Threshold behaviour of (e,2e) reactions in classical and quantum mechanics

Electron impact ionisation of one-electron atoms or ions is well described in the near threshold region by Wannier’s theory based on estimating the classical phase space available to two outgoing electrons in the Coulomb field of the residual ion. The relation between classical dynamics and quantum mechanics in the near threshold region becomes more complicated and interesting in ‘non-Wannier situations’, where classical ionisation is either strictly forbidden or strongly inhibited. This paper focusses on two examples of such situations, which have been receiving increasing attention in recent years, namely the s-wave model and the case where the exponent in Wannier"s law diverges.