Place: United States of America, Germany
Subject: biography, physics
German-born US physicist who provided the experimental evidence for the quantum theory of Max Planck and the quantum model of the atom developed by Niels Bohr. For this achievement, Franck and his co-worker Gustav Hertz (1887-1975) were awarded the 1925 Nobel Prize for Physics.
Franck was born in Hamburg on 26 August 1882. When he left school, his father (a prosperous banker) sent him to Heidelberg University in 1901 to read law and economics as a preparation for his entry into the family firm, considering the status of scientists to be very lowly indeed. Fortunately, at Heidelberg Franck met Max Born and a lifelong friendship began. Born, also from a wealthy Jewish family, had full parental approval for his career and this eventually convinced Franck's father to allow his son to follow a scientific career. At first it was to be geology, but this quickly turned to chemistry and then to physics when he went to Berlin University in 1902. It was there that Franck obtained his doctorate in 1906 for research into ionic mobility in gases.
Franck was awarded the Iron Cross during World War I, and from 1916 he worked at the Kaiser Wilhelm Institute of Physical Chemistry under Fritz Haber on the study of gases, becoming head of the physics division there in 1918. Two years later, Franck became professor of experimental physics at the University of Göttingen, where Born had just taken the chair of theoretical physics. At Göttingen, Franck and Hertz undertook the work on the quantum theory that gained them the 1925 Nobel Prize for Physics. Franck remained there until 1933, when Adolf Hitler came to power. Although allowed to retain his position because of his distinguished war record, Franck was told to dismiss other Jewish members of his Institute in the university. Franck refused to do this and left Germany, going first to Denmark and then to the USA, where he became professor of physics at Johns Hopkins University in Baltimore in 1935. This was followed by a move to Chicago in 1938, where Franck was appointed professor of physical chemistry.
During World War II, Franck became a US citizen and carried out metallurgical work related to the production of the atomic bomb. He became aware of the devastating power of this weapon and, in a document that became known as the Franck Report, he and other scientists suggested that it should first be demonstrated to the Japanese on unpopulated territory. Franck retired from the University of Chicago in 1949. Numerous honours were accorded him in these late years by academics and universities in both the USA and Europe. The city of Göttingen, as part of its 1000th anniversary in 1953, made Franck and Born honorary citizens and Franck died there on 21 May 1964 while visiting friends.
In his major contribution to physics, Franck investigated the collisions of electrons with noble gas (rare gas) atoms and found that they are almost completely elastic and that no kinetic energy is lost. With Hertz, he extended this work to other atoms. This led to the discovery that there are inelastic collisions in which energy is transferred in definite amounts. For the mercury atom, electrons accept energy only in quanta of 4.9 electronvolts. For such collisions to be inelastic, the electrons need kinetic energy in excess of this figure. As the energy is accepted by the mercury atoms, they emit light at a spectral line of 2,537Å/2.5 × 10−7 m. This was the first experimental proof of Planck's quantum hypothesis that E=hν where E is the change in energy, h is Planck's constant, and ν the frequency of light emitted. These experiments also tended to confirm the existence of the energy levels postulated by Bohr in his model of the atom. For this work, Franck and Hertz shared the 1925 Nobel Prize for Physics.
Franck also studied the formation, dissociation, vibration, and rotation of molecules. With Born he developed the potential-energy diagrams that are now common in textbooks of physical chemistry. From the extrapolation of data regarding the vibration of molecules obtained from spectra, he was able to calculate the dissociation energies of molecules. Edward Condon (1902-1974) interpreted this method in terms of wave mechanics, and it has since become known as the Franck-Condon principle.
During his later years at Göttingen and at Baltimore, Franck carried out experiments on the photodissociation of diatomic molecules in liquids and solids and this led to an interest in photosynthesis. Research in this field was dominated by organic chemists and biochemists and Franck found himself involved in much controversy. His research led him to believe in a two-stage mechanism within the same molecule for the photosynthetic process, when the established view was that two different molecules are involved.
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