Confined Clusters of Rare Gas Atoms: Structures and Phases

Abstract

Classical-mechanical isoenergetic molecular dynamics simulations model Xe atoms and XeN clusters trapped in a cage of atomic dimensions, a homogeneous spherical cavity with Morse interaction between its walls and the rare gas atoms. The main interests are the differences in structures and dynamics of rare gas clusters confined and in free space. As the total energy increases, the confined rare gas clusters undergo a distinct phase change from a quasi-two-dimensional fluid to a gadabsorbate exchange fluid. The size of the confined rare gas clusters plays an important role in this transition. The slowest-slide and eigenvector-following methods are used to investigate the saddles of the multidimensional potential surfaces of the clusters. Understanding the dynamical behavior of adsorbents in microporous solids has been an ever-growing research field. Due to the combination of their inertness and sensitivity of their NMR spectra to environment, 129Xe atoms serve as good probes to investigate the properties of molecular sieves. The atomic dimensions of some of the tunnels connecting the cavities confined adsorbates larger than a certain size to a single cavity. Several experiments’s2 indicate that the Xe clusters may be formed under high loading conditions. If the influence of the Xe atoms in adjacent cavities is negligible, the dynamics of the Xe clusters inside zeolite cavities can be studied by extending the methods used for rare gas clusters in free space3-’ which have been well developed and can be easily adapted to study these systems, Simulation ~tudies~-’~ of clusters in realistic cavities have given insight into the behavior of trapped atoms and clusters. Here, we simplify the cavity to study the relation between the cluster dynamics and the cavity structure parameters. An important characteristic of clusters is the multiplicity of locally stable structures they exhibit or, in words aptly borrowed from hydrology, the multiplicity of “catchment basins” that occur on their potential energy surfaces. Each of these regions has an associated minimum-energy configuration for the cluster. The essence of this study is to investigate the relation between the thermodynamic and dynamical properties of occluded clusters, in terms of the topography of their underlying potential surfaces. Our approach was first to determine the stationary points, notably minima and saddles, on the potential surfaces, identify their connectivity, and then relate the observed dynamics to these structure features. Because of the interaction between the rare gas atoms and cavity walls, there are several features which make this system different from the rare gas clusters in free space. The angular momentum of the cluster around its center of mass is no longer conserved, and therefore the rotational motion influences the intrawell dynamics. The linear momentum of the cluster is also not conserved because the center of mass of the cluster may move around the cavity. Here, we use isoenergetic molecular dynamics simulations with the velocity version of the Verlet methodI4 to study the dynamics of different numbers of Xe atoms inside a single cavity at various energies. We will see how the atomic dimensions of the cavities influence the dynamics of the rare gas clusters.