Differential cross sections for electron impact ionization of helium

Differential cross sections for electron impact ionization of helium are calculated in the Born approximation for ejected-electron energies below the $n=2\mathrm{}$ threshold of the residual ${\mathrm{He}}^{+}$ ion. The resonant features associated with the lowest-lying ${}_{}{}^1{}_{}{}^{}S$, ${}_{}{}^1{}_{}{}^{}P$, and ${}_{}{}^1{}_{}{}^{}D$ helium autoionizing states are analyzed using the Fano line-shape formula. The angular correlation between the two outgoing electrons is expressed as a Legendre-polynomial expansion, and the coefficients are evaluated for representative energy losses and momentum transfers. For an incident-electron energy of 256.5 eV, the predicted angular distribution is in satisfactory agreement with the coincidence measurements of Ehrhardt et al. except near the recoil maximum.