Detection and characterization of ultra-thin films with neutron reflectometry

Specular reflectometry, being a technique based on interference between coherent X-ray or neutron beams, is considered to have a fundamental limit in sensing the presence of films that are too thin for the maximum momentum transfer, Q(max), to which reflectivity has been measured. However, it is known both experimentally and from simulations that an ultra-thin film, with thickness t << 2pi/Q(max), can be detected if it exists sandwiched between two contrast-matched media. This possibility is qualitatively explained using phase-vector diagrams. The diagrams also show that the detection is through unmistakable shifts of the interference maxima and minima, and that the scattering-length density of the ultra-thin film determined by least-squares analysis is unique.