Fano resonance in XUV generated by helium with few-cycle intense laser pulses and its classical analogy

We integrate numerically the Schr\"odinger equation for the model helium atom irradiated by intense few-cycle laser pulse and find the emitted XUV spectra. They demonstrate resonant peaks at the frequencies of transitions from the doubly-excited autoionizing states (AISs) to the ground state. We study the properties of these peaks depending on the laser pulse duration and find that the decay of the AISs due to photoionization by the laser field affects them. Moreover, we consider the classical system of two coupled oscillators and find that both the quantum (the atom with AIS in the field) and the classical (the coupled oscillators with friction) systems demonstrate Fano-like resonant peak described by an essentially complex asymmetry parameter. We find a remarkable similarity in the behavior of these systems and conclude that the classical system of coupled oscillators with friction is an analogy of the AIS having an extra decay channel in addition to the autoionization one.