Molecular frequency metrology: ultra-high precision spectroscopy of the rotational transition of HD+

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Precision spectroscopy of molecular hydrogen ions provides an approach for determining several fundamental constants of atomic physics by alternative spectroscopic means. Until recently, the spectroscopy of these molecular ions was limited by Doppler broadening, which also limited the achievable precision. In the group of the applicant a new rotational spectroscopy technique was recently developed, which achieves the Lamb-Dicke regime. The spectral resolution was a factor 40 below the Doppler broadening. With this breakthrough important new perspectives open up in precision spectroscopy. In this project, the technique shall be developed further both in terms of resolution (by a factor 10) and in terms of accuracy (by a factor 100). Several experimental extensions shall be tested and characterized.As outcome, the project shall determine one rotational frequency and two hyperfine splittings in the molecule HD+, with fractional uncertainty 2×10^-11 relative to the rotational frequency. In parallel, the cooperation partners will develop the ab initio theory of the spin-dependent energy contributions further. By comparing the experimental and theoretical results we intend to test the ab initio quantum theory with a fractional uncertainty of 3×10^-11, 30 times more accurately than so far.The results will also be used to determine the ratio of electron mass to reduced proton-deuteron-mass, me/mp + me/md. The goal uncertainty, 3×10^-11, will allow to independently confirm the most precise value, obtained in Penning traps. If agreement is found, both values together can lead to an improved value for the CODATA fundamental constants compilation.