MEASUREMENT OF THE POSITRONIUM 1(3)S(1)-2(3)S(1) INTERVAL BY CONTINUOUS-WAVE 2-PHOTON EXCITATION

Using continuous-wave excitation to eliminate the problems inherent with pulsed laser measurements of nonlinear transitions, we have measured the 1(3)S1-2(3)S1 interval in positronium (Ps) to be 1233607216.4+/-3.2 MHz. The quoted 2.6 ppb (parts per 10(9)) uncertainty is primarily due to the determination of the Ps resonance relative to the Te2 reference line, with a 1.5 ppb contribution from a recent calibration of the Te2 line relative to the hydrogen 1S-2S transition. The uncertainty corresponds to 3.5 x 10(-5) of the alpha2R infinity QED contribution to the 1(3)S1-2(3)S1 interval. Our measurement is sufficiently accurate to provide a test of the as yet uncalculated alpha4R infinity QED corrections.