THEORY FOR THE ULTRAFAST MELTING AND FRAGMENTATION DYNAMICS OF SMALL CLUSTERS AFTER FEMTOSECOND IONIZATION

The ultrafast relaxation of small clusters immediately after ultrashor t single ionization is studied. We use an electronic model and molecul ar dynamics simulations to:analyze the ionization induced fragmentatio n dynamics of small Hg, clusters. Our results show that fragmentation consists mainly in emission of single atoms. We also find a remarkable . difference between this nonequilibrium unimolecular dissociation and that described by the Rice-Ramsperger-Kassel-Marcus theory. For fixed cluster size, we determine the time-dependent fragmentation probabili ty of a cluster ensemble as a function of the temperature before ioniz ation. A dramatic change of the fragmentation behavior occurs when the temperature;before ionization reaches the ''melting temperature'' of the neutral clusters. For all cluster sizes studied, the temperature d ependence of the average fragmentation times after ionization shows a remarkable correlation with the root-mean-square bond length fluctuati ons delta(T) of the Clusters before ionization. This new effect reflec ts sensitivity of the ultrashort-ionization induced fragmentation dyna mics to the melting dynamics of the neutral clusters. (C) 1998 America n Institute of Physics.