Non-Sequential Double Recombination High Harmonic Generation in   Molecular-like Systems
  We present a study of non-sequential double recombination (NSDR) high harmonic generation (HHG) in a molecular-like system. Using a Coulomb-corrected three-step model we are able to classically describe the observed NSDR HHG cutoffs precisely for all internuclear distances showing an intrinsic dependence on the location of the nuclei in the NSDR HHG process originating partly from the strong electron correlation in the NSDR HHG process. This dependence modifies the classically allowed return energies which in return changes the cutoffs observed in the HHG spectra. We also observe that the NSDR HHG signal changes its character for internuclear distances of $R\gtrsim 8-9$ a.u. which is proposed to stem from a change in the charge transfer dynamics within the molecule. For large internuclear distances of $R\gtrsim 13$ a.u. we observe a clear signature of the point of emission for the first electron emitted in the NSDR HHG signal and we also see signs of molecular exchange paths contributing to the HHG spectrum for these internuclear distances. 
Kenneth K. Hansen and Lars Bojer Madsen
physics.atom-ph