ABSTRACT: The beamline P08 at the PETRA III is dedicated to surface scattering. In additional to its six-circle diffractometer for studies on solid substrates, it hosts two instruments for scattering experiments on liquid surfaces and interfaces: the Langmuir trough Grazing Incidence Diffraction (GID) setup [1] as a bread-and-butter instrument for studies on a Langmuir trough, and the Liquid Interface Scattering Apparatus (LISA) from the University of Kiel for general sample environments. This talk will focus on the Langmuir trough GID setup. This fixed incidence setup is optimised for ultralow background grazing incidence diffraction (GID), small angle scattering (GISAXS) and X-ray fluorescence (GIXRF) measurements, and pseudo reflectivity acquisition [2]. I will specifically talk about the pseudo reflectivity method. It allows one to acquire a reflectivity curve with 10^13 dynamic range from liquid surfaces via one single GISAXS exposure, and hence eliminates the demand of varying incidence for layer structure acquisition. It is based on an extension of the capillary wave model (eCWM). This eCWM includes the effect of the bending rigidity in the thermal roughness factor of the scattering cross section, in addition to the standard capillary wave contributions. Hence it is applicable not only to simple liquid interfaces but also to the liquid interfaces with thin films. The theory provides a single analytical expression to describe both the specular reflection from a liquid interface and the diffuse scattering around it. The single expression for both implies that the reflectivity can be analytically derived from diffuse scattering signal, e.g. measured via GISAXS. Our experimental result (from 12ID, NSLS-II) evidences that this derived reflectivity, namely pseudo reflectivity, agrees with the reflectivity measured from conventional specular reflectometry.

References
[1] Shen, C. et al., F. A grazing incidence diffraction setup for Langmuir trough experiments at the high-resolution diffraction beamline P08 at PETRA III. Journal of Physics: Conference Series 2380 (2022).
[2] Shen, C. et al. Reconstructing the reflectivity of liquid surfaces from grazing incidence X-ray off-specular scattering data. J Appl Crystallogr 57 (2024).