational levels of the EF 1SIGMA(g)+ state have been measured by Doppler-free two-photon laser spectroscopy for all three molecules. The accuracy is 0.015 cm-1, and will improve to 0.003 cm-1 when wavelength calibrations of I2 reference transitions are completed. We also describe accurate measurements of high Rydberg np states in HD, and their analysis using multichannel quantum-defect theory. For HD these measurements give a result of 124 568.479(20) cm-1 for the ionization potential, in excellent agreement with theory. For H-2, the measurements of EF-state transitions give results slightly lower than previous experimental values, but the ionization potential remains in good agreement with theory. For D2 the results reported here, when taken in combination with recent measurements of transitions between excited states by Jungen et al. [J. Mol. Spectrosc. (to be published)], yield a value for the ionization potential that is again quite close to the theoretical result.