Non-resonant absorption enhancement in laser-excited simple metal clusters through electron-electron collisions

We study the response of small- and medium-sized sodium clusters (Na13-147) to dual-pulse laser fields of moderate intensity I=8 x 10(12) W/cm(2). The contribution of electron-electron collisions on the cluster dynamics and plasmon-enhanced coupling is analyzed by a systematic comparison of semiclassical Vlasov and Vlasov-Uehling-Uhlenbeck simulations. We find that electron-electron collisions significantly enhance the non-resonant absorption of laser energy and have substantial impact on the time scale on which frequency matching between the laser and the cluster surface-plasmon occurs. The total ionization and energy capture at optimal pulse delays, however, are practically unaffected. This underlines that enhanced absorption through resonant plasmon excitations remains very effective and is only weakly influenced by electron-electron collisions.