Spectroscopy of excited state positronium

Electric dipole transitions in the microwave range have been induced between the fine-structure levels of positronium in the excited state n = 2. As an indication of the transitions, we used the increase in Lyman-α radiation when the metastable 2 3S1-level is depopulated. The results for the transitions 2 3S1→2 3P0,1,2 are ν0 = 18499.65 ± 1.20 ± 4.00 MHz, ν1 = 13012.42 ± 0.65 ± 1.54 MHz and ν2 = 8624.38 ± 0.54 ± 1.40 MHz. The first error is statistical and the second systematic. The precision of the present measurement has improved by a factor of 3, compared to previous data. Recent bound state QED-calculations have been extended to the order R∞α4 ln α-1. The not yet completely calculated order R∞α4 is estimated to contribute less than 1 MHz. Our experimental results are in good agreement with theory. By applying a weak magnetic field, we were able to observe the transition 2 3S1→2 1P1 which is strictly forbidden by C-invariance in zero field. Our result, corrected for Zeeman- and motional Stark-effect, is ν3 11180.0±5.0±4.0 MHz. An upper limit for the C-violating matrix element of vertical stroke left angle 2 1P1vertical stroke Hcvertical stroke 2 3P1 right angle vertical stroke <76 MHz could be deduced. Our experiment used moderated slow positrons from the bremsstrahlung and pair production of a pulsed electron linear accelerator (TEPOS facility at the university of Giessen). (orig.)