Phase transition behavior observed in small silicon cluster ions

Small silicon clusters produced by laser vaporization supersonic expansion are heated to different internal temperatures during a subsequent photoionization step, depending upon the ionization laser wavelength. Following ionization and mass selection the cluster is photofragmented by a 532 nm pulse from a Nd:YAG laser, and the charged fragments are collected and mass analyzed by a time-of-light mass spectrometer. It is found that the fragmentation patterns for clusters larger than Si+10 have a dependence upon the cluster temperature. This dependence is ascribed to clusters which are fairly rigid structures at low temperatures, undergoing a transition to a fluxional state at higher temperatures. Such a transition is analogous to a phase transition in larger systems.