X‐ray reflectivity from curved liquid interfaces

• Published in: Journal of synchrotron radiation 2018-03, Vol.25 (2), p.432-438
• Main Authors: Festersen, Sven, Hrkac, Stjepan B., Koops, Christian T., Runge, Benjamin, Dane, Thomas, Murphy, Bridget M., Magnussen, Olaf M.
• Format: Article
• Language: English
• Subjects: Beams (radiation); Curvature; Diffractometers; Feasibility studies; free‐elecron laser; Incidence angle; liquid interfaces; methods; Physics; Reflectance; synchrotron; Vapors; X‐ray reflectivity
• ISSN: 1600-5775; 0909-0495; 1600-5775
• DOI: 10.1107/S1600577517018057

X‐ray reflectivity studies of the structure of liquid–vapour and liquid–liquid interfaces at modern sources, such as free‐electron lasers, are currently impeded by the lack of dedicated liquid surface diffractometers. It is shown that this obstacle can be overcome by an alternative experimental approach that uses the natural curvature of a liquid drop for variation of the angle of incidence. Two modes of operation are shown: (i) sequential reflectivity measurements by a nanometre beam and (ii) parallel acquisition of large ranges of a reflectivity curve by micrometre beams. The feasibility of the two methods is demonstrated by studies of the Hg/vapour, H2O/vapour and Hg/0.1 M NaF interface. The obtained reflectivity curves match the data obtained by conventional techniques up to 5αc in micro‐beam mode and up to 35αc in nano‐beam mode, allowing observation of the Hg layering peak. This paper reports the successful implementation of a method that enables the measurement of liquid–liquid and liquid–vapour X‐ray reflectivity (XRR) curves using simply a fixed, horizontal source beam, a vertical translation stage and a two‐dimensional detector. It will enable studies of liquid interfaces at sources lacking liquid diffractometers, especially ultra‐fast XRR measurements via pump–probe techniques at X‐ray free‐electron lasers.