Abstract
The problem of Freeman resonances [R. R. Freeman \textit{et al.}, Phys. Rev. Lett. \textbf{59}, 1092 (1987)] when strong field ionization is enhanced due to the transient population of excited states during the ionization, is revisited. An intuitive model is put forward which explains the mechanism of the intermediate population of excited states during nonadiabatic tunneling ionization via the under-the-barrier recollision and recombination. The theoretical model is based on perturbative strong-field approximation (SFA), where the sub-barrier bound-continuum-bound pathway is described in the second-order SFA, while the further ionization from the excited state by an additional perturbative step. The enhancement of ionization is shown to arise due to the constructive interference of contributions into the excitation amplitudes originating from different laser cycles. The applied model provides an intuitive understanding of the electron dynamics during a Freeman resonance in strong-field ionization, as well as means of enhancing the process and possible applications to related processes.
Highlights
The enhancement of strong-field ionization due to transient excitation of Stark-shifted bound states is well known from experiments in the multiphoton regime of ionization and is termed as Freeman resonances [1,2,3,4,5,6,7,8,9,10,11,12,13]
The process takes place at the laser field maximum, when the spatial distribution of the dressed excited state is concentrated at the distance α ∼ E0/ω2 away from the core, with the width ≈1/κ∗; for recombination the recolliding electron arrives at the core, because momentum transfer from the core is needed for recombination
We have proposed an intuitive model for Freeman resonances in the nonadiabatic tunneling ionization
Summary
The enhancement of strong-field ionization due to transient excitation of Stark-shifted bound states is well known from experiments in the multiphoton regime of ionization and is termed as Freeman resonances [1,2,3,4,5,6,7,8,9,10,11,12,13]. It has been recognized that recollision can happen within the sub-barrier dynamics during tunneling [23] The latter may contribute to the electron transition to the excited state as long as the electron gains sufficient energy during the nonadiabatic tunneling. In this paper we develop a theory for Freeman resonances in the nonadiabatic tunneling regime which is based on the concept of under-the-barrier recollision. The proposed model provides a physical explanation via the quantum orbit picture for the resonantly enhanced strong-field ionization involving excited states at a Freeman resonance.
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