Two-state approximation for aggregate states of clusters

The concepts of the phase and change of state for clusters are analyzed. Th e connection between a phase or state change and the thermodynamic paramete rs of a cluster is analyzed within the framework of a two-state model. This model aggregates the excited configurational states of each state of the c luster. The model allows one to estimate parameters of clusters and bulk in the range of the phase or state change. The approach has sufficient genera lity to be applicable not only to order-disorder changes and evaporation bu t also to such processes as ionization and vibronic (electronic-to-thermal) energy conversion. The last of these raises a question regarding one step in the interpretation of some recent experimental studies of cluster meltin g. (C) 2001 American Institute of Physics.