Dynamics of rare gases in zeolites: Instantaneous normal mode analysis

The instantaneous normal mode (INM) spectra of rare gases in zeolites is analyzed with a view to understanding the short-time dynamical behavior of fluids adsorbed in confining media. Xenon adsorption in all-silica polymorphs of four zeolites (silicalite, mordenite, zeolite-A, and zeolite-Y) is studied using molecular dynamics and Monte Carlo simulations. The participation ratio distribution is shown to be a particularly good indicator of the extent of ballistic behavior in the short-time dynamics. The fraction of imaginary modes in the INM spectrum is shown to be correlated with the self-diffusion coefficient; however, a significant number of imaginary modes would appear to be due to the negative curvature of the confining potential rather than to the existence of barrier crossing motions. The Einstein frequency shows interesting temperature-dependent behavior which is sensitive to the structure and framework density of the zeolite. The gradual emergence of Liquidlike behavior with increasing concentration is reflected in both the participation ratio distribution and the harmonicity ratio; these features of the INM spectrum are therefore expected to be useful for indexing the modification of dynamical behavior of a fluid on confinement.