Weakly relativistic electron dynamics and efficient X-ray photon emission driven by an ultraintense extreme-ultraviolet laser field

The electron dynamics of a tightly bound system exposed to an ultraintense extreme-ultraviolet laser field is investigated by solving numerically the relativistically corrected two-dimensional time-dependent Schrödinger equation. The Coulomb interaction between the single-active electron and the parent ion is included throughout the harmonic generation process. The nondipole effects are found to be more pronounced with the increase of the ionization potential. The structure of the below-threshold harmonics changes drastically for different field intensities. The time-frequency analysis of the electron acceleration gives clearly the time-resolved harmonic spectrum and its dependence on the intensity of the pump field. With proper driving field intensities and ramp durations, hyper-Raman lines appear in the harmonic spectra for the soft-core potential. But due to multiple excitations of the atomic bound states, the hyper-Raman lines are spiked and broadened. When nearly full ionization occurs at the tail of the driving field, the HR lines disappear, while the emission of the odd harmonic lines is found to be most efficient then.