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-a radiation when the metastable 2(3)S(1)-level is depopulated. The results for the transitions 2(3)S(1) --> 2(3)P(0,1,2) are v(0) = 18499.65 +/- 1.20 +/- 4.00 MHz, v(1) = 13012.42 +/- 0.65 +/- 1.54 MHz and v(2) = 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(infinity)alpha(4) In alpha(-1). The not yet completely calculated order R(infinity)alpha(4) is estimated to contribute less than 1 MHz. Our experimental results are in good agreement with theory. By applying a weakmagnetic field, we were able to observe the transition 2(3)S(1) --> 2(1)P(1) which is strictly forbidden by C-invariance in zero field. Our result, corrected for Zeeman- and motional Stark-effect, is v(3) = 11180.0 +/- 5.0 +/- 4.0 MHz. An upper limit for the C-violating matrix element of \[2(1)P(1)\H-not subset of\2(3)P(1)]\<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).