Imaginary potential for electrons of CaF2 from a Bloch-wave analysis

In a previous analysis of Bloch waves in electron diffraction experiments with a CaF2 single-crystal wedge absorption coefficients of the strong Bloch waves were determined. The experiments were performed with two types of crystal wedges, produced by (111) cleavage faces. One type had a wedge angle of 1090 and the other type an angle of 710. In the experiment with the blunt wedge the direction of incidence was close to [100] and in the sharp-wedge experiment close to [110]. The two sets of absorption coefficients were not consistent with an imaginary potential describing the absorption. From reexamination of the data a consistent absorption potential, expressed by an analytical formula, could be determined, which confirmed the absorption coefficients for the blunt wedge and modified the former values for the sharp wedge. Agreement between calculation and experiment could be achieved by convoluting the diffracted beams with the profile of the incident beam. This effect is important for the sharp wedge and negligible for the blunt wedge. (orig.)