A phaseshift analysis of experimental angular distributions of electrons elastically scattered from He, Ne and Ar over the range 0.5 to 20 eV

Abstract

Absolute differential elastic cross sections have been measured for electron scattering by helium, neon and argon atoms over the incident electron energy range of 0.5 to 20eV, and over the scattered electron angular range of 15 to 150. The experimental method uses crossed electron and modulated atom beams, 127 electrostatic energy analysis of incident and scattered electrons, quadrupole mass analysis of the target gas and single-particle counting techniques. It is shown how the use of a chi-squared optimisation analysis leads to better knowledge of apparatus behaviour and of cross section deter- minations. Absolute cross section values have been established by the methods of (i) the parametrisation of the relative elastic angular distributions in terms of the first three phases at 22 energies and 27 angles within the above ranges, (ii) a phaseshift analysis around the 19.3 eV ('S) resonance in helium and (iii) the measurement of elastic scattering signal ratios in Ne and Ar at any energy and scattering angle (here 19.2 eV and 60) for known relative source pressures. The errors in the differential cross section value at 19.2 eV and 60 in He determined by these methods are found to be *4%, *2'/0 and *llo/', respectively. However, only the first method gives a calibration over the full energy and angular range of this paper. The present cross section values for helium, neon and argon generally agree, within the combined experimental errors, with the data of Andrick and Bitsch but disagree with most other measured values. In the case of helium the p and d phaseshifts calculated from a variational method by Sinfailam and Nesbet give the best agreement with the present values while the calculated polarised-orbital values of Callaway and colleagues also agree closely. For the s-wave phase an error of 1.4% is generally not sufficient to indicate a preference for any one theoretical set of values.