Theoretical triple differential cross sections for the autoionisation of helium by electron impact: limits on the validity of first-order models

A first-order model for the autoionisation of helium by electron impact is proposed in which full account is taken of exchange and in which the principal resonances are each described by two angle-dependent parameters. It is also shown how second-order terms may be incorporated into the model. The wavefunctions for the incident, scattered and ejected electrons are calculated using the polarised-orbital approximation. Results are presented for the triple differential (e-2e) cross section for the (2s2)1S, (2s,2p)3P and (2s,2p)1P resonances at incident energies of 70 and 80 eV for a scattering angle of 150 and also for the double differential cross section corresponding to an integral over the scattering angle. The limits on the validity of this type of model are discussed together with the possible errors introduced by the use of a simplified representation of the autoionising states. (author)