The inertia of tunneling ionization and high‐order harmonic shifting in the nonlinear single‐atom response

We demonstrate that the ionization‐induced high‐order harmonic emission mechanism can be affected by a new frequency‐shifting effect which is present directly in the single‐atom response at fixed laser frequency and monotonically varying laser intensity. By analyzing the motion of newly‐born electrons we found that the phases of their collisions with ‘‘parent’’ ions are influenced by the dynamics of the driving field amplitude. At the leading edge of the laser pulse, where the field amplitude is increasing, the collisions at a given velocity occur periodically in time but are shifted monotonically in phase with each optical cycle, the sign of the shift depending on the delay time at which freed electrons are detached from the atoms after tunneling. The relationship between the dynamics of the |Ψ|2 function and the possibility to produce a frequency shift in the presence of a monotonously varying amplitude pulse is discussed.