MODIFICATIONS OF HIGH HARMONIC SPECTRA BY ION RESONANT TRANSITIONS

High-order harmonic generation is considered in a system consisting of an ion with an internal degree of freedom plus an outer electron. The theoretical treatment is both quantum-mechanical and classical. The e mphasis is on the core resonance effects, which can significantly modi fy the harmonic spectra, with appearance of anomalous peaks. Under som e assumptions, the spectral amplitude of the resonant harmonic of the system dipole moment can be obtained by evaluation of such amplitude w ithin a single-electron approximation and multiplication of the result by a correcting factor. The latter depends on the polarizability of t he ion and of a free electron at the harmonic frequency.