Plasmon-enhanced electron acceleration in intense laser metal-cluster interactions.

We have measured the energy and angular-resolved electron emission from medium-sized silver clusters (N approximately 500-2000) exposed to dual laser pulses of moderate intensity (I approximately (10(13-14) W/cm2). When the second pulse excites the plasmon resonantly, we observe enhanced emission along the laser polarization axis. The asymmetry of the electron spectrum is strongly increasing with electron energy. Semiclassical simulations reveal the following mechanism: Electrons bound in highly excited states can leave, return to, and traverse the cluster. Those electrons that return at zero plasmon deflection and traverse the cluster during a favorable plasmon half-cycle can experience maximum acceleration by the evolving polarization field. As a result of these constraints energetic electrons are emitted in direction of the laser polarization axis in subcycle bursts.