Exact determination of the phase in neutron reflectometry by variation of the surrounding media

Recently, it has been shown that the complex amplitude for neutrons specula rly reflected from a film can be determined exactly at all accessible wave- vector transfers, including those for which the Born approximation is inval id, through the use of buried reference layers of finite thickness. Given t he phase of reflection, it is then possible to solve the one-dimensional in verse-scattering problem directly, without resort to potentially ambiguous fitting procedures, to obtain a unique scattering length density depth prof ile for a laminar thin-film structure. Here we describe an extension of tha t phase determination method which utilizes controlled variations of the sc attering length density of the incident and/or substrate medium instead of reference layers of finite thickness. This technique is of practical import ance for thin-film systems involving either gas-liquid or solid-liquid inte rfaces in which the scattering length density of the liquid can be varied i n a known way, as in an aqueous media consisting of a mixture of H2O and D2 O. [S0163-1829(98)04447-6].