Many-electron strong-field physics

Double and multiple ionisation cannot be understood without the contribution of a non-sequential mechanism, which is due to electron-electron correlation. This paper discusses the experimental manifestations of this effect, from its first footprint in the total yields of multiply charged ions to the highly differential cross-sections for ion and electron production, and the theoretical models that have been set up for their description. Except for the ab initio solution of the time-dependent Schrodinger equation, most models implement the recollision mechanism. The kinematical constraints imposed by the former are extensively discussed. A fully quantum-mechanical formalisation of the recollision model in terms of the lowest-order S-matrix element for this process is introduced and its limitations are discussed. Various simplifications are proposed that drop more and more of the quantum features. This is contrasted with purely classical trajectory descriptions with no approximations to the dynamics.