Deep-Ultraviolet Frequency Metrology of H-2 for Tests of Molecular Quantum Theory

Molecular hydrogen and its isotopic and ionic species are benchmark systems for testing quantum chemical theory. Advances in molecular energy structure calculations enable the experimental verification of quantum electrodynamics and potentially a determination of the proton charge radius from H-2 spectroscopy. We measure the ground state energy in ortho-H-2 relative to the first electronically excited state by Ramsey-comb laser spectroscopy on the EF1 Sigma(+)(g) - X-1 Sigma(+)(g) (0, 0) Q1 transition. The resulting transition frequency of 2 971 234 992 965(73) kHz is 2 orders of magnitude more accurate than previous measurements. This paves the way for a considerably improved determination of the dissociation energy (D-0) for fundamental tests with molecular hydrogen.