| In recent years, more and more attention has been concentrated onpositron scattered by atoms. Great successes have been achieved bothexperimentally and theoretically, a number of theoretical calculations havegiven successful predictions that cover almost all aspects of thepositron–hydrogen system (Igarashi and Toshima 1994, Kernoghan et al1995, 1996, Mitroy and Stelbovics 1994, Ratnavelu et al 1996, Kuang andGien 1997, Kadyrov A S and Bray I 2002, Zhou et al in press). However, asto the many-electron atoms scattering with positron, there remain obviousdiscrepancies between the experimental measurements and the theoreticalpredictions, for the various reaction within the complex collision system.Nowadays, with the remarkably improvement on the experimentaltechnology, corresponding descriptions from the theory are expectedurgently, which comes into being the great challenge to our theoreticalworkers.In the past 20 years, some theoretical models have been developed tothe positron-atom collision, wherein the common models include thedistorted-wave Born approximation method (DWBA), the R-matrix method,the convergent close coupling method (CCC) and the coupled-channelsoptical potential method (CCO). The DWBA method are more valid forthe collision in the high energy range; The R-matrix method, which wasproposed by Burke,Noble and Scott and then developed by the Belfastgroup, is more suitable for the collision in the low energy range and whenthe incident energy reach to the intermediate range, a series ofpseudosturcture are caused due to the use of the large number ofpseudostates; The CCC method has advantage over the simple target atom,for example,the H and He , and there are some difficulty in treating themore complex target atoms. So it is necessary to develop a more commontheory model, which can descript the positron collision with themany-electrons atom explicitly.The CCO method, developed in 1980's by McCarthy et al, has beenwidely applied to the electron scattered by various atoms and molecules andhas achieved great success. In the recent year, Prof Zhou Ya-jun et al havesuccessfully developed the CCO method to the positron collision with atomH . In this paper, we propose a complex, local polarization to represent thepositronium formation (Ps) arrangement channel and the ionizationcontinuum channel, and develop the CCO method to the positron collisionwith the complex atoms.In the present paper, we investigate the positron-Li collision in the lowand intermediate energy, and calculated the Ps, ionization, elastic,excitation and the total cross section. In the intermediate energy collision,infinite reaction channels are open including the Ps and ionization channels,which must be considered effectively in order to descript the collisionprocess explicitly. In the low intermediate energy range, the ionization andPs channels play a prominent role in the positron-Li collision and must beconsidered carefully. We developed a complex, local polarization potentialrepresenting the Ps and ionization effects and add it to the polarizationpotential to the first order coupled potential, then through the CCO method,we can consider all the reaction channels effectively. To test the validity ofthe polarization potential model and the developed CCO method, wecalculate the important channels cross sections including the Ps, ionization,elastic, excitation and the total cross section. In this paper, we have done the following calculation: First: By using the obtained polarization model, we calculated the Psand ionization cross section of the positron-Li collision in the low andintermediate energy, and compared the results with the latest experimentalmeasurement and the recent theoretical calculations. We found that thepresent Ps results agree quite well with the latest experiment and also inaccord with the recent R-matrix and CCC calculations. For the ionizationcross section, though there are no experimental results available, ourcalculations agree with the explicit DWBA calculation perfectly. Thesuccessful predictions of the Ps and ionization cross sections validate ourpolarization potential.Secondly: we add the polarization potential to the first order coupledpotential and calculate the elastic, 2s-2p excitation cross section in the lowand intermediate energy positron-Li collision, furthermore, we calculatedtotal cross section considering or omitting the Ps and ionization effectrespectively. We found that, the effect of the Ps-formation and theionization on the total cross section is significant at lower energies and thiseffect is not negligible up to 30eV. For there are no experiments data aboutthe elastic, excitation and total cross section in the positron-Li collision, wejust compare our calculations with the available theoretical results. Wefound that, out results accord well with the explicit R-matrix calculationson the whole, and in the more high incident energy range, our resultsconverge to the other theoretical predictions. Through the theoretical calculation results, we could obtain thefollowing conclusions: 1. The complex, local polarization potential model successful predicted the Ps cross section in low energy positron-Li collision and the ionization cross section in the intermediate energy positron-Li collision, which validate the polarization model. In the low energy range, the Ps channel plays a prominent role in the positron-Li atom collision and this effect is not negligible even in the intermediate energy range. In the intermediate energy range, the ionizatin channel is important and must be carefully considered in the positron-Li collision. 2. We add the polarization potential to the CCO method formalism and successfully predict the elastic, 2s-2p excitation and the total... |
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