Spatial Distributions of the Vapor Species in Alkali Halide Effusion Beams

Studies of the individual polymeric species in alkali halide effusion beams have shown that the monomer and dimer spatial distributions are not equivalent. Under molecular flow conditions, the spatial distribution of molecules emerging from an effusion channel must characterize to some extent the nature of the vapor–solid interactions that occur in the transport process. On the basis of a coupled surface- and vapor-transport model the experimental results can be reproduced quantitatively by assuming the nature of the surface interactions of monomer and dimer differ. The monomer spatial distribution is predictable for a species that is chemisorbed and characterized by a long mean free diffusion path in the adsorbed state. For the dimer, the distribution suggests a heterogeneous reaction sequence involving a dissociative adsorption and a reconstitution from adsorbed monomer.