Time-resolved dynamics of cluster isomerization

In this paper we report on the time-resolved dynamics of the cube → ring isomerization of the Na4Cl4 cluster, 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 normal-mode selective vibrational excitations. The nuclear excitation was conducted from a cluster equilibrated state at 600 K (total vibrational energy Ev=7930 cm−1 ) to total energies in the range Ev=10 610 cm−1 (cube temperature T=800 K) to Ev=30 730 cm−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 time-dependent concentrations for nonselective excitation, we have obtained the Ev dependent four rate constants for the isomerization scheme cube ⇄ ladder ⇄ ring, establishing the relations between the results of the first passage time calculations and the detailed kinetic analysis. The rates in the energy domain Ev<20 000 cm−1 (T<1500 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 domain Ev<20 000 cm−1 (T<1500 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 (Ev>26 000 cm−1 ), when both the IVR and the isomerization time scales approach their limiting values of a vibrational period.