High-order harmonic generation from silver clusters: Laser-frequency dependence and the screening effect of d electrons

We present time-dependent density functional studies of harmonic generation from Ag₂ and Ag₈ in pulsed laser fields. The harmonic generation is strongly dependent on the laser frequency. The harmonics are emitted from the clusters much more efficiently when the applied laser field is in tune with the dipole resonance frequency of the system. Such resonance frequency dependence is substantially equal to a resonance phenomenon in a forced oscillator in a sense that the valence s-electrons are shaken effectively at the tuned laser frequency and the induced dipole moment continues to oscillate even though the laser field is switched off. Furthermore, we have found that the polarizable core d-electrons significantly screen the valence s-electrons such that the electron density of the s electrons induced in the laser field is canceled out. The screening effect of the d electrons becomes more important in the system of Ag₈ than Ag₂.