We report measurements of excited states in the three stable isotopic variants of molecular hydrogen: ${\mathrm{H}}_2$, ${\mathrm{D}}_2$, and HD. The lowest rovibrational levels of the ${\mathit{EF}}^1$${\mathrm{\Sigma }}_{\mathit{g}}^{+}$ state have been measured by Doppler-free two-photon laser spectroscopy for all three molecules. The accuracy is 0.015 ${\mathrm{cm}}^{\mathrm{-}1}$, and will improve to 0.003 ${\mathrm{cm}}^{\mathrm{-}1}$ when wavelength calibrations of ${\mathrm{I}}_2$ 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) ${\mathrm{cm}}^{\mathrm{-}1}$ for the ionization potential, in excellent agreement with theory. For ${\mathrm{H}}_2$, 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 ${\mathrm{D}}_2$ 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 ionization potential that is again quite close to the theoretical result.

• Received 20 January 1992

DOI:https://doi.org/10.1103/PhysRevA.46.3676