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arXiv 1201.3591physics.atom-ph
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正电子-原子相互作用对分子湮没γ谱的影响

Effect of positron-atom interactions on the annihilation gamma spectra of molecules

D. G. Green, S. Saha, F. Wang, G. F. Gribakin, C. M. Surko

摘要:

本文计算了甲烷及其氟代物、乙烷、丙烷、丁烷和苯等分子的正电子湮没γ谱。湮没γ谱反映了电子-正电子对在湮没瞬间的动量分布。从单个分子轨道的伽马光谱的贡献是从使用现代计算量子化学密度泛函理论工具计算的电子动量密度。计算,在其最简单的形式,有效地处理低能量(热化,室温)正电子作为一个平面波,并给出湮没伽马光谱比实验宽约40%,虽然主要的化学趋势再现。我们表明,这种有效的“窄化”的实验光谱是由于作用的分子势的正电子,主要是由于正电子从核排斥。它导致抑制小的正电子-核分离的贡献,其中电子动量大。为了研究核排斥效应以及短程电子-正电子和正电子-分子关联的影响,采用了原子轨道对分子轨道的线性组合描述。它有利于纳入修正因子,可以从原子多体理论计算,并考虑排斥和相关性。它们的列入在计算中给出的γ谱线宽,更好地符合实验。此外,它示出的电子动量密度的有效失真,当它是通过正电子湮没γ谱观察到的,可以近似由一个相对简单的比例因子。

英文摘要:

Calculations of gamma spectra for positron annihilation on a selection of molecules, including methane and its fluoro-substitutes, ethane, propane, butane and benzene are presented. The annihilation gamma spectra characterise the momentum distribution of the electron-positron pair at the instant of annihilation. The contribution to the gamma spectra from individual molecular orbitals is obtained from electron momentum densities calculated using modern computational quantum chemistry density functional theory tools. The calculation, in its simplest form, effectively treats the low-energy (thermalised, room-temperature) positron as a plane wave and gives annihilation gamma spectra that are about 40% broader than experiment, although the main chemical trends are reproduced. We show that this effective "narrowing" of the experimental spectra is due to the action of the molecular potential on the positron, chiefly, due to the positron repulsion from the nuclei. It leads to a suppression of the contribution of small positron-nuclear separations where the electron momentum is large. To investigate the effect of the nuclear repulsion, as well as that of short-range electron-positron and positron-molecule correlations, a linear combination of atomic orbital description of the molecular orbitals is employed. It facilitates the incorporation of correction factors which can be calculated from atomic many-body theory and account for the repulsion and correlations. Their inclusion in the calculation gives gamma spectrum linewidths that are in much better agreement with experiment. Furthermore, it is shown that the effective distortion of the electron momentum density, when it is observed through positron annihilation gamma spectra, can be approximated by a relatively simple scaling factor.

机构、总结、中文摘要均由AI分析而来,可能存在错误,请以论文正文为准。

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