Effect of time-dependent ionization on the harmonics generated by bound–bound transitions

Abstract

We present a study of the effect of time-dependent ionization on harmonic generation by bound–bound transitions. The introduction of tunneling and above-barrier ionization rates in a simple bound system, a two-level atom, induces an unexpected richness in the harmonic spectra, leading to the production of radiation at high frequencies. Such frequencies are not reached when no ionization or steady ionization is considered. Time-dependent ionization is, therefore, a key process in the understanding of generation of middle plateau frequency harmonics from atoms in the tunneling ionization regime and shows that bound–free transitions are not uniquely responsible for the higher-harmonic frequencies in realistic atoms. Following previous authors’ studies, we use an adiabatic approach to understand the presence of these higher frequencies.