DOUBLE CONICAL BEAM-ROCKING SYSTEM FOR MEASUREMENT OF INTEGRATED ELECTRON-DIFFRACTION INTENSITIES

The performance of a scanning system for a transmission electron micro scope is described, where the focused beam is scanned at a constant an gle around the optic axis. Below the specimen, the beam is de-scanned, the net effect being equivalent to precessing the specimen around a s tationary beam focused to a probe size of 0.1 mu m for typical precess ion and convergence angles. Unlike standard convergent beam electron d iffraction (CBED) patterns aligned on a zone axis, precessed patterns include many reflections intercepted by the Ewald sphere, where the di ffracted intensities are determined by integration through the Bragg c ondition. The double conical scanning system was designed to collect a large data set of integrated intensities that are more suitable for s tructure determination by electron diffraction, both by removing excit ation errors due to curvature of the Ewald sphere and also by reducing non-systematic dynamical effects. Experimental patterns obtained from the Si{110} axis are discussed, followed by analysis of the intensiti es diffracted into the higher order Laue zones of a rare earth pyroger manate (Er2Ge2O7) with a large unit cell. The scanning system is equal ly suitable for collection of precessed spot patterns from radiation-s ensitive specimens such as organic crystals.