Ulf Lorenz

Structural Evolution of Colloidal Crystal Films in the Process of Melting Revealed by Bragg Peak Analysis

In situ X-ray diffraction studies of structural evolution of colloidal crystal films formed by polystyrene spherical particles upon incremental heating are reported. The Bragg peak parameters, such as peak position, integrated intensity, and radial and azimuthal widths were analyzed as a function of temperature. A quantitative study of colloidal crystal lattice distortions and mosaic spread as a function of temperature was carried out using Williamson-Hall plots based on mosaic block model. The temperature dependence of the diameter of polystyrene particles was obtained from the analysis of Bragg peaks, and the form factor contribution extracted from the diffraction patterns. Four stages of structural evolution in a colloidal crystal upon heating were identified. Based on this analysis, a model of the heating and melting process in the colloidal crystal film is suggested.

Statistical properties of a free-electron laser revealed by Hanbury Brown–Twiss interferometry

We present a comprehensive experimental analysis of statistical properties of the self-amplified spontaneous emission (SASE) free-electron laser (FEL) FLASH at DESY in Hamburg by means of Hanbury Brown and Twiss (HBT) interferometry. The experiments were performed at the FEL wavelengths of 5.5 nm, 13.4 nm, and 20.8 nm. We determined the 2-nd order intensity correlation function for all wavelengths and different operation conditions of FLASH. In all experiments a high degree of spatial coherence (above 50%) was obtained. Our analysis performed in spatial and spectral domains provided us with the independent measurements of an average pulse duration of the FEL that were below 60 fs. To explain complicated behaviour of the 2-nd order intensity correlation function we developed advanced theoretical model that includes the presence of multiple beams and external positional jitter of the FEL pulses. By this analysis we determined that in most experiments several beams were present in radiating field and in one of the experiments external positional jitter was about 25% of the beam size. We envision that methods developed in our study will be used widely for analysis and diagnostics of the FEL radiation.

Double hexagonal close-packed structure revealed in a single colloidal crystal grain by Bragg rod analysis

A coherent X-ray diffraction study of a single colloidal crystal grain composed of silica spheres is reported. The diffraction data contain Bragg peaks and additional features in the form of Bragg rods, which are related to the stacking of the hexagonally close-packed layers. The profile of the Bragg rod shows distinct intensity modulations which, under the specific experimental conditions used here, are directly related to the stacking sequence of the layers. Using a model for the scattered intensity along the Bragg rod for an exact stacking sequence of a finite number of hexagonally close-packed layers, it is found that a double hexagonal close-packed stacking sequence is present in the colloidal crystal grain. This analysis method opens up ways to obtain crucial structural information from finite-sized crystalline samples by employing advanced third-generation X-ray sources.

Revealing Three-Dimensional Structure of an Individual Colloidal Crystal Grain by Coherent X-Ray Diffractive Imaging

We present results of a coherent x-ray diffractive imaging experiment performed on a single colloidal crystal grain. The full three-dimensional (3D) reciprocal space map measured by an azimuthal rotational scan contained several orders of Bragg reflections together with the coherent interference signal between them. Applying the iterative phase retrieval approach, the 3D structure of the crystal grain was reconstructed and positions of individual colloidal particles were resolved. As a result, an exact stacking sequence of hexagonal close-packed layers including planar and linear defects were identified.

In situ X-ray crystallographic study of the structural evolution of colloidal crystals upon heating

The results of a real-time X-ray crystallographic study of the melting transition of polystyrene colloidal crystals during heating are presented.