Metallic-like droplets produced by irradiating rare-gas clusters with free electron laser pulses

Unlike metallic elements, rare-gas atoms remain nonmetallic even if they are condensed. However, once rare-gas clusters are irradiated by extreme ultraviolet free electron laser (EUV-FEL) pulses with slightly higher energy than their ionization potential, they exhibit metallic-like behaviors because the ionization takes place sequentially within the clusters owing to the huge valence electron ionization cross section in the EUV regime. In this work Ar and Xe clusters were produced by pulsed supersonic jets, and the cluster beams were crossed synchronously with focused EUV-FEL beams ejected from the SCSS test accelerator in Japan. We measured time-of-flight (TOF) mass spectra and kinetic energy distribution (KED) of daughter ions produced via Coulomb explosion by using three dimensional (3D) momentum imaging spectrometer. The metallic-like nature is evidenced not only by widely spread charge distributions within the cluster but also by enhanced positive surface charges surrounded by quasi-free electron clouds.