MEASUREMENT OF THE N = 2 FINE STRUCTURE IN POSITRONIUM AND POSITRONIUM(N = 2) FORMATION FROM CLEAN METAL SURFACES (EXCITED STATES)

Results of two experiments involving the first excited state of positronium (Ps) are reported. The first experiment involves an attempt to measure the 2('3)S(,1)-2('3)P(,2) fine structure interval in Ps with a precision of about 100 ppm testing the as yet uncalculated radiative corrections to the n = 2 energy levels to order (alpha)('4) Ryd. This precision would be an order of magnitude improvement over the measurement of the same fine structure interval first made by Mills, Berko, and Canter. In this experiment a magnetically guided beam of slow positrons (<100 eV) is focused inside a section of X-band waveguide in which Ps(n = 2) is formed by positron collision with the inside wall of the waveguide. Ps(n = 2) is detected by observing the delayed coincidence between the 2P-1S Lyman-(alpha) decay photon and one of the annihilation photons from the ground state. The 2('3)S(,1)-2('3)P(,2) resonance ((TURN)8625 MHz) is measured by the observed resonant increase in the delayed coincidence rate as a function of the frequency of the applied microwave power. Data obtained in this experiment shows a resonance amplitude near the expected resonant frequency, but about three times smaller than that observed in the first experiment. This small resonance signal competing with a strongly fluctuating background noise made it ultimately infeasible to measure the fine structure interval with the anticipated precision.;In the second experiment, and electrostatically guided slow positron beam of variable energy (0-1 keV) was used to investigate the formation of Ps(n = 2) from clean metal surfaces of a Cu(100) crystal and an annealed W foil under positron bombardment in ultra-high vacuum conditions. Ps(n = 2) was detected using the same delayed coincidence technique as in the fine structure experiment. The fraction of Ps(n = 2) produced per incident positron was measured as a function of incident positron energies in the range of 0 to 1 keV. The results of these experiments show a maximum Ps(n = 2) fraction of (1.28 (+OR-) 0.06) x 10('-3) for Cu(100) and (1.76 (+OR-) 0.22) x 10('-3) for W. These results also show a low energy threshold for Ps(n = 2) formation near 3 eV consistent with the model of Ps(n = 2) formation as a charge capture process at the metal surface by backscattered positrons.