Precise determinations of ionization potentials and $EF$ state energy levels of $\ce{H2}$, $\ce{HD}$, and $\ce{D2}$,” & , Physical Review A 46, 3676–3690 (1992).


We report measurements of excited states in the three stable isotopic variants of molecular hydrogen: $\ce{H2}$, $\ce{D2}$, and $\ce{HD}$. The lowest rovibrational levels of the $\mathit{EF}~{}^{1}{\Sigma}_g^{+}$ state have been measured by Doppler-free two-photon laser spectroscopy for all three molecules. The accuracy is $0.015~\text{cm}^{-1}$, and will improve to $0.003~\text{cm}^{-1}$ when wavelength calibrations of $\ce{I2}$ reference transitions are completed. We also describe accurate measurements of high Rydberg $np$ states in $\ce{HD}$, and their analysis using multichannel quantum-defect theory. For $\ce{HD}$ these measurements give a result of $124\,568.479(20)~\text{cm}^{-1}$ for the ionization potential, in excellent agreement with theory. For $\ce{H2}$, 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 $\ce{D2}$ 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 ionization potential that is again quite close to the theoretical result.

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