NOVEL VERSATILE X-RAY REFLECTOMETER FOR ANGLE AND ENERGY-DISPERSIVE CHARACTERIZATION OF LIQUID AND SOLID-SURFACES AND INTERFACES

The design and development of a highly flexible reflectometer which al lows characterization of surfaces and interfaces of solids and liquids at grazing angles of incidence and exit is described. The reflectomet er is capable of operating either in the angle dispersive or energy di spersive mode. In the angle dispersive mode the high resolution capabi lity of the energy dispersive Ge detector is exploited to avoid the us e of a monochromator crystal and also to keep the electronic noise ext remely low: These factors allow the reflectivity to be measured over a dynamic range of up to 10(8). X-ray wavelengths are exchanged without any new alignment of the set-up. In the case of layered structures, t he resolution of the instrument limits the determination of characteri stic lengths to an upper value of 3000 +/- 1 Angstrom. The angle dispe rsive mode allows measurements of the reflectivity of surfaces on an a bsolute scale, while the energy dispersive mode allows qualitative inv estigations of the time dependent structural changes close to the samp le surface. Several examples to demonstrate the performance of the ins trument in the two modes are presented: The presence of a thin adsorbe d water layer on the surface of a polished, hydrophilic (100) Si wafer and the formation of a monolayer of surfactant lipid molecules at the air/water interface have been demonstrated using the angle dispersive mode. The time resolved measurement of the structural transition of P b stearate multilayers on Si substrate is reported as an example for t he energy dispersive mode.