Timehyphen;resolved dynamics of cluster isomerization

摘要

In this paper we report on the timehyphen;resolved dynamics of the cube rarr; ring isomerization of the Na4Cl4cluster, which was interrogated by constant energy molecular dynamics simulations. The isomerization was induced by several excitation modes of the nuclear motion, i.e., nonselective, bond selective, ion selective, and normalhyphen;mode selective vibrational excitations. The nuclear excitation was conducted from a cluster equilibrated state at 600 K (total vibrational energyEv=7930 cmminus;1) to total energies in the rangeEv=10thinsp;610 cmminus;1(cube temperatureT=800 K) toEv=30thinsp;730 cmminus;1(T=2300 K). The reaction rates for isomerization were initially obtained from the mean first passage times for the ring formation. Concurrently, we have simulated the time evolution of the concentrations of the cube, ladder, and ring isomers by the thermal quenching method. From the timehyphen;dependent concentrations for nonselective excitation, we have obtained theEvdependent four rate constants for the isomerization scheme cube rlarr2; ladder rlarr2; ring, establishing the relations between the results of the first passage time calculations and the detailed kinetic analysis. The rates in the energy domainEv20thinsp;000 cmminus;1(T1500 K) exhibit no appreciable dependence on the initial excitation mode, and deviations from statistical behavior are negligible. We have also explored the intracluster vibrational energy redistribution (IVR) times, their dependence on the excitation mode and on the excitation energy. In the energy domainEv20thinsp;000 cmminus;1(T1500 K), the separation of the time scales between fast IVR and slow isomerization is applicable, whereupon the kinetics exhibits a statistical behavior. This conclusion is compatible with the vibrational level structure of cubic Na4Cl4, where no frequency mismatch prevails. Deviations from statistical behavior are manifested by the breakdown of the conventional kinetic scheme at high energies (Ev26thinsp;000 cmminus;1), when both the IVR and the isomerization time scales approach their limiting values of a vibrational period.