ИСТИНА

The emission spectra of harmonics are calculated for Ga+ and In+ ions exposed by a femtosecond laser pulse of intensity up to 300 TW/cm2. Temporal evolution of a multielectron ion from the initial ground state is described by a finite superposition of discrete stationary states. The expansion coefficients (complex amplitudes of level population) are obtained numerically from the homogeneous differential equation set with dipole transition matrix elements between stationary states obtained in the Hartree-Fock model. The ionization of excited states by the laser field and spontaneous decay are taken into account. In the semiclassical approach the emission spectrum of ions in a laser field is determined by the Fourier transform of their induced average dipole moment. The increasing of harmonic yield was studied at the multiphoton resonance on the transition from the ground state to highly excited autoionizing states, both of negative and positive parity. In the latter case the two-photon resonance transition occurs at harmonic and laser field frequencies. Calculations with the realistic electronic structure of ions show (see Fig. 1) an increase in the intensity of the resonant harmonic, but not so sharp (orders of magnitude relative to neighboring ones) as in the experiments with plasma targets [1, 2] and in the simplified model calculation [3]. A possible reason is the truncation of the expansion basis, which in particular does not include continuum states. Methods are discussed which allow to account for theomitted states. References [1]. Singh M., et al. Optica 8 (8) 1122−1125 (2021). [2]. Singh M., et al. Phys. Rev. Lett. 130 (7) 073201(6) (2023). [3]. Strelkov V. V. arXiv:2221.03695.