The high-order harmonic-generation (HHG) spectrum of Be is investigated in the multiphoton regime by solving the full-dimensional, two-active-electron, time-dependent Schrödinger equation in an intense ($\approx {10}^{13}$ W/cm${}^2$), 30-cycle laser field. As the laser frequency ${\omega }_L$ varies from $1.7$ to $1.8$ eV (which is in the tunable range of a Ti:sapphire laser), the seventh harmonic becomes resonant sequentially with the transition between the ground state and two doubly excited autoionizing states, $2p4s$(${}^{1}P$) (at ${\omega }_L=1.734$ eV) and $2p5s$(${}^{1}P$) (at ${\omega }_L=1.785$ eV), while the third harmonic becomes resonant with the $2s2p$(${}^{1}P$) singly excited state (at ${\omega }_L=1.766$ eV). At each of these resonant frequencies, the HHG power spectrum is found to increase by an order of magnitude over a range of harmonics that form a plateau, extending from the resonant harmonic up to a cutoff at the 25th harmonic. In contrast to the well-known rescattering plateau cutoff law appropriate in the tunneling regime (which predicts a cutoff at the fifth or seventh harmonic), the multiphoton regime plateau we find for Be originates from atomic resonance effects. Off resonance, the Be HHG spectrum decreases monotonically with harmonic order. By taking the ratio of the integrated harmonic power of the seventh harmonic to that of the fifth harmonic, one can isolate the resonant effects of the two doubly excited states in the HHG spectrum from those of singly excited resonance states. These ratios exhibit resonance profiles for driving laser-pulse durations much longer than the lifetimes of these autoionizing states. The energy widths of these resonance features are comparable to the widths of the laser pulse and are much smaller than the autoionizing state widths. These results demonstrate an important role for electron correlations in enhancing harmonic-generation rates in the multiphoton regime.

• Received 18 September 2013

DOI:https://doi.org/10.1103/PhysRevA.88.053412