Angular distribution for the elastic scattering of electrons from Ar + ( 3 s 2 3 p 5 2 P ) above the first inelastic threshold

The measured angular differential cross section (DCS) for the elastic scattering of electrons from Ar{sup +}(3s{sup 2}3p{sup 5} {sup 2}P) at the collision energy of 16 eV is presented. By solving the Hartree-Fock equations, we calculate the corresponding theoretical DCS including the coupling between the orbital angular momenta and spin of the incident electron and those of the target ion and also relaxation effects. Since the collision energy is above one inelastic threshold for the transition 3s{sup 2}3p{sup 5} {sup 2}P-3s3p{sup 6} {sup 2}S, we consider the effects on the DCS of inelastic absorption processes and elastic resonances. The measurements deviate significantly from the Rutherford cross section over the full angular range observed, especially in the region of a deep minimum centered at approximately 75 deg. Our theory and an uncoupled, unrelaxed method using a local, spherically symmetric potential by Manson [Phys. Rev. 182, 97 (1969)] both reproduce the overall shape of the measured DCS, although the coupled Hartree-Fock approach describes the depth of the minimum more accurately. The minimum is shallower in the present theory owing to our lower average value for the d-wave non-Coulomb phase shift {sigma}{sub 2}, which is due to the high sensitivity of {sigma}{sub 2} to the different scattering potentials used in the two models. The present measurements and calculations therefore show the importance of including coupling and relaxation effects when accurately modeling electron-ion collisions. The phase shifts obtained by fitting to the measurements are compared with the values of Manson and the present method.