Speaker
Description
Molecular hydrogen ions (MHIs) represent a class of bound quantum systems with significant potential for advancing our knowledge in multiple scientific domains, including the determination of fundamental constants, test of quantum physics, and the search for new interparticle forces. Furthermore, the comparison of transitions in MHIs and their antimatter counterparts provides an opportunity for novel tests of CPT invariance [1].
Among the various isotopologues of MHIs, in recent years only heteronuclear $\mathrm{HD}^+$ has been a focus of experimental investigation, yielding significant data on its rovibrational transition frequencies as well as its spin frequencies [2,3,4,5]. In particular, our measurements of two rovibrational transitions provide data on the spin structure consistent with the most precise ab initio calculation [4,5].
Expanding the scope of research to include other isotopologues of MHIs is crucial [6], with homonuclear $\mathrm{H}_2^+$ being a valuable choice. However, spectroscopic studies of $\mathrm{H}_2^+$ have historically faced difficulties because of lack of electric-dipole transitions, necessitating the development of alternative spectroscopic approaches. These challenges have prevented the realization of laser spectroscopy of $\mathrm{H}_2^+$ until recently.
We have now succeeded in measuring a rovibrational electric-quadrupole (E2) transition in $\mathrm{H}_2^+$ [7]. While the spectral lines exhibited Doppler broadening, in an additional study we demonstrated the feasibility of Doppler-free E2 spectroscopy, using $\mathrm{HD}^+$ as a test molecule. We achieved unprecedented line resolution of $2.6\times10^{12}$, improving on a previous demonstration by six orders of magnitude [8].
A characterization of Doppler-free transitions in H$_2^+$ at metrological level would be a milestone, as it would lead to a spectroscopically determined electron-proton mass ratio. Therefore, our current efforts are focused on implementing Doppler-free vibrational spectroscopy of $\mathrm{H}_2^+$. We will present up-to-date results and their interpretation at the meeting.
Funding was provided by the European Research Council (ERC) under the EU's Horizon 2020 research and innovation programme (grant agreement No. 786306, ``PREMOL”) and from both the DFG and the state of North-Rhine-Westphalia (Grant Nos. INST-208/774-1 FUGG and INST-208/796-1 FUGG).
[1] E.G. Myers, Phys. Rev. A 98, 010101 (2018).
[2] S. Alighanbari, et al. Nature 581, 152 (2020).
[3] S. Patra, et al. Science 369, 1238 (2020).
[4] I.V. Kortunov, et al. Nat. Phys. 17, 569 (2021).
[5] S. Alighanbari, et al. Nat. Phys. 19, 1263 (2023).
[6] S. Schiller and J.-Ph. Karr, to appear in Phys. Rev. A (2024).
[7] M.R. Schenkel, et al. Nat. Phys. (2024). https://doi.org/10.1038/s41567-023-02320-z
[8] M. Germann, et al. Nat. Phys. 10, 820 (2014).
