Sub-barrier pathways to Freeman resonances

Abstract

The problem of Freeman resonances [Freeman, Phys. Rev. Lett. 59, 1092 (1987)PRLTAO0031-900710.1103/PhysRevLett.59.1092] when strong-field ionization is enhanced due to transient population of excited Rydberg states during ionization is revisited. An intuitive model is put forward which explains the mechanism of intermediate population of excited states during nonadiabatic tunneling ionization via 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, with further ionization from the excited state by an additional perturbative step. The enhancement of ionization is shown to arise due to 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 a means of enhancing the process and possible applications to related processes.