Precise measurement of the D₂ S₁(0) vibrational transition frequency

Precise measurements of vibrational transition frequencies in isotopologues of molecular hydrogen provide a means to test fundamental physical questions and can also help determine physical constants more precisely. Many of the most precise measurements have focused on the heteronuclear isotopologue HD, which has comparatively strong dipole allowed vibrational transitions due to its slight asymmetry. These transitions are forbidden in the homonuclear isotopologues H₂ and D₂ due to their lack of a permanent dipole moment, and previous studies have instead focused on measuring their much weaker electric quadrupole transitions. In this talk, I will present a measurement of the D₂ S₁(0) vibrational transition frequency that circumvents this limitation. Instead of relying on the weak quadrupole moment to drive the transition, we instead apply a strong static electric field to induce a permanent dipole moment, enhancing the transition strength by a factor of nine. This approach can also be used to measure transitions with no electric quadrupole moment such as Q₁(0), which is expected to be 19 times stronger than S₁(0).

A. Fast & S. A. Meek, Mol. Phys. 120, e1999520 (2022) doi:10.1080/00268976.2021.1999520