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München, Germany

Eckert M.,Deutsches Museum
Acta Crystallographica Section A: Foundations of Crystallography

The discovery of X-ray diffraction is reviewed from the perspective of the contemporary knowledge in 1912 about the nature of X-rays. Laues inspiration that led to the experiments by Friedrich and Knipping in Sommerfelds institute was based on erroneous expectations. The ensuing discoveries of the Braggs clarified the phenomenon (although they, too, emerged from dubious assumptions about the nature of X-rays). The early misapprehensions had no impact on the Nobel Prizes to Laue in 1914 and the Braggs in 1915; but when the prizes were finally awarded after the war, the circumstances of Laues discovery gave rise to repercussions. Many years later, they resulted in a dispute about the myths of origins of the community of crystallographers. Source

Eckert M.,Deutsches Museum
Annalen der Physik

The discovery of X-ray diffraction by crystals was a result of experiments about the interference of X-rays passing through crystals conducted by deposited by Arnold Sommerfeld on May 4, 1912. The guiding idea was that interferences arise in consequence of the space lattice structure of the crystals, because the lattice constants are ca. 10 times greater than the conjectured wavelengths of the X-rays. Sending a beam of X-rays through a crystal, and the regular three-dimensional arrangement of crystal atoms will sort out those that are seen in the Laue spots from the mixture of wavelengths in the primary beam by interference. Further studies showed that better success might be achieved by placing the plate behind the crystal, as for a transmission grating. The diffraction spots that surrounded the central spot of the primary beam could be explained as an interference pattern due to the crystal's space lattice, each spot was caused by X-rays that corresponded to a certain lattice constant and wavelength. Source

Eckerta M.,Deutsches Museum
European Physical Journal H

Sommerfeld's extension of Bohr's atomic model was motivated by the quest for a theory of the Zeeman and Stark effects. The crucial idea was that a spectral line is made up of coinciding frequencies which are decomposed in an applied field. In October 1914 Johannes Stark had published the results of his experimental investigation on the splitting of spectral lines in hydrogen (Balmer lines) in electric fields, which showed that the frequency of each Balmer line becomes decomposed into a multiplet of frequencies. The number of lines in such a decomposition grows with the index of the line in the Balmer series. Sommerfeld concluded from this observation that the quantization in Bohr's model had to be altered in order to allow for such decompositions. He outlined this idea in a lecture in winter 1914/15, but did not publish it. The First World War further delayed its elaboration. When Bohr published new results in autumn 1915, Sommerfeld finally developed his theory in a provisional form in two memoirs which he presented in December 1915 and January 1916 to the Bavarian Academy of Science. In July 1916 he published the refined version in the Annalen der Physik. The focus here is on the preliminary Academy memoirs whose rudimentary form is better suited for a historical approach to Sommerfeld's atomic theory than the finished Annalen-paper. This introductory essay reconstructs the historical context (mainly based on Sommerfeld's correspondence). It will become clear that the extension of Bohr's model did not emerge in a singular stroke of genius but resulted from an evolving process. © EDP Sciences, Springer-Verlag 2014. Source

Walch H.,Ludwig Maximilians University of Munich | Gutzler R.,Ludwig Maximilians University of Munich | Sirtl T.,Ludwig Maximilians University of Munich | Eder G.,Ludwig Maximilians University of Munich | Lackinger M.,Deutsches Museum
Journal of Physical Chemistry C

Adsorption of the brominated aromatic molecule 1,3,5-tris(4-bromophenyl) benzene on different metallic substrates, namely Cu(111), Ag(111), and Ag(110), has been studied by variable-temperature scanning tunneling microscopy (STM). Depending on substrate temperature, material, and crystallographic orientation, a surface-catalyzed dehalogenation reaction is observed. Deposition onto the catalytically more active substrates Cu(111) and Ag(110) held at room temperature leads to cleavage of carbon-bromine bonds and subsequent formation of protopolymers, i.e., radical metal coordination complexes and networks. However, upon deposition on Ag(111) no such reaction has been observed. Instead, various self-assembled ordered structures emerged, all based on intact molecules. Also sublimation onto either substrate held at ∼80 K did not result in any dehalogenation, thereby exemplifying the necessity of thermal activation. The observed differences in catalytic activity are explained by a combination of electronic and geometric effects. A mechanism is proposed, where initial charge transfer from substrate to adsorbate, followed by subsequent intramolecular charge transfer, facilitates C-Br bond homolysis. © 2010 American Chemical Society. Source

Eckert M.,Deutsches Museum
European Physical Journal: Special Topics

Sommerfeld was deeply interested in the nature of X-rays. In 1900 he concluded from diffraction experiments on slits that, if X-rays are electromagnetic radiation, their “impulse width” should be of the order of magnitude of the size of molecules. In 1905 Sommerfeld regarded it “a shame that after ten years after Röntgen’s discovery one still does not know what is going on with X-rays”. When he became Röntgen’s colleague a year later, he perceived this shame even more as a challenge. By that time it was discovered that X-rays come in two varieties. One sort of X-rays was independent of the anti-cathode material and could be explained by Sommerfeld in 1909 as “Bremsstrahlung”, i.e. as an electromagnetic radiation caused by the deceleration of electrons at their impact in the anti-cathode. The other part had the character of a fluorescent radiation. The “Bremsstrahlung” was polarized and displayed an angular distribution of intensity with a characteristic shape dependent on the energy of the electrons at the impact in the anti-cathode; the other part was unpolarized and characteristic for the material of the anti-cathode. Sommerfeld’s “Bremsstrahlen”-theory could not be elaborated without further assumptions about the impact of electrons in the anticathode. Sommerfeld closed his theory by a quantum hypothesis: He linked the time required to stop an electron, t, and the energy released in this process, E, to Planck’s quantum of action, h, via tE = h. This so-called h-hypothesis became the subject of Sommerfeld’s presentation at the first Solvay Conference. Although met with criticism, the quantum effort at Munich raised curiosity. Sommerfeld attempted to verify this hypothesis theoretically and experimentally in his institute with Walther Friedrich, his experimental assistant. Friedrich, a doctoral student from Röntgen’s institute, was persuaded however by Max Laue, then Sommerfeld’s Privatdozent, to perform another experiment which led to the discovery of X-ray diffraction in crystals. Their sensational result, obtained in spring 1912, was among the subjects for the second Solvay Conference in 1913. Sommerfeld regarded this experiment the most important achievement of his institute – and silently buried the h-hypothesis which turned out to be one of the dead-ends of the early quantum theory. © 2015 EDP Sciences, Springer-Verlag. Source

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