High-precision measurement of the (6p²)³P₁ → (6p7s)³P₀ electric dipole transition amplitude in ²⁰⁸Pb

Improved ab initio wavefunction models for heavy, multivalence atomic systems have direct relevance to tests of atom-based symmetry violation and other fundamental physics searches. Unlike alkali systems, very few benchmark measurements of group IV atoms exist at the ~1% level of accuracy necessary to guide and further refine the theory. Transition amplitude measurements are an excellent way to test the accuracy of these ab initio wave function calculations, and our 2019 measurement of the strongly forbidden (6p²)³P₀ → (6p²)³P₂ electric quadrupole transition amplitude in ²⁰⁸Pb [1] showed excellent agreement with the predicted value of the Safronova group. Using a novel, broadband Faraday rotation spectroscopy technique with microradian accuracy, we have precisely measured the ratio of the (6p²)³P₁ → (6p7s)³P₀ to the (6p²)³P₀ → (6p²)³P₁ (E1/M1) transition amplitudes in lead. A crucial feature of this work is the ability to account for the different thermal populations of the (6p²)³P₀ and (6p²)³P₁ levels, which is achieved with a thermally-stable vacuum furnace monitored with an array of precision s-type thermocouples. The success of this technique opens up the possibility for a new series of transition amplitude measurements in Pb and other multivalence systems for further tests of ab initio theoretical models of such atomic systems.

[1] – D.L. Maser et al., Phys. Rev. A 100, 052506 (2019)