Examples of electrostatic electron optics: The Farrand and Elektros microscopes and electron mirrors☆
Introduction
Sporadic attempts have been made to build electron microscopes with electrostatic lenses, with commercial exploitation in mind. Since electrostatic lenses turn into electron mirrors if the potential barrier is high enough, the idea of constructing an electron microscope incorporating an electron mirror soon followed. As we shall see below, the AEG Forschungsinstitut in Berlin was a pioneer in this area, as we might expect since their earliest work was concerned with a related field, electron emission microscopy. In the early years, it was not clear whether magnetic or electrostatic lenses would dominate electron microscope design and among those who believed in the future of the electrostatic electron microscope was C.L. Farrand. Gertrude Rempfer and her husband were deeply involved in Farrand's project and, in Section 2 of this article, the story of the Farrand instrument is recounted; a few words on the Elektros microscope conclude this part.
In the third part, we turn to the optics of electron mirrors, for this was a major preoccupation of Gertrude Rempfer after she left Farrand and moved to Portland. The optics of mirrors is not a straightforward extension of the optics of lenses since one of the assumptions on which the derivation of the paraxial equation of motion is based is no longer valid: at the turning point, where the axial component of the electron velocity falls to zero and changes sign, the gradient of the rays is no longer small. Some other form of equation than the familiar paraxial ray equation must be found. The various ways of achieving this and hence of establishing the corresponding aberration integrals are listed in Section 3.
Electron mirrors are not covered by Scherzer's “theorem” and can hence be used to correct the spherical and chromatic aberrations of round lenses. Section 4 is devoted to this approach to aberration correction, which was another of Gertrude Rempfer's interests
Finally, we provide some pointers for future historians of the mirror electron microscope, both the laboratory instruments that go back to the Mahl–Pendzich design of 1943 and the rare attempts to market commercial models, notably by Litton Instruments in the USA and by JEOL, who launched their JEM–M1 model in 1968. Although a number of reviews of mirror microscopy have appeared, none of these mentions all the different projects (and no doubt I too have overlooked some). I hope that this arid bibliography of the subject may provide a starting point for a thorough re-examination of this somewhat neglected subject. The most recent wide-ranging review is that published by Luk'yanov and Spivak in 1973!
Section snippets
The electrostatic microscopes of the Farrand Optical Company and of Elektros
By far the most detailed account of the Farrand venture has been prepared by John Reisner [1] and the following paragraphs are based upon it. In 1944, C.L. Farrand launched an electron microscope project and, soon after, he retained the services of Reinhold Rüdenberg as consultant, after helping the latter to recover his patent rights. In 1945, however, the two men working on the electron microscope were “transferred to defense-related projects to protect them from the draft”; this left
Early mirror optics
As soon as the modes of action of electrostatic lenses, and especially of einzel lenses, were understood, it became obvious that, if the potential barrier at the lens was high enough, the latter would behave as an electron mirror: incident electrons would be reflected and return towards the source. The short paper by Henneberg and Recknagel [10] is the first to discuss the relation between the lens and mirror action of a lens and this was soon followed by papers by Recknagel [11], [12] and in
Mirrors as aberration correctors
The early calculations on the aberrations of mirrors had shown that such elements could in principle be used to circumvent Scherzer's result of 1936, to the effect that the chromatic and spherical aberrations of conventional round lenses cannot be eliminated by ingenious lens design. This was appreciated by Zworykin et al. [71], who devoted several pages of their influential treatise to mirrors and mirror correctors (see pp. 564–570, 575–577, 630–631, and 643–645). The difficulty was always to
Mirror electron microscopes
We have seen that such a microscope was constructed by Mahl and Pendzich in the wartime years [19] and a new mirror microscope was described by Orthuber in 1948 [131], again constructed in the AEG Research Institute, though the Mahl–Pendzich instrument is not referred to. In the 1950s and 1960s, such microscopes attracted considerable interest and two configurations were studied. In one, the optic axis was straight and the reflected electrons occupied the same narrow region as the incident
Acknowledgments
I am greatly indebted to several librarians and colleagues, who have enabled me to complete elusive references. In particular, my thanks go to Madame Josette Come–Garry of the Collège de France, Frau Katrin Quetting of the Fritz-Haber-Institut der MPG in Berlin and her colleagues in Halle, The Auskunftsteam at the University of Potsdam, Ms Nevenka Huntic of the Rayleigh Library in Cambridge, Ms Lilianna Nalewajska of the Warsaw University Library, Professors Seitkerim Bimurzaev, Evgeniy
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Cited by (5)
NanoMi: An open source electron microscope hardware and software platform
2022, MicronCitation Excerpt :Table 1 summarizes the advantages and drawbacks of electrostatic, magnetic coil-excited and permanent magnet-excited magnetic lenses. For a low-voltage (U0 ≤50 kV) microscope with modest resolution where easy build process is a critical requirement, electrostatic einzel lenses appear to provide a suitable solution (Liebl, 2007; Rempfer, 1985; Hawkes and Kasper, 1994; Rose, 2009; Baranova and Yavor, 1989; Hawkes, 2012). Similarly, deflectors and stigmators utilizing electric fields are relatively simple to implement.
Flat electron mirror
2021, UltramicroscopyCitation Excerpt :When the mirror is positioned in the diffraction plane of an imaging system, as is the case for grating mirrors for QEM, the high aberration coefficients will result in loss of resolution at the image plane. In an electron resonator system, a second electron mirror that provides a concave reflection field can correct for these aberrations [12–15] when positioned in the conjugate plane of the flat mirror. A schematic design of such system [7,16] is shown in [ Fig. 1].
Principles of Electron Optics: Second Edition
2017, Principles of Electron Optics: Second EditionPrinciples of Electron Optics, Volume 1: Basic Geometrical Optics
2017, Principles of Electron Optics, Volume 1: Basic Geometrical Optics
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A tribute to the late Gertrude Rempfer.