Resonances in positronium-rubidium and positronium-cesium scattering

Scattering of ortho positronium (Ps) by cesium and rubidium atoms has been investigated employing a three-Ps-state coupled-channel model with Ps(1s,2s ,2p) states using a time-reversal-symmetric regularized electron-exchange m odel potential. We find a narrow S-wave singlet resonance at 5.057 eV of wi dth 0.003 eV in the Ps-Rb system and at 5.067 eV of width 0.003 eV in the P s-Cs system. Singlet P-wave resonances in both systems are found at 5.3 eV of width 0.4 eV. Singlet D-wave structures are found at 5.4 eV in both syst ems. The pronounced P- and D-wave resonances in these systems lead to easil y detectable local minima in the low-energy elastic cross sections. We also report results for elastic and Ps-excitation cross sections for Pa scatter ing by Rb and Cs. (C) 2001 Elsevier Science B.V. All rights reserved.