James Franck (1882-1964) studied the effects of an electron upon an atom. Along with Gustave Hertz, he was awarded the Nobel Prize in physics in 1925.
James Franck was a physicist whose experimental work with atoms and electrons proved Niels Bohr's theory that atoms are quantized—that they transmit and absorb energy in discrete quantities or packages. Along with collaborator Gustav Hertz, he was awarded the 1925 Nobel Prize in physics. Franck was also known for his outspoken opposition to the use of the atomic bomb, which he helped develop during World War II.
Franck was born in Hamburg, Germany, on August 26, 1882, to Jacob Franck, a German Jewish banker, and Rebecka Nachum Drucker. Although Jacob Franck was deeply religious—he observed Jewish holidays with fasting and chanting—his spiritual devotion did not, on the whole, pass on to James, who would later declare science and nature as his true love and religion. He attended school at the Wilhelm Gymnasium in Hamburg before enrolling at the University of Heidelberg. Franck's father wanted him to study law and economics with the hope that his son would take over the family business. Out of a sense of duty, Franck complied, but after attending law lectures for a short time, he determined to follow his own path and enrolled in the faculty of chemistry.
Heidelberg was where Franck met Max Born, the German physicist with whom he formed his closest friendship. After two terms studying chemistry, Franck enrolled in the doctoral program at the University of Berlin. Under the influence of its physics professor, Emil Warburg, he became interested in physics and switched fields. He began a study to determine the mobility of ions using a method invented by Cambridge physicist Ernest Rutherford.
After graduating with a D.Phil. in 1906, Franck continued to pursue the same lines of research, exploring the forces between electrons and atoms at the physics faculty of the University of Frankfurt-on-Main. He returned to Berlin in 1908 to become an assistant to Professor Heinrich Rubens. There, Franck began collaborating with the German physicist Gustav Hertz on a series of experiments that would provide direct proof of Bohr's theoretical model of atomic structure, demonstrate the quantized energy transfer from kinetic, or moving, energy to light energy, and establish both of their reputations.
Bohr had postulated that an atom's nucleus, or core, is surrounded by "orbits" of negatively charged electrons. Bohr theorized that these orbits revolve around the nucleus at set distances known as shells. The number of electrons and, thus, the number of shells vary according to the type of atom. Atoms ranking high on the periodic table of elements contain more electrons than simple elements such as hydrogen, which has just one proton and one electron. These extra electrons are contained in extra shells, according to a definite pattern. The first shell contains two electrons; the second, eight; the third, eighteen; the fourth; thirty-two; and fifth, fifty, and so on. As soon as the first shell is full, electrons begin to fill up the second shell, then the third, up to the last shell.
In their natural, unexcited state, the electrons try to stay as close to the nucleus as possible, that is, in an inner shell. Bohr suggested that electrons would jump from one shell to another if energy were applied to them. The distance they would jump would depend on the amount of energy supplied; when the energy source were withdrawn, they would fall back to their original position. The energy emitted by electrons falling back in toward the nucleus would be exactly equivalent to that absorbed by them when jumping to an outer shell. Most importantly, atoms receiving energy could not absorb just any amount but only the specific amount they would need to make a leap. Thus, Bohr spoke of the atom as being "quantized."
Franck and Hertz did not set out to prove Bohr's theory. In fact, they were not even familiar with his work at the time they were carrying out their experiments. Rather, they were interested in measuring the energy needed to ionize atoms of mercury. To this end, they bombarded atoms of mercury vapor with electrons moving at controlled speeds. Below a certain speed, the electrons would bounce off the atoms with perfect elasticity, indicating that the electrons did not possess sufficient energy to ionize the mercury atom, that is, to transfer enough energy to the mercury to enable its electrons to jump from one atomic shell into another. Above a certain speed, Franck and Hertz discovered that resonance occurred. At this point, energy was transferred from the electrons to the atoms, causing the mercury gas to glow. They found that energy had been transferred from the electrons to the atoms in discrete amounts. The energy value of the light emitted from the ionized atoms was equivalent to the energy given to them by the electrons. This experiment proved that the quantized energy had changed from the kinetic energy of the moving electrons to the electromagnetic energy given off by the glowing mercury. It also provided direct experimental evidence for Bohr's theory of the quantized atom, a crucial step in the development of twentieth-century physics.
This experiment was also significant because it led to the realization that the light spectrum of an atom holds the key to its atomic structure. The discontinuous bands of light in an atomic spectrum, each representing a particular energy level, correspond to the range of possible jumps that an excited electron could make as it drops from the outer shells, where the absorption of energy had sent it, back to its original inner shell.
Franck's work was unexpectedly interrupted with the outbreak of the First World War. He signed up and became an officer. He served through 1918, working with a group of physicists who prepared and later directed chemical warfare. Franck received the Iron Cross for his valor; he also received a serious leg injury, which almost claimed his life. Returning to academia in 1918, he was named as the head of the physics division at the Kaiser Wilhelm Institute for Physical Chemistry, later renamed the Max Planck Institute. There, Franck pursued his work on electron impact measurements. It was also at the institute that he met Niels Bohr, with whom he developed a lasting friendship. Franck always regarded Bohr as a physicist second to none and consulted him regularly. "I never felt… such hero worship as [I did] to[ward] Bohr," he said in an interview excerpted in Redirecting Science: Niels Bohr, Philanthropy, and the Rise of Nuclear Physics.
In 1920, with the influence of Born, Franck was appointed professor and director of the Second Physical Institute of the University of Göttingen. The friendship between Franck and Born blossomed into a close working relationship, with Franck the experimenter complementing Born the theorist. During their twelve years at Göttingen, the pair used one another as sounding boards for their ideas, discoveries, and publications, although they collaborated on only a few joint papers. The only contention between them was Franck's habit of holding frequent consultations with Bohr, a practice that tended to slow down their work. More than sixty letters between Franck and Born have survived from the 1920s.
In the spring of 1921, at Bohr's invitation, Franck paid a visit to Copenhagen in time for the March opening of Bohr's Institute of Theoretical Physics. By now, his reputation preceded him and his visit made front page news in Denmark. His meeting with the Swedish physicist Oskar Klein and Norwegian Svein Rosseland convinced him to continue his experimental work on Bohr's theories.
Back at the University of Göttingen a couple of months later, Franck concentrated on building a research facility of international repute. He afforded his students considerable academic freedom. Scientific discussions between teacher and pupils would occur as often during a walk or bicycle ride as in the laboratory. The standards for admission to his school were extremely high but once accepted, a student was assured of his unwavering support and friendship, both professionally and personally.
Franck continued to investigate collisions between atoms, the formation and disassociation of molecules, fluorescence, and chemical processes. In 1925, building on three previously unconnected theories, he published a paper dealing with the elementary processes of photochemical reactions. In it he set out the connection between electron transition and the motion of nuclei, and described a general rule for vibrational energy distribution. This rule was later expressed by the American physicist Edward U. Condon in terms of quantum mechanics (a mathematical interpretation of particle structures and interactions) and became known as the Franck-Condon principle, which is applied to a large number of chemical and spectroscopic phenomena. In 1926 Franck published a book summarizing his work in this area.
Also in 1926, Franck traveled to Sweden to accept the 1925 Nobel Prize in physics, awarded jointly to him and Hertz for their experiments proving Bohr's atomic theory. He returned to Göttingen to begin his next project, the study of photosynthesis, but had no sooner begun his experiments when Adolf Hitler's arrival on the German political stage changed his life.
When Hitler's anti-Semitic Nazi regime took control of Germany, a new law was declared that barred Jews from the civil service, excepting those who had served in the First World War. Although Franck's position was secure, he could not in good conscience continue to work for a regime dedicated to racism, so on April 17, 1933, he sent letters to the minister of education and to the rector of the university, announcing his resignation and decrying the government's discriminatory policy. Hoping to remain in Germany, Franck searched for another position. Two possibilities presented themselves, one being the chair of physics at the University of Berlin, which would shortly be open. Though it was a position Franck would have coveted under other circumstances, it would have meant working for the government. The other possibility was the directorship of the Kaiser Wilhelm Institute for Physical Chemistry, a position that retiring director Fritz Haber hoped Franck would accept. Internal problems in the institute, however, prevented Franck from assuming this post as well. Franck decided to accept a visiting lectureship at the Johns Hopkins University in America. After the three month period of that position he returned to Göttingen to contemplate his uncertain future. Tentative offers were made from universities in the United States, but they did not promise the permanency Franck was seeking. He decided to accept an offer from Bohr for a year's work at his Institute of Theoretical Physics.
Franck arrived in Copenhagen in April 1934, and, with his assistant Hilde Levi, set to studying the fluorescence of green plants, an extension of his previous work studying energy exchanges in complex molecular systems. Under Bohr's direction, he also began administering experimental nuclear research at the Institute. He was frustrated by poor facilities and slow coworkers and, as stated in Redirecting Science, wrote of this period: "My nuclear physics exhausts itself at present in work which is just about to be completed when someone else publishes it in Nature." Working with a master theorist such as Bohr also proved difficult for Franck. "Bohr's genius was so superior. And one cannot help that one would get so strong inferiority complexes in the presence of such a genius that one becomes sterile," he later said in an interview quoted in Redirecting Science. After being used to having his own laboratory and students, it was hard for Franck to get used to working in Bohr's shadow.
The combination of numerous frustrations spurred Franck to accept an offer to settle in the United States. In late 1935, he became a professor at Johns Hopkins University, where he spent three years before moving to the University of Chicago to fill its chair of physical chemistry. With the help of the Samuel Fels Fund, a laboratory dedicated to research into photosynthesis was built, which Franck directed until his retirement in 1949, though he continued to work there for many years subsequently. He became an American citizen in the early 1940s.
When the Second World War broke out, Franck played a leading role in the Manhattan Project, the American government-sponsored atomic bomb project. Like the other German scientists on the team, he was driven by a desire to beat Hitler to the production of a nuclear weapon. But he firmly believed that the bomb should be used as a mode of deterrence, not as a means of aggression. When the U.S. finally developed the bomb and subsequently deployed it against the Japanese, Franck was a harsh critic.
In 1942, a crisis struck in Franck's private life with the death of his wife, Ingrid Josephson, who had been sick for many years. He coped with the loss by immersing himself in his work. He chaired a committee of scientists charged with exploring the social and political implications of detonating an atom bomb. That committee's findings, titled the Franck Report, was submitted to the U.S. Secretary of War, Henry Stimson, in 1945, and warned the United States Government against the use of the bomb as a military weapon. The report also speculated on the dangers of embarking upon an arms race and also urged the U.S. to restrict nuclear testing to areas where human life would not be endangered. The Franck Report has been seen as a testament to Franck's integrity, conviction, and sense of scientific responsibility.
With the end of the war, Franck returned to his post at the University of Chicago where he continued his work with photosynthesis. He was particularly curious as to how plants are able to transform visible light into a form of energy that they use for sustenance and growth. He began experiments on the emanation of electromagnetic radiation of chlorophyll, a key ingredient in the photosynthesis process. Happy to be back at work, Franck experienced joy in his personal life as well. In 1946, he married Hertha Sponer, a professor of physics at Duke University in North Carolina, whom Franck knew from Göttingen and Berlin. They had two daughters, Dagmar and Elizabeth.
Franck was honored with numerous awards during his long career. In addition to the Nobel Prize, he was awarded the highest honor of the German Physical Society, the Max Planck Medal in 1953. Two years later, he received the Rumford Medal of the American Academy of Arts and Sciences. He became a foreign member of the Royal Society of London in 1964 and a member of the U.S. National Academy of Sciences.
During a visit in 1964 to Göttingen, the city where he had spent his most productive years and which had made him an honorary citizen in 1953, Franck died suddenly on May 21. He was eighty-three. He was remembered by his colleagues as a brilliant experimentalist, a dedicated scientist, and a kind and generous man.
Further Reading on James Franck
Aaserud, Finn, Redirecting Science: Niels Bohr, Philanthropy and the Rise of Nuclear Physics, Cambridge University Press, 1990.
Biographical Memoirs of the Royal Society, Volume 11, Royal Society (London), 1965, pp. 53-74.
Born, Max, My Life: Recollections of a Nobel Laureate, Scribner's, 1975.
Cline, Barbara Lovett, Men Who Made a New Physics, University of Chicago Press, 1987, p. 108.
Levitan, Tina, The Laureates: Jewish Winners of the Nobel Prize, Twayne, 1960, p. 74.
Segre, Emilo, From X-rays to Quarks, W. H. Freeman and Co., 1980, p. 137.
Weber, Robert L., Pioneers of Science: Nobel Prize Winners in Physics, Institute of Physics, 1980, p. 75.
Bulletin of the Atomic Scientists, October, 1964, pp. 16-20.
Correspondence between Franck and Bohr is available on microfilm as part of the Bohr General Correspondence in the Niels Bohr Library of American Physics in New York.
The James Franck Papers, Joseph Regenstein Library, University of Chicago.
Kuhn, Thomas S., six sessions of interviews with Franck, July, 9-14, 1962, housed at the Archive for the History of Quantum Physics, microfilm 35, section 2 (available at the American Philosophical Society, Philadelphia, and the Niels Bohr Library of the American Institute of Physics, New York).