Dynamical resonance shift and unification of resonances in short-pulse laser cluster interaction

Pronounced maximum absorption of laser light irradiating a rare-gas or metal cluster is widely expected during the linear resonance (LR) when Mie-plasma wavelength $\lambdaM$ of electrons equals the laser wavelength $\lambda$. On the contrary, by performing molecular dynamics (MD) simulations of an argon cluster irradiated by short 5-fs (fwhm) laser pulses it is revealed that, for a given laser pulse energy and a cluster, at each peak intensity there exists a $\lambda$ -- shifted from the expected $\lambdaM$ -- that corresponds to a {\em unified dynamical} LR at which evolution of the cluster happens through very effective unification of possible resonances in various stages, including (i) the LR in the initial time of plasma creation, (ii) the LR in the Coulomb expanding phase in the later time and (iii) anharmonic resonance in the marginally over-dense regime for a relatively longer pulse duration, leading to maximum laser absorption accompanied by maximum removal of electrons from cluster and also maximum allowed average charge states for the argon cluster. Increasing the laser intensity, the absorption maxima is found to shift to a higher wavelength in the band of $\lambda\approx (1-1.5)\lambdaM$ than permanently staying at the expected $\lambdaM$. A naive rigid sphere model also corroborates the wavelength shift of the absorption peak as found in MD and un-equivocally proves that maximum laser absorption in a cluster happens at a shifted $\lambda$ in the marginally over-dense regime of $\lambda\approx (1-1.5)\lambdaM$ in stead of $\lambdaM$ of LR. Present study may find importance for guiding an optimal condition laser-cluster interaction experiment in the short pulse regime.