Ionization dynamics in expanding clusters studied by XUV pump-probe spectroscopy

The expansion and disintegration dynamics of xenon clusters initiated by the ionization with femtosecond soft x-ray extreme ultraviolet (XUV) pulses were studied with pump-probe spectroscopy using the autocorrelator setup of the Free-Electron LASer in Hamburg (FLASH) facility. The ionization by the first XUV pulse of 92 eV photon energy (8 x 10(12) W cm(-2)) leads to the generation of a large number of quasi-free electrons trapped by the space charge of the cluster ions. A temporally delayed, more intense probe (4 x 10(13) W cm(-2)) pulse substantially increases a population of nanoplasma electrons providing a way of probing plasma states in the expanding cluster by tracing the average charge of fragment ions. The results of the study reveal a timescale for cluster expansion and disintegration, which depends essentially on the initial cluster size. The average charge state of fragment ions, and thus the cluster plasma changes significantly on a timescale of 1-3 ps.