SPECTROSCOPY OF EXCITED-STATE POSITRONIUM

Electric dipole transitions in the microwave range have been induced b etween the fine-structure levels of positronium in the excited state n = 2. As an indication of the transitions, we used the increase in Lym an-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 an d the second systematic. The precision of the present measurement has improved by a factor of 3, compared to previous data. Recent bound sta te QED-calculations have been extended to the order R(infinity)alpha(4 ) In alpha(-1). The not yet completely calculated order R(infinity)alp ha(4) is estimated to contribute less than 1 MHz. Our experimental res ults are in good agreement with theory. By applying a weakmagnetic fie ld, 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, corr ected 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 experi ment used moderated slow positrons from the bremsstrahlung and pair pr oduction of a pulsed electron linear accelerator (TEPOS facility at th e university of Giessen).