MULTIFRAGMENTATION OF MOLECULES AND CLUSTERS

The multiple ionization and fragmentation of small polyatomic molecule s and clusters, e.g. H2O, CH4, and C-60 by fast H+, He+, and Oq+ ions were studied utilizing a position-and time-sensitive multiparticle det ector which allows the coincident measurement of the momenta of correl ated fragments. Thereby, a kinematically complete image of the molecul ar break-up can be derived for each individual event. Of special inter est are the ''Coulomb explosion'' processes, like H2O --> H++H++Oq+ wh ere the measured dissociation energies and angular correlations show s ignificant deviations from the pure Coulomb explosion model. For large r molecules and clusters, new phenomena may be expected. For example, using a classical molecular dynamics simulation, we have studied the b reak-up of hot Ar-n (n greater than or equal to 100) clusters. For ini tial temperatures around 500 K various analytical tools consistently i ndicate critical behaviour in the expansion phase.