Fine-scale oscillations in the wavelength and intensity dependence of high-order harmonic generation: Connection with channel closings

We investigate the connection of recently identified fine-scale oscillations in the dependence of the yield of the high-order harmonic generation (HHG) on wavelength lambda of a few-cycle laser pulse [K. Schiessl et al., Phys. Rev. Lett. 99, 253903 (2007)] to the well-known channel-closing (CC) effect. Using the Lewenstein model of HHG, we identify the origin of the oscillations as quantum interference of many rescattering trajectories. By studying the simultaneous variations with intensity and wavelength, different models for the interference of channel-closing peaks can be tested. Contrary to theoretical predictions for short-range potentials, the peaks are located neither at nor just below the CC condition, but a significant shift is observed. The long Coulomb tail of the atomic potential is identified as the origin of the shift.