Kinematic and dynamical CBED for solving thin organic films at low temperature; experimental tests with anthracene

Low-dose, low-temperature kinematic and dynamical convergent-beam electron diffraction (CBED) patterns from thin organic crystalline films have been used for the measurement of structure-factor amplitudes and phases. Kinematic conditions are identified by the observation of uniform intensity within the CBED discs and used to determine structure-factor magnitudes. CBED patterns from thicker regions affected by multiple scattering give structure-factor signs, which are varied for best fit. The use of a small probe (and the Kohler SAD mode) minimizes bending artifacts. A new method of thickness determination is evaluated. The approach is tested using experimental data from the centrosymmetric anthracene structure, the results compared with direct methods, and a potential map derived from experimental data. The faint peaks due to H-atom positions may be distinguished. Key issues influencing the validity of the method such as the appropriate dimension of the structure-factor matrix, sample thickness and crystal orientation are discussed.