Model nuclear relaxation time of inhomogeneous fluids near a substrate: Liquid 3He in confined geometries or physisorbed molecular systems

Abstract

We extend a previous analysis for the temperature dependence of the longitudinal nuclear spin relaxation time T 1of pure liquid 3He in confined geometries to the case where the various walls are coated with a film of 4He. This leads us to consider relaxation not only of 3He spins on the walls as we did earlier, but also relaxation by coupling between the different layers of 3He of inhomogeneous structure. We characterize the inhomogeneous 3He as a sequence of layers of different densities and thus different static susceptibilities and different fluctuation frequencies. We suggest that the same idea might help to understand the behavior of T 1(T) for adsorbed molecular fluids of BET type III, such as NH3, where one adsorbed monolayer grows into islands of multilayers, for which we propose a model structure, varying with temperature, whose overall T 1 bears much resemblance to that of 3He near a wall. We believe that both cases illustrate the more general behavior of T 1(T) in an inhomogeneous fluid; here the inhomogeneous density has its source in the attraction to the substrate, but any density gradient could in principle yield the same effect. Our phenomenological model is compared with existing data and further experiments are suggested.