Elastic scattering of electrons by molecular and atomic hydrogen

Abstract

Absolute experimental cross sections are presented for elastic differential scattering of electrons by molecular hydrogen at impact energies between 100 and 2000 eV and at scattering angles between 5 and 50 degrees . The results are compared with the relative experimental cross sections of previous experiments and with the data of different theoretical calculations. Total elastic cross sections are considered as well. The present absolute molecular hydrogen cross sections, together with the 100 and 200 eV ratio measurements of the differential elastic cross sections of H2 and H of Lloyd et al. (1974) were used to obtain a new set of absolute elastic cross sections for atomic hydrogen. These experimental data are compared with the absolute experimental sections obtained by Williams (1975) and with theory.

New and Old Measurements of Electron Scattering in Atomic Hydrogen

“…But similar difficulties had appeared in the earlier elastic DCSs of from 9 · 4 to 50 eV and of Lloyd et al (1974) Shyn and Cho (1995) at 30 eV and scattering angles larger than about 100 • are considerably higher than earlier values. Similar trends appeared in the 100 and 200 eV DCS of van Wingerden et al (1977) where there are larger uncertainties and the relative values at larger angles differ from theory. At 200 eV van Wyngaarden and Walters (1986) claimed the best theory produced elastic DCSs that were probably accurate to 5% and consequently that the data of Williams and Willis (1975) seemed too high, particularly at 100 eV.…”

“…Similar trends appeared in the 100 and 200 eV DCS of van Wingerden et al (1977) where there are larger uncertainties and the relative values at larger angles differ from theory. At 200 eV van Wyngaarden and Walters (1986) claimed the best theory produced elastic DCSs that were probably accurate to 5% and consequently that the data of Williams and Willis (1975) seemed too high, particularly at 100 eV. However, the differences between their theory and those measured data varied only from 13% to 37%.…”

“…• their measured ratios of the n = 1 to n = 2 DCS results are closer to the data of van Wingerden et al (1977) rather than of Williams and Willis (1975) and so diverged further from the van Wyngaarden and Walters (1986) 'twenty-pseudostate close-coupling' calculated values. Both experiments used different calibration methods which added support to their values.…”

“…At 100 eV the experimental DCSs of Williams and Willis (1975) are supported by the data of Doering and Vaughan (1986) (not shown in the figure); the two data sets agree at large angles, within rather large error limits of about 30%, with the Doering and Vaughan results higher than those of Williams and Willis. The multi-pseudostate close coupling values of [Callaway and Williams (1975) (triangles)], at 54 eV [Williams and Willis (1975) (open circles)] and at 100 eV [Williams and Willis (1975) van Wyngaarden and Walters (1986) and the distorted wave second-order exchange values of Madison et al (1991) (not shown in the figure for clarity) are in agreement with measurement at the smallest angles, but then become smaller than measurement as the scattering angle increases such that at 130 • the data sets are about three standard deviations apart. It seems that the absolute calibration (for the 20 • datum point) is adequate but the relative angular values decrease less rapidly than theory.…”

“…The contributions which set the scene for this paper on electron scattering from atomic hydrogen concern the theoretical advances by Madison et al (1991) and Bray and Stelbovics (1992), the analyses of theory by van Wyngaarden and Walters (1986) and Scholz et al (1991) and the comments by McConkey et al (1988). The convergent close coupling method (Bray and Stelbovics 1992), the multi-pseudostate close coupling method , the intermediate energy R-matrix method (Scholz et al 1991) and the exact second-order distorted wave method (Madison et al 1991) have built on decades of advances in theoretical and computational methods.…”