Ionization and recombination times of high-order harmonic generation with single-photon ionization

We theoretically study high-order harmonic generation (HHG) involving an extreme ultraviolet (XUV) pulse and an intense infrared driving field, where the electron is ionized by absorbing a single XUV photon. Using a developed classical-trajectory model that includes Coulomb effects and the improved initial conditions, it is demonstrated that the resulting harmonic emission times match well with those obtained by applying the Gabor transform to data from numerical solutions of time-dependent Schrödinger equations for helium and hydrogen atoms. This confirms a classical HHG scheme under single-photon ionization: The electron, ionized by absorbing one XUV photon, oscillates in the infrared field and may recollide with the parent ion, emitting high-frequency radiation. Therefore, the classical model can determine the ionization and recombination times of the electron in single-photon-ionization HHG. Our work shows great promise for resolving electron dynamics using high-order harmonic spectroscopy under single-photon ionization.