Tracing Nonlinear Cluster Dynamics Induced by Intense XUV, NIR and MIR Laser Pulses

The ionization of nanoscale clusters by intense laser pulses is fundamentally different from the ionization of atoms or small molecules. Laser energy is efficiently absorbed by clusters, transforming them into nanoplasmas within femtoseconds. An overview of recent experiments is presented, in which the dynamics of clusters induced by intense laser pulses were traced on femtosecond to nanosecond timescales. The development of an intense high-harmonic generation (HHG) source in combination with pump-probe techniques using laser pulses from the extreme-ultraviolet (XUV) to the terahertz (THz) regime have made it possible to trace and control nonlinear cluster ionization and relaxation directly in the time domain. Very efficient population of Rydberg states by electron-ion recombination was found during the cluster expansion. We have discovered that these Rydberg atoms and ions can relax via a so far unobserved correlated electronic decay (CED) mechanism, during which one electron relaxes from a higher to a lower atomic bound state and transfers its excess energy to a neighboring electron that escapes from the cluster, leaving distinct signatures in the electron kinetic energy spectrum.