Investigation of molecular diffusion across organic multilayers using neutron specular reflectivity

Langmuir-Blodgett (LB) multilayers of fatty acid salt molecules have been i nvestigated by neutron specular reflectivity (NSR) at the reflectometer ADA M at the Institute Laue-Langevin in Grenoble, France. Due to the high flux at the sample site, the NSR curve could be recorded over more than six orde rs of magnitude, corresponding with q(z) less than or equal to 0.3 Angstrom (-1). Because of the incoherent hydrogen scattering, the detectable coheren t NSR curve is limited to 10(-5). The incoherent background of the multilay er systems could be determined for the first time by means of a spin-polari zed NSR experiment. The observed intermixing at freshly prepared LB multila yers increases during thermal annealing below the phase transition temperat ure, whereas the vertical structure remains. This is explained by thermally induced vertical reorganisation of molecules across the multilayer system. The molecular exchange can be described as an analogue to an order-disorde r transition. The evaluated activation energy of about 1.7 eV is related to the breaking of van der Waals bonds between one particular molecule and it s hexagonal neighbourhood. Time-resolved measurements at particular tempera tures allow us to estimate a vertical "diffusion constant" in the range of 10(-23) m(2)/s. (C) 1999 Elsevier Science S.A. All rights reserved.