title: High-precision spectroscopy of the forbidden 2 3s1→2 1p1 transition in quantum degenerate metastable helium.
abstract: We have measured the forbidden 2 (3)S(1)→2 (1)P(1) transition at 887 nm in a quantum degenerate gas of metastable (4)He 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 2 (1)P(1) 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 2 (1)P(1) state of 0.551 (0.004)(stat) ((-0.000)(+0.013))(syst)  ns, which is the most accurate determination to date and in excellent agreement with theory.
authors.affiliation: LaserLaB, Department of Physics and Astronomy, VU University Amsterdam, De Boelelaan 1081, 1081 HV Amsterdam, Netherlands.
authors.forename: R P M J W
authors.initials: RP
authors.lastname: Notermans
authors.affiliation: LaserLaB, Department of Physics and Astronomy, VU University Amsterdam, De Boelelaan 1081, 1081 HV Amsterdam, Netherlands.
authors.forename: W
authors.initials: W
authors.lastname: Vassen