High-Precision Spectroscopy of the Forbidden 2(3)S(1) -> 2(1)P(1) Transition in Quantum Degenerate Metastable Helium

We have measured the forbidden 2S13→2P11 transition at 887 nm in a quantum degenerate gas of metastable He4 atoms confined in an optical dipole trap. The determined transition frequency is 338 133 594.4 (0.5) MHz, from which we obtain an ionization energy of the 2P11 state of 814 709 148.6 (0.5) MHz. This ionization energy is in disagreement by >3σ with the most accurate quantum electrodynamics calculations available. Our measurements also provide a new determination of the lifetime of the 2P11 state of 0.551(0.004)stat(-0. 000+0.013)systns, which is the most accurate determination to date and in excellent agreement with theory. © 2014 American Physical Society.