Precision Measurement Of The Ionization And Dissociation Energies Of The Deuterium Molecule

The hydrogen molecule is an important system for the development of molecular quantum mechanics. In particular, the dissociation energy is a benchmark quantity for ab initio calculations. Latest calculations including nonadiabatic QED corrections can now reach a precision of better than 1 MHz for the dissociation energy of molecular hydrogen (M. Puchalski et. al, Phys. Rev. Lett. 121, 073001 (2018)), opening up the prospect of contributing to the more accurate determination of the proton charge radius, if validated experimentally. We present a spectroscopic determination of the ionization and dissociation energies of D$_2$ with an uncertainty below 1~MHz by combining separate measurements of the intervals between the X and GK states, and the GK and high-$n$f Rydberg states with MQDT-assisted extrapolation to the ionization limit. The X-GK interval is measured by Doppler-free two-photon spectroscopy using 178~nm radiation, generated by frequency up-conversion using a KBBF crystal. The laser pulses are generated in a seeded, chirp-compensated, Ti:Sa oscillator-amplifier system where the seed is a Ti:Sa laser at 714~nm locked to a frequency comb. The GK-$n$f Rydberg state intervals are measured in a resonant three-photon excitation scheme, using pulsed VUV and VIS lasers to reach the intermediate GK state and a single-mode continuous-wave (cw) Ti:Sa laser for transitions to $n$f Rydberg states. The frequency of the cw laser was calibrated with a frequency comb referenced to an atomic clock. To reach an accuracy below 1~MHz, AC- and DC-Stark effects, as well as first- and second-order Doppler effects are analyzed and compensated