|
The formation of the recently observed low-energy structure in the photo-electron spectra of atoms and small molecules in strong micrometer-wavelength pulses is traced back to soft recollisions of the released electrons. In contrast to the well-known hard recollisions, which are responsible for above-threshold ionization and high-harmonic generation, hereby the electrons "misses" the ionic core and rather turns around aside it. Such recollisions may cause an energy bunching, since electrons with different initial drift momenta can acquire impacts, which exactly counterbalance these differences. We derive an analytical expression for the peak energy, which explains the experimentally observed scaling with intensity and frequency. Furthermore we predict the occurrence of higher-order peak structures. |