Near-threshold correlation studies of the 2¹P and 3³D states of helium excited by electron impact

Abstract

Excitation of the 3 3 D state of helium by electrons with an energy of 370 meV above threshold is studied experimentally using the electron-photon polarization correlation method and theoretically using the convergent close-coupling (CCC) and R-matrix with pseudo-states (RMPS) methods. Stokes parameters and associated coherence parameters are presented. Despite difficulties associated with a sharp resonance in the cross section at this energy, generally good agreement is obtained between experiment and the calculations. At small scattering angles the results show a violation of the propensity rule for angular momentum transfer. CCC and RMPS calculations have also been performed at 22 and 24 eV for the 2 1 P state and are compared with earlier angular correlation data.

Electron impact excitation of the 31D state of helium - an electron-photon coherence study at small scattering angles

“…Excitation of the triplet states proceeds exclusively through exchange and this must be responsible for the different L ⊥ behaviours. The only exception to this general behaviour observed for triplet states is for the 3 3 D state at an incident energy of 23.45 eV, where the cross section is dominated by a negative ion resonance which influences the momentum transfer and the other charge cloud parameters (Crowe et al 2000). A very recent theoretical analysis of s-p excitation/de-momentum partial waves and penetration of the target by the incident electron is small.…”

The scattering of 40 eV electrons from magnesium: a polarization correlation study for the 31P state and differential cross sections for elastic scattering and excitation of the 31P and 33P states

“…Since the inception of electron-photon correlation measurements following the groundbreaking paper, 'Impact excitation and polarization of the emitted light', by Fano and Macek (1973), a considerable amount of experimental time has been devoted to investigations using this technique, with helium being the most popular target. In this laboratory we have concentrated on measurements over a large range of scattering angles ( 120 • ), often at energies close to the excitation threshold (Crowe et al 2000) on excitation to higher angular momentum states (Cvejanović and Crowe 1997a) and transitions involving a change of multiplicity . These conditions provide severe tests of scattering theory.…”