正负电子被汞原子非弹性散射的光学势模型方法计算

文中运用先学势模型计算了正负电子被重原子汞散射的总截面（能量０．１ｅＶ－４００ｅＶ），与实验进行了比较，并对理论值与实验值的差异进行了讨论，证明了光学势模型对重原子也是适用的。     

8—9.5 keV正电子致Ti的K壳层电离截面的实验研究

低能正电子碰撞原子内壳层电离截面的实验数据目前还很缺乏,从而影响了对近年来发展的各相关理论模型的检验,限制了慢正电子束流技术在诸多领域中的应用.本文采用慢正电子束流装置产生的8—9.5 keV正电子束碰撞纯厚Ti靶,利用硅漂移探测器(SDD)收集正电子碰撞Ti靶产生的X射线,同时采用高纯锗探测器在线获得与靶碰撞的入射正电子数,从而得到Ti的K壳层实验产额,并基于蒙特卡罗模拟程序PENELOPE获得模拟产额.将实验产额分别与内壳层电离截面数据库采用经典光学数据模型(ODM)和扭曲波玻恩近似理论模型(DWBA)的蒙特卡罗模拟产额进行对比,发现基于ODM理论模型的模拟产额与实验值有较大的偏差,基于DWBA理论模型的模拟产额与实验结果符合较好.根据实验产额和基于DWBA理论模型的模拟产额的比较结果,对蒙特卡罗模拟程序使用的DWBA理论模型数据库进行修正后再进行模拟和比较,从而得到可靠的8—9.5 keV正电子致Ti原子K壳层电离截面数据.     

Elastic scattering of slow positrons on atoms

The results of calculations of the elastic scattering cross section of positrons on noble gas and alkali atoms are presented. The calculations are performed within the one-electron Hartree-Fock approximation with multielectron correlations in the so-called random phase approximation with exchange taken into account. Virtual positronium formation is taken into account and proved to be very important. Arguments are presented that the positron polarization potential is repulsive for alkali atoms. The results obtained are in a reasonable agreement with experiment and with some previously reported calculations.     

Total cross section prediction of the collisions of positrons and electrons with alkali atoms using Gradient Tree Boosting

A relatively new computational technique, namely gradient tree boosting (GTB), is presented for modeling the total cross sections of the scattering of positrons and electrons by alkali atoms in the low and intermediate energy regions. The calculations have been performed in the framework of gradient tree boosting (GTB). The GTB has been running based on the experimental data of the total collisional cross sections to produce the total cross sections for each alkali atom as a function of the incident energy of the projectile as well as the atomic number and the static dipole polarizability of the atom. Moreover our GTB model is used to predict the experimental data for total collisional cross sections that are not used in the training session. The calculated and predicted total collisional cross sections are compared with the experimental data. We find that the GTB technique shows a good match to the experimental data. To our knowledge, this is the first application of the GTB technique to the data of positron and electron collisions with alkali atoms at low and intermediate energies.     

Coupled-channel calculations of positron-atom scattering at intermediate energies

We present the cross sections for positron collisions with the alkali atoms Li, Na and K in the close-coupling approximation within the framework of a single-active-electron model. Our target basis sets are represented by Li(2s, 3s, 2p, 3p), Na(3s, 4s, 3p, 4p), and K(4s, 5s, 4p, 5p) respectively. The effects of coupling to the lowest lying Ps channels have been investigated by augmenting the target basis sets with the Ps(1s, 2s, 2p) eigenstates. The calculations have been made for impact energies below 50 eV. The interaction between the positron and the alkali target atom is represented by a model potential obtained using a single-active-electron approximation. The effect on elastic scattering and excitation due to the coupling between the direct and the Ps formation channels is found to be of great significance at low impact energies. The case of K is of particular interest. The inclusion of the Ps channels quantitatively reproduces the maximum in the total cross section at about 6 eV as reported in a recent experiment by Parikh et al. We find that the calculated total cross sections for Na and K are in good agreement with the experimental data of Parikh et al. and Kwan et al. respectively in the energy range of 2–50 eV provided allowance is made for the incomplete elastic discrimination in the experiment. The Ps formation reduces the K(4s4p) cross section at all energies below 30 eV and a reduction is also seen in the elastic cross section for energies below about 7 eV. Similar effects are observed in the case of Li. In the case of Na for impact energies between 2 and 20 eV Ps formation significantly reduces the Na(3s3p) excitation cross section, while the elastic scattering cross section increases. The calculated total Ps formation cross sections for Na and K are in good agreement with the preliminary experimental data of Kwan, Stein and co-workers.     

Structures in positron–atom and molecule total cross sections

Resonances in positron scattering from several atomic and molecular species are predicted by theoretical models. However, only positron–molecule binding was experimentally confirmed so far. Sharp structures in positron total cross sections were also measured; however, their detection is still in a very preliminary stage owing to instrumental limitations. Here we briefly review the present knowledge about structures in low-energy positron scattering. Based on further measurements and re-analysis of the experimental data collected at the University of Trento, we also present evidence of sharp structures in the total cross sections for several atomic and molecular targets. We find that the behaviour of some of those features can be described using the equation of a Fano resonance line-shape, despite the fact that an actual resonant scattering process might not be involved. Possible mechanisms that can give rise to such structures still need to be identified. Further independent experimental work using high-resolution spectrometers is needed in order to confirm the existence of structures in positron scattering cross sections.     

Logarithmically accurate total cross sections of fast electron scattering by atoms

A simple analytical formula for the total integrated cross section of fast electron scattering by atoms is obtained. This formula, being logarithmically accurate, is derived from the approximate rule of sum that does not take into account exchange effects in the atomic wave function (Hartree approximation). It is shown that exchange effects can be neglected in that scattered electron energy range where the first Born approximation is valid. The results of calculation for He, Ne, and Ar atoms agree well with experimental data.     

Positron scattering and ionization of neon atoms—theoretical investigations

Although positron scattering with inert gas atoms has been studied in theory as well as in experiment, there are discrepancies. The present work reports all the major total cross sections of e+-neon scattering at incident energies above ionization threshold, originating from a complex potential formalism. Elastic and cumulative inelastic scatterings are treated in the complex spherical e+-atom potential. Our total inelastic cross section includes positronium formation together with ionization and excitation channels in Ne. Because of the Ps formation channel it is difficult to separate out ionization cross sections from the total inelastic cross sections. An approximate method similar to electron-atom scattering has been applied to bifurcate ionization and cumulative excitation cross sections at energies from threshold to 2000 eV. Comparisons of present results with available data are made. An important outcome of this work is the relative contribution of different scattering processes, which we have shown by a bar-chart at the ionization peak.     

一种考虑几何屏蔽效应的计算“电子-分子”散射总截面的可加性规则修正方法

在考虑分子内原子间的几何屏蔽效应随电子入射能量变化的基础上, 提出了一种能够在中、高能区准确计算“电子-分子”散射总截面的可加性规则修正方法. 利用这一修正后的可加性规则并使用“电子-C, H, O, N原子”散射总截面的实验数据, 在50—5000 eV内计算了电子被NO, N₂O, NO₂和C₂H₆分子散射的总截面, 且将计算结果与实验结果及其他理论结果进行了比较. 结果表明, 利用这一方法修正过的可加性规则进行计 关键词： 电子散射 可加性规则 总截面 几何屏蔽效应     

Photoionization of the outer electrons in noble gas endohedral atoms

We suggest a prominent modification of the outer shell photoionization cross section in noble gas (NG) endohedral atoms NG@C _n under the action of the electron shell of fullerene C _n . This shell leads to two important effects: a strong enhancement of the cross section due to fullerene shell polarization under the action of the incoming electromagnetic wave and to prominent oscillation of this cross section due to the reflection of a photoelectron from the NG by the fullerene shell. Both factors lead to powerful maxima in the outer shell ionization cross sections of NG@C _n , which we call giant endohedral resonances. The oscillator strength reaches a very large value in the atomic scale, 25. We consider atoms of all noble gases except He. The polarization of the fullerene shell is expressed in terms of the total photoabsorption cross section of the fullerene. The photoelectron reflection is taken into account in the framework of the so-called bubble potential, which is a spherical δ-type potential. It is assumed in the derivations that the NG is centrally located in the fullerene. It is also assumed, in accordance with the existing experimental data, that the fullerene radius R _C is much larger than the atomic radius r _A and the thickness Δ_C of the fullerene shell. As was demonstrated recently, these assumptions allow us to represent the NG@C _n photoionization cross section as a product of the NG cross section and two well-defined calculated factors. The text was submitted by the authors in English.     

Inner-shell ionization of atoms by electron,positron and photon impacts

Plane wave Born approximation with Coulomb, relativistic and exchange corrections is employed to obtain L1-, L2- and L3-subshell ionization cross sections of several atoms due to electron and positron impacts for projectile energy varying from the threshold of ionization to 60 times the threshold energy. Photoionization cross sections for all the three L-subshells of the atoms are also calculated using the hydrogenic approximation for the atomic wave functions. For L3-subshell the present cross sections due to electron impact are in good agreement with a number of experimental data for different atoms over the entire energy range investigated. For L1- and L2-subshells the present calculations yield qualitative agreement with the experimental data. The agreement between the present results and the limited experimental data for positron impact is also satisfactory. The hydrogenic approximation for the L-subshell photoionization is found to be good at small photon energies but it underestimates the cross sections at large photon energies.     

Dynamics of positron scattering from lithium,sodium and potassium atoms in hot and dense plasmas

The dynamics of positron scattering from the ground state of lithium, sodium and potassium atoms in hot and dense plasmas has been investigated by applying a two-state approximation that includes elastic scattering and rearrangement scattering. The wave functions and energies of the target alkali atoms have been determined quite accurately within the framework of the ’method of model potential’. An inclusive study was made on the effects of plasma screening on the differential and total cross sections for elastic scattering and positronium formation in the ground state for the incident positron energy lying within 10 eV to 500 eV. For the unscreened case, our present results agree reasonably with the results of other calculations. It was found that the cross sections suffer considerable change due to the effects of the background plasma.     

Search for long-range forces by means of ultracold neutrons (1)

A method of using a gravitational spectrometer to search for long-range forces between neutrons and atoms is proposed. The constraints on the strength of long range forces within the range of 10⁻¹⁰–10⁻⁴ cm can be obtained from the experiments on measurements of the total cross section of interaction of ultracold neutrons with atoms of noble gases (He, Ne, Ar, ⁸⁶Kr) and the data on the coherent neutron scattering length of the nucleus. The first result of such type analysis is presented.     

Light atom scattering from adsorbates at low coverage

A multiple scattering theory is developed for the scattering of light atoms from a disordered adsorbate on a smooth surface. Using gas phase potentials for adatom potentials, excellent agreement is found with data for He scattering from adsorbed Xe and surprisingly good agreement with the extensive data for He scattering from adsorbed CO. The attractive adatom potential must be included in the calculation if quantitative comparison with experimental data is to be made. An important contribution to the total cross section of an adatom are quantum mechanical oscillations similar to glory oscillations in gas phase scattering. The dependence of the total cross section on the incident angle is strongly influenced by the varying number of adatoms seen by the probe.     

A modification potential method of calculating total cross sections of electrons scattering from complex molecules C2H6, C2F6, C6H6 and C6F6 at 100 eV-5000 eV

A complex optical model potential modified by incorporating the concept of bonded atom, which takes into consideration the overlapping effect of electron clouds between two atoms in a molecule, is first employed to calculate the total cross sections for electrons scattering from such complex molecules as C总交叉断面 电子散射 添加规则 原子分子碰撞 电子云total cross section, electrons scattering, additivity rule, atomic and molecular collisionProject supported by the National Natural Science Foundation of China (Grant No 10574039).2005-11-102005-11-102005-11-30A complex optical model potential modified by incorporating the concept of bonded atom, which takes into consideration the overlapping effect of electron clouds between two atoms in a molecule, is first employed to calculate the total cross sections for electrons scattering from such complex molecules as C2H6, C2F6, C6H6 and C6F6 using the aclditivity rule model at Hartree-Fock level over the energy range from 100 eV to 5000 eV. The total cross sections are quantitatively compared with those obtained by experiments wherever available, and they are in good agreement with each other over a wide energy range. It is shown that the modified potential together with the additivity rule model is completely suitable for the calculation of total cross sections of electrons scattering from such complex molecules as C2H6, C2F6, C6H6 and C6F6 above 200 eV-300 eV.     

Positron scattering by atoms

We describe the present status of coupled-state calculations for positron scattering by ‘one-electron’ atoms. We show how pseudostates are used to represent the continuum channels. Illustrative results from positron scattering by atomic hydrogen and the alkali metals are presented.     

Inner-shell electron capture by impact of positron on the multi-electron atomic targets

The first-order Coulomb-Born approximation has been applied to the study of positronium formation through K-shell electron capture in the collision of positron with multi-electron atomic targets. The single-zeta Roothaan-Hartree-Fock wave functions are used to describe the electron initial bound states. The differential and total cross sections are computed for the impact of positron on helium, carbon, neon, sodium and argon atoms, with the formation of positronium in its ground state. For helium atoms, the calculated total cross sections are compared with the available experimental data and other theoretical calculations. The comparison shows a good agreement between the present calculations and the measurements.     

Scattering of e± from N2 in the energy range 1 eV–10 keV

The differential, integrated elastic, inelastic, total, momentum transfer, viscosity, and ionisation cross sections for electron and positron scattering from the homonuclear diatomic nitrogen molecule over an incident energy range of 1?eV–10?keV are calculated using the additivity rule. Dirac partial wave analysis is employed to calculate the cross sections of the constituent atoms of molecules, using a complex optical model potential (OPM). Comparison of our results of the additivity rule with the available experimental data and other theoretical predictions is presented. Our findings produce reasonable results in intermediate and high energies.     

Interference effects on the photoionization cross sections between two neighbouring atoms: nitrogen as an example

Interference effects on the photoionization cross sections between two neighbouring atoms are considered based on the coherent scattering of the ionized electrons by the two nuclei when their separation is less than or comparable to the de Broglie wave length of the ionized electrons. As an example, the single atomic nitrogen ionization cross section and the total cross sections of two nitrogen atoms with coherently added photoionization amplitudes are calculated from the threshold to about 60~\AA (1~\AA=0.1~nm) of the photon energy. The photoionization cross sections of atomic nitrogen are obtained by using the close-coupling R-matrix method. In the calculation 19 states are included. The ionization energy of the atomic nitrogen and the photoionization cross sections agree well with the experimental results. Based on the R-matrix results of atomic nitrogen, the interference effects between two neighbouring nitrogen atoms are obtained. It is shown that the interference effects are considerable when electrons are ionized just above the threshold, even for the separations between the two atoms are larger than two times of the bond length of N₂ molecules. Therefore, in hot and dense samples, effects caused by the coherent interference between the neighbours are expected to be observable for the total photoionization cross sections.