Two-state approximation for aggregate states of clusters

The concepts of the phase and change of state for clusters are analyzed. The connection between a phase or state change and the thermodynamic parameters 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 cluster. The model allows one to estimate parameters of clusters and bulk in the range of the phase or state change. The approach has sufficient generality to be applicable not only to order-disorder changes and evaporation but 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 melting. (C) 2001 American Institute of Physics.