Absolute measurements with an accuracy of 22% and theoretical results in a distorted-wave Born approximation (DWBA) are reported for the triple-differential cross section for 26.6-eV electron-impact ionization of helium. An apparatus is used that allows all scattering angles to be independently varied for both coplanar and noncoplanar geometries. The measurements are compared with a DWBA calculation that includes exchange distortion in the calculation of the distorted waves, as well as with earlier calculations by Crothers [J. Phys. B 19, 463 (1986)] and Pan and Starace [Phys. Rev. Lett. 67, 185 (1991)]. Emphasis is placed on understanding the mechanisms for near-threshold ionization.PACS number(s): 34.80.Dp
Triply ditkrential cross sections in coplanar geometry for the ionization of alomic hydrogen by electron impact have been measured and calculated at incident electron energies of 54.4, 27.2, 17.6 and 15.6 eV. The theory is the same as that used successfirlly at higher energies arid is shown to repioduce nearly all features of the CTOS sections in this low-energy regime as well. Moreovcr, the physical origin of peaks and dips in the triply differential cross sections has been explained. We conclude that kinematical effects, exdiarige effects, correlation and the coherence between electron-electron and electron-proton scattering are all important at t h e e enerejes.
Triple differential cross sedions measured in a coplanar symmetric geometry on helium and neon at LOO eV and 500 eV incident energy are compared with a distorted-wave Bom approximation of Zhang el 01. The agreement is better far 500eV than far IOOeV and better for helium than for neon.Addiiionaiiy, measursmmis on helium ai lower mergics have been performed. Tie angular correlations strongly vary with the impact energy. At 500 eV, the peak around 45" which has its origin in binary collisions is largely dominating. With decreasing energy, its relative importance diminishes while the peak around 120" takes OVCL Both peaks are nearly equal at 50 eV and the distribution reduces to a single peak around 95' at 26.6 eV.There are no theoretical results for comparison at these energies.
Details are given of a new apparatus and method of measurement of absolute triple differential cross sections (TDCS) in crossed-beam experiments. Supplementary tests and an analysis of error sources show that the absolute error in the measurements is less than 22%. The relative error between individual data points is less than 10%. Preliminary results for an impact energy of 26.6 eV on helium are confirmed and extended to other near-threshold energies and angles, including non-coplanar ones. The method can readily be extended to higher energies and other gas targets. Results are compared with a number of (absolute) calculations.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.