The most precise experimental value for the dissociation energy of molecular hydrogen has been measured by an international team led by Wim Ubachs at VU Amsterdam and Frédéric Merkt at the Swiss Federal Institute of Technology (ETH Zurich). The measurement delivers an order of magnitude improvement over the previous best and is a significant deviation from the most recent theoretical calculations. Resolving this discrepancy could lead to improvements in molecular quantum theory and could result in a better measured value for the proton radius.
The hydrogen dissociation energy is the amount of energy required to separate the two atoms in a hydrogen molecule. To measure the value, physicists have previously broken down the process into a thermodynamic cycle of intermediate transitions where the molecule is first ionized, and then split into a proton and a hydrogen atom. An electron is then added to the proton, forming two neutral hydrogen atoms. Physicists then calculate the molecular dissociation energy by summing the excitation energies for each of these transitions.
The dissociation energy can also be calculated theoretically; accounting for relativistic and quantum-electrodynamic effects. Comparing the theoretical result with experimental values has previously allowed for stringent tests of quantum theory. Experiment and theory have agreed with each other in previous studies, even as the accuracy of experiments improved.
New transition sequence
To improve accuracy further, Ubachs, Merkt and colleagues obtained a more precise value for the ionization energy – which is the stage of the dissociation process with the largest experimental uncertainty. They achieved this using a new sequence of transitions, which involved measuring a high-energy transition with vacuum-ultraviolet light at VU, then a lower-energy transition with an ultraprecise continuous-wave near-infrared laser spectroscopy at ETH. Combining the results, the researchers calculated a dissociation energy with a relative uncertainty of under 10⁻⁹.
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The result is in line with previous experimental results, yet it appears to deviate from the most recent theoretical calculations by more than three times the experimental uncertainty. Writing in Physical Review Letters, the physicists note that adjustments may be needed to how relativistic electron motion and quantum vacuum fluctuations are treated in calculations. Alternatively, the mismatch could point a more fundamental problem in molecular quantum theory.
Resolving the discrepancy could offer a new way to determine the radius of a proton and could also lead to better measurements of the proton-to-electron mass ratio. Precise measurements of these quantities have the potential to reveal news physics.
