Laser-induced nonlinear excitation of collective electron motion in a cluster

Abstract

We consider the nonlinear collective electron dynamics inside a largecluster irradiated by a strong linearly polarized short (≲0.1 ps) laser pulse. The equation of the centre-of-mass motion of the electron clouddriven by the strong laser field is derived using the approximation of anincompressible medium. The analysis of this equation demonstrates the presenceof odd harmonics of the fundamental frequency in the cluster dipole moment, andin both the internal and the scattered electric field. Both neutral and ionizedclusters are considered. For clusters with radii R ≃ 100 Åirradiated by a femtosecond titanium–sapphire laser with a peakintensity of I ≃ 1016 W cm−2,the internal electric field strength near the tripled fundamental frequencyis shown to be of the same order as the field of the fundamental. Thereason is that both for metallic and for laser-ionized van der Waalsclusters the Mie surface-plasmon energy ℏωMis around 5 eV, which is close to three times the energy of a titanium–sapphirelaser-field quantum. On the other hand, the condition for first-order resonancewith the Mie frequency is not met during the presence of the main laser pulse, butonly temporarily, either at the first onset of inner ionization on the leading edge ofthe pulse (for van der Waals clusters) or during the subsequent Coulombexplosion. In both cases, the ion density is reduced. The presence of a strong thirdharmonic leads, in particular, to the enhanced production of multiply charged ionsin clusters irradiated by a strong laser field, as compared with isolatedatoms. This point is discussed in the light of recent experimental results onthe production of multiply charged ions in laser–cluster experiments.Third-harmonic generation by a cluster in a strong laser field, as a function ofboth the cluster and the laser-field parameters, is also considered.