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 cl assical phase space available to two outgoing electrons in the Coulomb fiel d of the residual ion. The relation between classical dynamics and quantum mechanics in the near threshold region becomes more complicated and interes ting in 'non-Wannier situations', where classical ionisation is either stri ctly forbidden or strongly inhibited. This paper focusses on two examples o f 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.