Mechanism of atomic autoionization

Analysis of the coupling matrix elements between quasibound and free states of doubly excited helium provides an unambiguous mechanistic interpretation of autoionization by electron correlation. The analysis is based on the dependence of the integrand of the matrix elements on the distances $r_1$ and $r_2$ and the angle ${\theta }_{12}$, and on the separation of the total amplitude into a part containing only one-electron operators and a part comprising $\frac{1}{r_{12}}$. The former gives rise to the normally dominant part, a generalized $S_N-2\mathrm{}$ mechanism; the latter, usually a smaller part, is the direct electron-electron, binary-collision contribution.