Electron correlations and pre-collision in the re-collision picture of high harmonic generation

We discuss the seminal three-step model and the recollision picture in the context of high harmonic generation in molecules. In particular, we stress the importance of multi-electron correlation during the first and the third of the three steps of the process: (1) the strong field ionization and (3) the recombination. We point out how accurate account for multi-electron correlations during the third, recombination, step allows one to gauge the importance of pre-collision: the term coined by J. H. Eberly to describe unusual pathways during the first, ionization, step.