Wigner time delay in the spillover plasmon resonance region of Na clusters

An earlier theoretical work on sodium metal clusters, Na_x, has showed a significant plasmon resonance spillover to the continuum near their ionization thresholds [1]. The study also predicted the existence of an attractive force arising from the self-consistent field induced by many-body interactions that can cause photoemission delay in attoseconds. The present work explores this spillover energy region with an aim to study the ultrafast ionization in the time domain. It particularly focusses on the Wigner time delay approach [2] involving the energy derivative of the ionization phase. The ground state of Na_x is described using a jellium-based density functional approach (DFT) with a gradient corrected exchange-correlation functional (LB94). The photoionization dynamics is accounted for using the linear-response time-dependent DFT [3].