Origin of the complex dynamics in structural isomerization of small clusters: The effects of potential topography

Abstract

The dependence of lifetime distribution in isomerization dynamics of Ar7-like clusters on the potential topography is reported. Using the scaled Morse potential V=∑i<j(e−2(ρij−ρ0)−2e−(ρij−ρ0)), where ρij is an internuclear distance, and ρ0, the equilibrium distance of the isolated diatomic molecule, is the only independent parameter that uniquely specifies the system Hamiltonian, we have examined the dynamics for two typical values of ρ0. In the high enough energy region, which is called the liquidlike phase, the cluster of a small ρ0 (called a compact cluster) has an exponential form for the lifetime distribution. In contrast, dynamics on the potential with a large ρ0 (a loose cluster) exhibits a large deviation from the exponential form, its mixing dynamics notwithstanding; it displays a large hole in the short lifetime region and a steep clifflike structure in the even shorter lifetime region embedded in the hole. Since the deviation is so clear, the present system offers an excellent example with which to investigate the dependence of phase space structure on the potential topography. We have identified the mechanisms of formation both of the hole and cliff. The hole is particularly important to understand how mixing dynamics can deviate from the statistically simple exponential distribution. By comparing a variety of quantities that characterize the system dynamics, we propose a conceptual picture to understand the basic difference between the dynamics on compact and loose potentials, in which the complexity of the dynamics is ascribed to the bifurcation of the reaction tubes and to the turning-point distributions clinging to the branching places of such bifurcated tubes.