Time-dependent multiconfiguration self-consistent-field study on resonantly enhanced high-order harmonic generation from transition-metal elements

We theoretically study high-harmonic generation (HHG) from transition-metal elements Mn and Mn+ using full-dimensional, all-electron, first-principles simulations. The HHG spectra calculated with the time-dependent complete-active-space self-consistent-field (TD-CASSCF) and occupation-restricted multiple-active-space (TD-ORMAS) methods exhibit a prominent peak at similar to 50 eV, successfully reproducing resonant enhancement observed in previous experiments [Opt. Express 20, 25239 (2012)]. Artificially freezing 3p orbitals in simulations results in its disappearance, which shows the essential role played by 3p electrons in the resonant harmonics (RH). Further transition-resolved analysis unambiguously identifies constructively interfering 3p-3d (m = 0, +/- 1) giant resonance transitions as the origin of the RH, as also implied by its position in the spectra. Time-frequency analysis indicates that the recolliding electron combines with the parent ion to form the upper state of the transitions. In addition, this study shows that the TD-CASSCF and TD-ORMAS methods can be applied to open-shell atoms with many unpaired inner electrons.