We report the measurement of the w (1s2p 1P1 ! 1s2 1S0) resonance line transition energy in helium-like titanium. Our result, 4749.85(7) eV, deviates from the most recent ab initio prediction by three times our experimental uncertainty and by many times more than the previously estimated uncertainty in the prediction. This measurement is one of few sensitive to two-electron quantum electrodynamics (QED) contributions. Systematic errors such as Doppler shifts are minimised in our experiment by trapping and stripping Ti atoms in an Electron Beam Ion Trap (EBIT) and by applying absolute wavelength standards to calibrate the dispersion function of a curved-crystal spectrometer. The relative uncertainty of the present result (15 ppm) at the strategic location of Z = 22 permits a leveraged assessment of the most recent theoretical computations of three-body QED in atomic systems, revealing that when the values for all reported w-line transitions in two- electron ions for Z > 15 are considered, a general divergence becomes apparent around Z = 20 and increases approximately as Z3.
Citation: Physical Review Letters
Pub Type: Journals
quantum electrodynamics, highly-charged ions, helium-like titanium, electron beam ion trap, comparison of theory and experiment