New results are presented for Ps(1s) scattering by H(1s), He(1 1 S) and Li(2s). Calculations have been performed in a coupled state framework, usually employing pseudostates, and allowing for excitation of both the Ps and the atom. In the Ps(1s)-H(1s) calculations the H − formation channel has also been included using a highly accurate H − wave function. Resonances resulting from unstable states in which the positron orbits H − have been calculated and analysed. The new Ps(1s)-He(1 1 S) calculations still fail to resolve existing discrepancies between theory and experiment at very low energies. The possible importance of the Ps − formation channel in all three collision systems is discussed.

A measurement method of absolute hydrogen atom density in plasmas by (2+1) -photon laser-induced fluorescence spectroscopy Rev. Sci. Instrum. 72, 2298 (2001; 10.1063/1.1367356 Coherent states composed of Stark eigenfunctions of the hydrogen atom Am.A computational scheme has been developed and used to investigate the influence of the plasma environments on modified atomic autoionization for isolated atoms/ions by using the complex coordinate rotation method which is proved to be a very simple and powerful tool to analyze the position and the width of a resonance. The Debye screening potential is employed to describe the effects of the plasma environments. Stark shifts and widths on the ground state of hydrogen are reported for field strength up to F = 0.12 a.u. Slater-type basis wave functions are used to describe the system and angular-momentum states up to L = 11 are included when the external electric field is turned on. Converged results are obtained by using different maximum angular-momentum states. The modified autoionization for various Debye lengths ranging from infinite to a small value of 0.86 are reported. It has been observed that for a given temperature and under the influence of a given external electric field, the resonance energy and the autoionization width increase for increasing electron density in the plasma. A discussion on the physical implication of our results is made.