正电子与氢原子碰撞的理论研究
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摘要
正电子与原子散射是原子物理领域的一个基本问题。近年来，随着实验技术的不断进步以及理论模型的不断发展，真空以及等离子体环境中的正电子与原子的散射问题受到越来越广泛的关注。正负电子偶素的形成是正电子与原子散射过程中的特有现象。这个过程是一个典型的两个中心的碰撞问题，由正负电子偶素引入的第二个中心对整个碰撞过程的影响很大。所以如何描述它，并且在中低入射能量范围内把它对整个散射过程中的影响考虑得更周详是处理正电子与原子散射问题必须要考虑的问题，也是一直困扰理论工作者的一个难题。而在散射过程中，当入射正电子以较低能量入射时，或者靶原子的偶极极化率很大时，入射正电子对靶原子的极化作用是很强的，这个极化相互作用对整个散射过程的影响也很大，也是理论工作者必须要考虑的一个问题。本文的核心问题就是要来探讨正负电子偶素形成以及入射正电子对靶的极化效应对整个散射过程的影响。论文所使用的方法是动量空间耦合通道光学势方法。在计算中，通过一个等价局域的极化势来描述正负电子偶素形成以及电离连续态的作用。论文选择三个散射体系来进行研究：正电子与亚稳态2s氢原子散射体系，正电子与激发态2p氢原子散射体系以及德拜等离子体环境中正电子与氢原子散射体系。
本论文将动量空间耦合通道光学势方法推广到对正电子与亚稳态2s氢原子散射过程中的2s-3s激发过程的研究中，得到了与已有理论结果符合很好的结果。通过三种理论模型结果的比较，探讨了正负电子偶素形成极化势以及电离极化势在这个激发过程中的重要作用：极化势在低能入射时对散射过程的影响很大，这种影响随着能量的增加而减弱；对于中度以及大角度散射区域的影响较明显等等。
对于正电子与激发态2p氢原子散射体系中的非正常宇称共振态问题，论文中应用动量空间耦合通道光学势方法对其进行了研究。计算了正电子与2p激发态氢原子散射过程中的角动量L=1，2的非正常宇称共振态P~e，D~o的共振参数。不仅验证了之前其它理论方法所预言的共振位置，还发现了一些新的共振位置。讨论了正负电子偶素形成对于非正常宇称共振态的产生所起到的重要作用：正负电子偶素激发态与带正电，质量近似无穷大的质子间相互作用而形成一个吸引的偶极极化势会形成非正常宇称共振态。
考虑到德拜屏蔽势对粒子间相互作用的修正，论文建立了一个在德拜等离子体环境下处理正电子与氢原子散射过程中正负电子偶素形成通道的屏蔽近似模型。计算了不同德拜长度下的正负电子偶素形成截面，首次给出了激发态正负电子偶素形成截面以及总截面，首次给出了考虑正负电子偶素束缚态结构受屏蔽影响变化的基态，激发态以及总的正负电子偶素形成截面。发现德拜屏蔽势对于正负电子偶素形成过程影响很大，一些真空中在正电子与氦原子，碱金属原子散射中的现象出现，比如低能时正负电子偶素形成截面值的增大和总的正负电子偶素形成截面中最小值位置的出现。率先给出结论：在低能情况下，考虑正负电子偶素束缚态结构的变化会使截面值变小，但并不会影响屏蔽效应使截面增加的总的趋势。
最后，对接下来的工作进行了展望：鉴于屏蔽效应对正负电子偶素形成的重要影响，对德拜等离子体环境中正电子与原子碰撞的各种截面进行计算并讨论屏蔽的影响将会很有意义。计划还将对德拜等离子体环境中正电子与原子碰撞中的共振现象进行研究。
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.
本论文将动量空间耦合通道光学势方法推广到对正电子与亚稳态2s氢原子散射过程中的2s-3s激发过程的研究中，得到了与已有理论结果符合很好的结果。通过三种理论模型结果的比较，探讨了正负电子偶素形成极化势以及电离极化势在这个激发过程中的重要作用：极化势在低能入射时对散射过程的影响很大，这种影响随着能量的增加而减弱；对于中度以及大角度散射区域的影响较明显等等。
对于正电子与激发态2p氢原子散射体系中的非正常宇称共振态问题，论文中应用动量空间耦合通道光学势方法对其进行了研究。计算了正电子与2p激发态氢原子散射过程中的角动量L=1，2的非正常宇称共振态P~e，D~o的共振参数。不仅验证了之前其它理论方法所预言的共振位置，还发现了一些新的共振位置。讨论了正负电子偶素形成对于非正常宇称共振态的产生所起到的重要作用：正负电子偶素激发态与带正电，质量近似无穷大的质子间相互作用而形成一个吸引的偶极极化势会形成非正常宇称共振态。
考虑到德拜屏蔽势对粒子间相互作用的修正，论文建立了一个在德拜等离子体环境下处理正电子与氢原子散射过程中正负电子偶素形成通道的屏蔽近似模型。计算了不同德拜长度下的正负电子偶素形成截面，首次给出了激发态正负电子偶素形成截面以及总截面，首次给出了考虑正负电子偶素束缚态结构受屏蔽影响变化的基态，激发态以及总的正负电子偶素形成截面。发现德拜屏蔽势对于正负电子偶素形成过程影响很大，一些真空中在正电子与氦原子，碱金属原子散射中的现象出现，比如低能时正负电子偶素形成截面值的增大和总的正负电子偶素形成截面中最小值位置的出现。率先给出结论：在低能情况下，考虑正负电子偶素束缚态结构的变化会使截面值变小，但并不会影响屏蔽效应使截面增加的总的趋势。
最后，对接下来的工作进行了展望：鉴于屏蔽效应对正负电子偶素形成的重要影响，对德拜等离子体环境中正电子与原子碰撞的各种截面进行计算并讨论屏蔽的影响将会很有意义。计划还将对德拜等离子体环境中正电子与原子碰撞中的共振现象进行研究。
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.
引文
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