Theory for the ultrafast melting and fragmentation dynamics of small clusters after femtosecond ionization

The ultrafast relaxation of small clusters immediately after ultrashort single ionization is studied. We use an electronic model and molecular dynamics simulations to:analyze the ionization induced fragmentation 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 probability of a cluster ensemble as a function of the temperature before ionization. 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 dependence of the average fragmentation times after ionization shows a remarkable correlation with the root-mean-square bond length fluctuations delta(T) of the Clusters before ionization. This new effect reflects sensitivity of the ultrashort-ionization induced fragmentation dynamics to the melting dynamics of the neutral clusters. (C) 1998 American Institute of Physics.