Calculation Of Neutron Spectra For Hydrogen In Zeolites: Rotational Motions And Transnational Motions In The Born-Oppenheimer Limit

Amy Lisa Graves, Swarthmore College
G. J. Martyna

Abstract

Inelastically scattered neutrons can probe the location and dynamical behavior of molecular hydrogen absorbed within zeolites. Hydrogen may, at certain temperatures, display mixed quantum/classical dynamics associated with its rotational and center-of-mass (CM) motions. When hydrogen diffusion occurs within a zeolite containing a unique adsorptive site, the spectrum is a simple convolution of (quantum) rotational and (classical) CM contributions. We present a jump diffusion model with a faujasite-type host which illustrates this behavior. A more general case would be a host which induces a spatially-dependent distribution of rotational frequencies. In order to address this more general situation, we present a formalism in which the standard time correlation function for neutron intensity: [formula] is approximated in the “Born–Oppenheimer” limit. A preliminary test of this formalism is performed for jump diffusion within a one-dimensional model system.