Rare-gas clusters in intense VUV, XUV and soft x-ray pulses: signatures of the transition from nanoplasma-driven cluster expansion to Coulomb explosion in ion and electron spectra

We investigate the wavelength-dependent ionization, heating, and expansion dynamics of medium-sized rare-gas clusters (Ar-923) under intense femtosecond short-wavelength free-electron laser pulses by quasi-classical molecular dynamics simulations. A comparison of the interaction dynamics for pulses with (h) over bar omega = 20, 38 and 90 eV photon energy at fixed total excitation energy indicates a smooth transition from plasma-driven cluster expansion, where predominantly surface ions are expelled by hydrodynamic forces, to quasi-electrostatic behavior with almost pure Coulomb explosion. Corresponding signatures in the time-dependent cluster dynamics, as well as in the final ion and electron spectra, corroborate that this transition is linked to a crossover in the electron emission processes. The resulting signatures in the electron spectra are shown to be even more reliable for identifying the cluster expansion mechanisms than ion energy spectra. It is shown that the prevailing ionization mechanism and the dominant expansion process can be roughly estimated by a simple frustration parameter.