Jonathan Gilligan

In recent years, advances in theoretical treatments have made molecular hydrogen attractive for studies of fundamental physics in molecules. The relativistic and radiative corrections to the ground-state binding energy have been calculated, but few measurements have been able to test these predictions. With frequency-tripled pulse-amplified light from a continuous-wave single-frequency dye laser, it is possible to make precise measurements of two-photon transitions in the deep ultraviolet. Several two-photon transitions from the ground state to the EF state of the stable isotopes of molecular hydrogen were measured with accuracies around 0.015 cm$\sp{-1}$. Saturated absorption spectra of I$\sb2$ were acquired simultaneously to allow the accuracy to be improved to 0.003 cm$\sp{-1}$ in the future by measuring the absolute frequencies of visible transitions in I$\sb2$. A second experiment used laser double resonance to measure the energies of transitions in HD from the EF state to singlet Rydberg p states ranging from n = 40 to 80. A quantum defect analysis of these transitions was used to extrapolate to the series limit. Combining the series limit with the measured EF state energy gives a value of 124 568.479(19) cm$\sp{-1}$ for the ionization potential, in good agreement with ab initio calculations. The measurements of transitions to the EF state in D$\sb2$ allows a previous measurement of the ionization potential of D$\sb2$ to be improved by a factor of four to $\pm$0.027 cm$\sp{-1}$. The ionization potentials of H$\sb2$, D$\sb2$, and HD have now been measured with accuracies of 0.014-0.027 cm$\sp{-1}$. These accuracies are better than those of the ab initio calculations. The agreement between the measurements and the ab initio values confirms the calculated relativistic and radiative corrections.