Positron Collisions With Hydrogen

Positron scattering from atoms is one of the basic issues in the area of atomicphysics. In recent years, with the development of the experimental technique and theimprovement of the theoretical models, study of positron collisions with atoms in vac-uum and plasmas is receiving increasing attention. Positronium formation is one of theunique phenomenon in the process of positron scattering from atoms, which is a clas-sical two-center nature of the problem. The efect of the second center associated withpositronium formation has a significant impact on the whole scattering process. Hence,how to describe it and take it into account of explicitly is the necessity and a challengeto treat the problem of positron scattering from atoms in the low and intermediate energyregion. There is also a fundamental issue to the theorists due to an enhanced long-rangepolarization potential in the case of the larger polarizability of the atomic target or thelow-energy incident positron. In the present work, the main aim is to explore the in-fluence of the positronium formation and the polarization of the target in the scatteringprocess by using the momentum-space coupled-channel optical method. In this model,the positronium formation channels and the ionization continuum are described by anequivalent local polarization potential. We choose three scattering systems to study, in-volving the positron-metastable2s state hydrogen scattering system, positron-excited2pstate hydrogen scattering system and positron-hydrogen collisions in Debye plasma.Momentum-space coupled-channel optical method is developed to investigated the2s-3s excitation for positron-metastable hydrogen collisions, and the present results weobtained are in good agreement with the available theoretical calculations. The compar-ison of the results from the three models shows that the polarization potential plays asignificant role in this excitation process. The polarization potential produces significantefects upon the scattering process at lower energies and the influence is decreasing as thepositron incident energy is increasing. The efect is more obvious at middle and backwardscattering angles, etc.As for the unnatural parity resonance states in the positron-excited2p state hydrogenscattering system, momentum-space coupled-channels optical method is used to treat thisissue in positron-H(2p) scattering. The resonance parameters of unnatural parity reso-nance states P~e，D~owith angular momenta L=1,2are calculated. The present results are in good agreement with the available theoretical calculations. In addition, some new reso-nances are identified. Positronium formation plays an important role in forming unnaturalparity resonance states, which is a result of electron in its2p positronium formation stateinteraction with the positively charged and infinitely massive proton.Taking account of the influence of the Debye screening on the interaction betweenthe particles, a screening approximation model is set up to treat the positronium forma-tion in positron-hydrogen collisions in Debye plasma environment. Positronium forma-tion cross sections are calculated for various Debye lengths. Ps(n=2) and total Ps(n=1+2)formation cross sections are displayed for the first time in the literature. Taking accountof the influence of the plasmas environment on the structure of positronium bound statesto calculate the positronium cross sections has not been studied so far. The screenedCoulomb potential is found to significantly afect the positronium formation process. In-teresting features which are similar to those in positron-alkali atom and positron-He (23S)collisions in vacuum are observed with the reduction of Debye length, such as the en-hancement of the Ps formation cross sections at low energies and the appearance of thelocal minima in the total Ps(n=1+2) formation cross sections. The efect of the change ofthe structure of the positronium bound states under the screening is reported for the firsttime, which makes the cross section to decrease in the low energy. But this efect doesnot influence the whole increasing trend by the Coulomb screening.At last, we do some preliminary researches. In view of the Debye screening’s signif-icant impact on positronium, we plan to develop CCO method to calculate various crosssections for positron-atom scattering in Debye plasmas. The influence of the ionizationcontinuum and positronium formation is described by a polarization potential. The reso-nance for positron-atom collision in Debye plasmas is also planned to study.