Under the condition of ten different incident energies ranging from 3 eV to 80 eV above the ionization potential of helium and the outgoing electrons having equal energies,by making use of 3C model and modified 3C model,the triple differential cross sections of electron-impact single ionization of the ground state of helium in the perpendicular geometry are calculated.The result is compared with corresponding experimental result to systematically investigate the influences of various screening effects on the triple differential cross sections for helium.The collision mechanisms of the triple differential cross sections are explored.The result shows that the effects of dynamic screening in the final state can directly affect the structures of the triple differential cross sections at lower incident energy,which will unavoidably affect the angular distribution and relative amplitude of side peaks at angles φ =90° and φ =270°.The screening effects of residual electron in the final state of He+ have a similar significant effect on the amplitude of triple differential cross section of helium and angular distributions and relative amplitudes of side peaks at angles φ =90° and φ =270°.When the incident energy is over 84.6 eV,the screening effect of residual electron in the final state of He+ has a slight effect on the amplitude of triple differential cross section,which can be overlooked.But the effects of dynamic screening in the final state on side peaks at angles φ =90° and φ =270° need considering.In addition,by taking advantage of DS3C-Z model,the results of collision mechanism of various peaks at angles φ =180°,φ =90° and φ =270° show that the middle peak at angle φ =180° is produced by a process called triple scattering mechanism and then the side peaks at angles φ =90° and φ =270° are produced by a process called double scattering mechanism.Such a collision mechanism has a direct influence on the generation and variation law of triple differential cross sections.%用3C模型和修正后的3C模型在低能、两个出射电子等能分享几何条件下,对电子垂直入射碰撞电离氦原子的三重微分散射截面进行了理论计算,并把计算结果与实验测量结果进行了比较,系统研究了(e,2e)反应中各种屏蔽效应对氦原子三重微分散射截面的影响,同时对截面中形成各峰的碰撞机理做了详细的探讨.研究结果表明:在入射能较低时,各种屏蔽效应对氦原子的三重微分散射截面幅度以及角分布均存在一定影响,并且形成各峰的碰撞机理直接影响截面的变化规律.
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