束缚电子偶素交换效应的形式计算
Formal calculation of exchange effects on confined positronium
摘要：
正电子素原子（Ps）被广泛用作表征非金属材料中的空隙或空位的探针，其中Ps湮灭寿命被拾取强烈修改，这取决于捕获腔的大小和适当的外部电子密度。这些材料的特性和Ps湮灭寿命之间的连接通常是基于模型，不考虑完整的电子不可逆性的要求，这必须考虑到一个正确的描述拾取湮灭过程。在这份报告中，我们提供了一个正式的理论框架，其中Ps和周围的电子之间的交换效应以自然的方式引入，给出了一个清晰和通用的图片的各种贡献的Ps拾取湮灭。此外，我们的研究结果提供了一个简单的解释，降低的接触密度（在正电子位置的Ps-电子密度）作为电子不可压缩性的直接后果，在与以前的解释的基础上的Ps波函数的空间变形的差异。计算进行内的“对称性适应微扰理论”的方法，并与实验数据的Ps寿命的一些聚合物和分子固体的结果进行了比较。最后，引入适当的近似，我们也恢复早期的建模，并给出了一个简单的解释Ps性能在亚纳米空隙。
英文摘要：
Positronium atoms (Ps) are widely used as a probe to characterize voids or vacancies in non-metallic materials, where Ps annihilation lifetime is strongly modified by pickoff, depending on the size of the trapping cavity and on the appropriate outer electron density. The connection between these material characteristics and Ps annihilation lifetimes is usually based on models that do not consider the requirements of full electron indistinguishability, which must be taken into account for a correct description of pickoff annihilation processes. In this report we provide a formal theoretical framework in which exchange effects between Ps and surrounding electrons are introduced in a natural way, giving a clear and versatile picture of the various contributions to the Ps pickoff annihilation. Moreover, our results provide a simple explanation of the lowering of the contact density (the Ps-electron density at the positron position) as a direct consequence of the electrons indistinguishability, at variance with previous interpretation based on spatial deformations of Ps wavefunction. Calculations are performed within the "symmetry adapted perturbation theory" approach, and the results are compared with experimental data on Ps lifetimes of some polymers and molecular solids. Finally, introducing suitable approximations, we also recover early modeling and give a simple interpretation of Ps properties in subnanometric voids.
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