Novel versatile X-ray reflectometer for angle and energy dispersive characterization of liquid and solid surfaces and interfaces

Abstract

The design and development of a highly flexible reflectometer which allows characterization of surfaces and interfaces of solids and liquids at grazing angles of incidence and exit is described. The reflectometer is capable of operating either in the angle dispersive or energy dispersive mode. In the angle dispersive mode the high resolution capability of the energy dispersive Ge detector is exploited to avoid the use of a monochromator crystal and also to keep the electronic noise extremely 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, the resolution of the instrument limits the determination of characteristic lengths to an upper value of 3000 ± 1 A ̊ . The angle dispersive mode allows measurements of the reflectivity of surfaces on an absolute scale, while the energy dispersive mode allows qualitative investigations of the time dependent structural changes close to the sample surface. Several examples to demonstrate the performance of the instrument in the two modes are presented: The presence of a thin adsorbed 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 Pb stearate multilayers on Si substrate is reported as an example for the energy dispersive mode.