H2,D2, and HD ionization potentials by accurate calibration of several iodine lines

Abstract

In a recent laser spectroscopic study of the ${\mathrm{H}}_2$, ${\mathrm{D}}_2$, and HD molecules [J. M. Gilligan and E. E. Eyler, Phys. Rev. A 46, 3676 (1992)], the energies of transitions between the X ${}_{}{}^1{}_{}{}^{}\mathrm{\Sigma }_{\mathrm{g}}^{+}{}_{}{}^{}$ ground state and the metastable EF ${}_{}{}^1{}_{}{}^{}\mathrm{\Sigma }_{\mathit{g}}^{+}{}_{}{}^{}$ state were measured precisely with respect to seven transitions in the iodine molecule. We report the absolute wavelengths of these iodine transitions with an accuracy of 250 kHz (5 parts in ${10}^{10}$) by interferometric comparison with the iodine-stabilized He-Ne standard at 633 nm. The full accuracy of the transitions to the EF state can now be realized. Improved ionization potentials for ${\mathrm{H}}_2$, ${\mathrm{D}}_2$, and HD are obtained using these results in combination with other previous measurements connecting the EF state to high Rydberg states. These ionization potentials are generally, but not completely, in agreement with ab initio theory. We also compare our measurements of the iodine wavelengths with less accurate values predicted from previously determined empirical molecular constants of the iodine B←X transition. We discuss the possibility of determining these constants more accurately and thereby improving the overall accuracy with which the widely used visible and near-infrared spectrum of ${\mathrm{I}}_2$ is known.