by Ben Johnson
Nobel Prize in Physics 1925 together with Gustav Hertz
"for their discovery of the laws governing the impact of an electron upon an atom".
James Franck was a German-American physicist born on August 26, 1882 in Hamburg, Germany. He received the Nobel Prize for physics in 1925 for his research on the interaction of electrons and atoms.
Franck attended school in Hamburg where most of his classes focused on the classics and languages but failed to hold his interest. Moving to Heidelberg for university, he received his first taste of science when taking classes in geology and chemistry and met Max Born who would remain a friend and colleague throughout his career and life. Making the change to the University of Berlin in 1902, Franck pursued a Ph.D. in physics which he obtained in 1906 under the tutelage of Emil Warburg for investigating the dynamics of ions during gas discharges. Franck returned to Berlin as assistant to Heinrich Reubens after a short stint at the University of Frankfurt am Main and started to lecture in 1911.
In the years following, Franck began work with Gustav Hertz who had recently come to Berlin from Munich. The two conducted experiments on the interaction of electrons with atoms, later to become known as the Franck-Hertz experiment. In 1914, they published their research on mercury atoms [J. Franck, G. Hertz, Verhandlungen der Deutschen Physikalischen Gesellschaft, Berlin 16, 512 (1914)] and were awarded the Nobel Prize for this work in 1925.
The experimental set-up consisted of a glass tube containing free mercury atoms in the form of vapor. Electrons were accelerated between two electrodes and the electric current’s dependence on voltage measured. After first determining suitable electric field gradients and vapor pressures, the two scientists could show that the electric current increased in the expected way with voltage until 4.9 V. At this point the current decreased rapidly before be- ginning to climb again. At repeated whole number multiples of 4.9 V, that is 9.8 V, 14.7 V, etc, similar minima in the electric current were observed.
In order to interpret their observations in terms of modern concepts, Franck and Hertz explained that the mercury could accept and give off energy only in discrete amounts, in this case equal to 4.9eV. With the potential between the electrodes below 4.9V, the electrons could only obtain enough kinetic energy to participate in elastic collisions with the mercury atoms. Above this voltage the electrons were energetic enough to excite the mercury atom in a single interaction, at 9.8 V in two interactions and so on. After these collisions, the mercury atom should return to its ground state and release energy in the form of radiation with a wavelength of 2536̊ Angstrom. Franck and Hertz found such a line in the emission spectrum of mercury and later published these findings. Although we know today that the 4.9 eV is the difference between the two lowest electron levels in mercury, at the time the excitation was erroneously interpreted to be caused by the ionization of the atom and subsequent recombination.
In addition to providing a new method for the calculation of Planck’s constant, h, the Franck-Hertz experiment is often viewed as the first evidence supporting the model of the quantum atom introduced by Niels Bohr in 1913. The model was, however, not mentioned in Franck and Hertz’s initial publication and Bohr was the first to connect the two. The results of the experiment could, however, also be seen to offer support for one of the then-current competing models of the atom and not only for the Bohr theory.
In fact, at the beginning of the 20th century the corpuscular nature of matter had only recently been proven. Although the idea had existed for millennia, credit for the initial discussion is usually given to Leucippus and Democritus in the 5th century B.C., the modern debate on the existence of atoms had already lasted a century at the time of the Franck- Hertz experiment. In the beginning of the 19th century, John Dalton formulated the first atomic theory and later, in 1827, the dynamics of molecular behavior were first observed when Robert Brown studied the movement of pollen grain particles in water, later to be known as Brownian motion. In the closing years of the 19th century, these dynamics received an initial mathematical description, with important work done by Thorvald N. Thiele, but not until 1905 was Einstein able to formulate a correct, quantitative theory. This was subsequently verified in 1908 by Jean Baptiste Perrin and settled the debate about the particle nature of matter only a few years before the work of Franck and Hertz. Four years prior to Perrin’s work, Thomson had published his “plum pudding” atomic model describing negatively charged electron rings surrounded by a sphere of positive charge. With this work he was able to drive the debate about the nature of the particles constituting matter, although the existence of these particles had not yet been scientifically established. In 1911, Ernest Rutherford published his own atomic model with a central, massive, charged nucleus surrounded by a sphere of electrification of the opposite charge and comparatively little mass.
In the time before quantum mechanics, Rutherford’s model was immediately open to criticism, including how the outer, lighter charged particles were to be kept from falling into the central nucleus. If in motion, an electron would surely radiate energy according to classical electrodynamics before spiraling into the nucleus. Furthermore, the spectrum of this emitted energy would be continuous in nature and not composed of discrete lines as was observed.
Due, in part, to Niels Bohr answering some of these questions in his 1913 publications, today we consider Rutherford’s model to be the correct one. But in their publications at the time, Niels Bohr and other scientists were careful to consider both the Rutherford and Thomson models as possibilities, if only implicitly in some cases. Put into this light, the achievements of Franck and Hertz helped clarify the nature of matter beyond Bohr’s quantum postulation because they also supported Rutherford’s nuclear atomic model. Thus, the Frank-Hertz experiment contributed to the closing of the profound debate about the corpuscular nature of matter. But it would be more than a decade before the results of Franck, Hertz, Bohr and many others could be explained in the overarching theory of quantum mechanics in 1926.
Soon after Franck and Hertz published their results, the beginning of World War I in 1914 saw Franck on the eastern front overseeing the use of poison gas in combat. After being seriously injured in 1917, Franck was awarded the Iron Cross for his bravery and in 1918 returned to Berlin. Here he continued his research on inelastic collisions of electrons with atoms and molecules at the Kaiser Wilhelm Institute for Physical Chemistry, then headed by Fritz Haber. He remained in Berlin until Max Born was appointed chair of theoretical physics at Göttingen and insisted that the chair of the Second Institute of Experimental Physics be filled by Franck. Born and Franck then proceeded to developed the physics at Göttingen into a world renowned institution. Born made major contributions to quantum mechanics, in particular his probabilistic interpretation of Schrödinger’s wave function for which he would receive the Nobel Prize in 1954. Franck became respected for his didactic skills and for further scientific developments such as the Franck-Condon Principle describing electron transition intensities in molecules.
Franck was also celebrated for his integrity and expression in times of political turmoil. Although of Jewish decent, he was at first not subject to the Nazi Civil Service Law of 1933 due to his service in World War I, but decided nevertheless to renounce his position at Göttingen and publicly declare his opposition to the new policies. This led to concern among friends and colleagues and sharp reactions from Nazi supporters. In the same year, Franck emigrated to Baltimore, Maryland, USA, where he became a professor at Johns Hopkins University before finally finding his way to the University of Chicago where he continued research on photosynthesis which he had begun in Göttingen.
Receiving his American citizenship in 1941, Franck became involved in the Manhattan Project in 1942 and after Germany’s defeat in 1945 again involved himself in matters of politics. His famous appeal to the American government, known as the Franck Report, called for a demonstration of nuclear weapons before their use against Japan. Although Franck was able to secure the support of several notable scientists involved in nuclear research, his advice was not heeded, with well-known historical consequences.
In 1947, Franck became professor emeritus at the University of Chicago and remained active in photosynthesis research. In 1911 he married Ingrid Josefson with whom he had two daughters. After her death, he married Hertha Sponer, a former colleague and physics professor at Duke University, in 1946. James Franck died on May 21, 1964 during a visit to Göttingen.
 Nobel Prize Winners, Tyler Wasson, Ed., The H.W. Wilson Company, New York (1987)
 Aufrecht im Sturm der Zeit: Der Physiker James Franck, Jost Lemmerich, GNT-Verlag, Diepholz,
Stuttgart, Berlin (2007)
 The Scientist’s Atom and the Philosopher’s Stone, Alan Chambers, Springer Verlag, Dordrecht, Heidelberg, London, New York (2009)
 Joseph John Thomson, On the Structure of the Atom, Philosophical Magazine 7, 237-265 (1904)
 Ernest Rutherford, The Scattering of Alpha and Beta Particles by Matter and the Structure of the
Atom, Philosophical Magazine 21, 669-688 (1911)
 Niels Bohr, On the Constitution of Atoms and Molecules I, Philosophical Magazine 26, 1-24 (1913)
 James Franck and Gustav Hertz, U ̈ber Zusammenstöße zwischen Elektronen und Molekülen des Quecksilberdampfes und die Ionisierungsspannung desselben, Verhandlung der Deutschen Physikalischen Gesellschaft, Berlin 16, 457-467 (1914)
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