ABSTRACT
The study of liquid-liquid interfaces requires special instrumental considerations. In the case of a liquid surface diffractometer, it is necessary that the instrument tilts the beam with respect to the sample. This geometry may be easily realized on a laboratory source by moving the anode [35], but often the investigations possible are limited due to a lack of intensity. Synchrotron X-ray sources can provide orders of magnitude of intensity more, and hence are very attractive for studying weakly scattering systems such as liquids. However, the experimental geometry is more complicated since it is necessary to deflect the beam down, which requires the use of a deflecting X-ray optic and some careful experimental considerations. Early designs by Als-Nielsen and Pershan [36] achieved this by using a setup similar to a triple-axis neutron spectrometer. Many specialized designs for synchrotron instruments since then have used a single crystal to tilt the incoming X-ray beam down onto the sample (including those described in references [7,37-40]). In this arrangement, the beam moves on the surface of a cone and therefore both the sample and the detector have to be moved vertically and horizontally to follow the beam. These movements can involve up to six motors. This design works very well, but since the sample height has to be corrected for each new angle of incidence, a waiting time is required to allow vibrations at the liquid sample surface to decay before data collection can continue (see Fig. 9.4). Additionally, it is essential to check and correct for tracking errors after a careful alignment. 9.6.2 High-Energy Liquid Diffractometer
In a novel development suitable for liquid reflectivity at high energies, a double-crystal tilt stage has been implemented in an instrument at the ESRF [41]. Using a matching pair of deflecting crystals in transmission Laue geometry usually at 71 keV, it is possible to tilt the incident beam without moving the sample. The setup provides very accurate angular movement-a key requirement for working at high energies. Unfortunately, Laue geometries are not feasible at lower energies due to lack of transmission through the crystals.
