50 years of electron biprism -50 years of exciting electron physics-

Abstract

In 1953, Gottfried Moellenstedt invented the electron biprism, more or less by accident, with the serendipity characteristic to him: Aiming at dark-field imaging in an electron microscope, he stretched a thin tungsten wire across the objective aperture to block off the zero beam. However, instead of a dark field image, because of inadvertent charging of the wire under the beam, he found two images of the ZnO-needles serving as an object. Instead of trying the dark field imaging over and over again, he asked what the effect would be if the obtained two images were superimposed. Is coherence given? Moellenstedt, educated by Walther Kossel in diffraction of electron waves, had a vision: Together with his PhD-student Heinrich Dueker, he developed the electron biprism, consisting of a 1µm thin wire deliberately chargeable by means of a voltage source, as a beam splitter for coherent superposition of the electron waves passing the filament on the right and on the left. In 1955 they published the first results showing biprism interference fringes [G. Moellenstedt and H. Dueker, Naturwiss. 42, 41 (1955)], which, since then, gave access to the understanding of electron waves and their use for analysis of object structures up to atomic dimensions.