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James Franck, the ionization potential of helium, and the experimental discovery of metastable states
Abstract
In 1920, James Franck together with Fritz Reiche and Paul Knipping found strong experimental evidence that the lowest-lying triplet state in helium is metastable—an atom in this state cannot make a spontaneous transition to the ground state. Even though their evidence was entirely experimental, they tied their results almost inextricably to Alfred Landé’s 1919 model of the helium atom, and in the process, misunderstood the new theoretical selection rules of Adalbert Rubinowicz and Niels Bohr. In an additional complication, experiments of the English physicists Frank Horton and Ann Catherine Davies contradicted Franck's. Although Franck's result has held up, the reasons for the discrepancies remain unclear.
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A table of energy levels based on the most accurate observations now available is given for the neutral 4He atom. The wavelengths contained in a revised list of 4He I lines from 320 A to 21132 A are also based on the best measurements, over half of them having been calculated from the wavenumber separations of appropriate levels. Several previously disturbing features of the 4He term scheme are obviated by the revised level values. In a discussion of the experimental results for this spectrum some comparisons with theory are made.
In 1911, James Franck and Gustav Hertz began a collaboration to investigate the nature of collisions of slow electrons with gas molecules that led to a series of carefully planned and executed experiments, culminating in their discovery of inelastic collisions of electrons with mercury vapor atoms in 1914. This paper tells the story of their collaboration and the eventual reinterpretation of their results as a confirmation of Niels Bohr's new atomic theory, largely as a result of experiments done in North America during the Great War.
Apart from hydrogen, helium is the most abundant chemical element in the universe, and yet it was only discovered on the Earth in 1895. Its early history is unique because it encompasses astronomy as well as chemistry, two sciences which the spectroscope brought into contact during the second half of the nineteenth century. In the modest form of a yellow spectral line known as D3, ‘helium’ was sometimes supposed to exist in the Sun's atmosphere, an idea which is traditionally ascribed to J. Norman Lockyer. Did Lockyer discover helium as a solar element? How was the suggestion received by chemists, physicists and astronomers in the period until the spring of 1895, when William Ramsay serendipitously found the gas in uranium minerals? The hypothetical element helium was fairly well known, yet Ramsay's discovery owed little or nothing to Lockyer's solar element. Indeed, for a brief while it was thought that the two elements might be different. The complex story of how helium became established as both a solar and terrestrial element involves precise observations as well as airy speculations. It is a story that is unique among the discovery histories of the chemical elements.
Um das Serienspektrum des Heliums und der Alkalien dynamisch zu erklären, braucht man sich mit Rücksicht auf die Bohrsche Frequenzbedingung v = E
n
- E
n
/h nut mit den Energiewerten E
n
zu beschäftigen, die ein Außenelektron (Valenzelektron) auf seiner n-quantigen Bahn um die von Z - I Innenelektronen dicht umgebene Kernladung +Ze besitzt. Kern und inhere Elektronen wirken auf das äußere wie ein punktförmiger Kern der Ladung +e(I + x), worin x aus der Wechselwirkung der Elektronen zu berechnen ist. In Anbetracht der Schnelligkeit des Elektronenumlaufes ersetzte A. Sommerfeld1) die Wechselwirkung zwischen inneren und äiußeren Elektronen näiherungsweise durch die Wirkung, welche zwischen den konzentrischen Kreisbahnen auftritt, wenn man sich dieselben entsprechend der Verweilzeit kontinuierlich mit Elektrizität belegt denkt. x wird dann eine Größe zweiter Ordnung in dem Radienverhältnis der inneren Elektronenbahnen zur äußeren
In today's quantum mechanics and quantum field theory, the observable signature of a symmetry is often sought in the form of a selection rule: a missing radiation frequency, a particle that does not decay in another one, a scattering process which fails to take place. The connection between selection rules and symmetries is effected thanks to the mathematical discipline of group theory. In the present paper, I will offer an overview of how the productive synergy between selection rules and group theory came to be. The first half of the work will be devoted to the emergence of the idea of spectroscopic selection rules in the context of the old quantum theory, showing how this notion was linked with an interpretive scheme of theoretical nature which, once combined with group theory, would bear many fruits. In the second part of the paper, I will focus on the actual encounter between selection rules and group theory, and on the person largely responsible for it: Eugene Wigner. I will attempt to reconstruct the path which led Wigner, of all people, to be the agent effecting this connection.
Frank Horton was born at Handsworth, Birmingham, on 20 August 1878. His parents were Albert Horton and Kate Louisa Horton ( née Carley) and he was the eldest son and second child of a family of seven, five sons and two daughters; a most devoted family. Whether Frank’s future career was directly influenced by the fact that his father was a schoolmaster and later an Inspector of Schools is not known for certain, but the future Professor and Vice-Chancellor may well have approached maturity with this background bias, which may have strongly turned his thoughts to administration in later life. As a child he got his early education at King Edward’s School, Birmingham, where it was soon apparent that he showed real academic promise, so that he was encouraged to enter Mason College, Birmingham, where teaching and research prestige were already so high as to make the early attainment of University status a virtual certainty. His undergraduate career in Mason College, led in 1899 to a First Class External Honours degree in the University of London, in physics and in chemistry after which he chose to tread the path of physics research under the aegis of Professor J. H. Poynting. It was under Poynting’s influence that the young Horton was first given the opportunity of showing, in the field of experimental research, those qualities of meticulous and painstaking care which, through all his life, were to characterize his work, whether in the laboratory, the lecture room, the administrative office or in committee.
In this paper we analyse the approach to interpreting atomic spectra in the framework of classical physics from the discovery of the electron in 1897 to Bohr's atomic model of 1913. Taken as a whole, efforts in this direction are part of a remarkable intellectual endeavour in which the classical theoretical framework seems to have been exploited to its full potential. By demonstrating the limits and weaknesses of classical physics in solving the problem of spectral emissions, these attempts opened the way to a complete break from traditional thought and the introduction of the new quantum ideas.