James Franck: A versatile scientist

JAMES FRANCK was born on August 26, 1882 at Hamburg, Germany, as the son of a banker. He attended the local school and then went to the University of Heidelberg to study Chemistry and Geology. After two semesters, he moved to the University of Berlin, then the centre of Physics in Germany. The colloquium addressed by Rubens, Warburg and Planck played a formative influence in Franck's academic career. He was awarded the Ph.D. degree for his work on ionic mobility in gases (1906).

Turbulent career

The following year (1907) he became research assistant to Professor Rubens and served for some time in World War I. Afterwards he worked till 1920 under Fritz Haber at the Kaiser Wilhelm Institute for physical chemistry. He was appointed to a chair at the University of Goettingenr. Max Born (1882-1970) accepted the chair of theoretical physics on the condition of Franck being appointed to a chair in the university. With him Franck formed a life-long friendship, which resulted in building the nucleus of a scientific community at Goettingen.

Although a Jew, he was initially allowed to continue in office; but he refused to accept the dismissal of his colleagues. In April 1933, he resigned his post and published a courageous statement of protest against the new laws. Within a few months the whole group including Max Born had to leave Germany.

After spending an year in Copenhagen, Franck immigrated to the U.S. He was professor at Johns Hopkins University (1935-38) and professor of physical chemistry at the University of Chicago from 1938. Here he established a laboratory for photosynthesis, which he directed until his retirement in 1949. This work involved Franck in all the complexities of biochemistry but it attracted him by its fundamental importance. He was awarded in 1955 the Rumford Medal by the American Academy of Arts and Sciences for his contribution to photosynthesis.

Study of ion mobilities

At Goettingen, Gustaz Hertz (1887-1975), nephew of the famous physicist Heinrich Hertz, joined him in fundamental experiments on ion mobilities. In their original experiment (1914), electrons with variable energy were made to collide with mercury atoms in a vapour. They showed that electrons could impart energy to a mercury atom only if they had a kinetic energy exceeding 4.9 ev; and exactly this quantum of energy was taken up by the mercury atom, causing it to emit the line 2536 AU.

Thus Franck & Hertz confirmed experimentally that the electrons in an atom exist in discrete energy levels that had just been postulated by Niels Bohr (1885-1962). This experiment is the first deceive proof of the connection of the quantum of energy (E) with the frequency of light emitted (V): E=hv, where h is a universal constant. This fundamental contribution was acknowledged late due to the outbreak of World War I, by the award of the Nobel Prize to Franck and G. Hertz (1926). He received the Max Planek Medal of the German Physical Society.

Franck-Condon principle

During the period 1921-1933, he took up the study of atoms in collision, the formation and dissociation of molecules and their vibration and rotation. Born and Franck developed the use of the now familiar potential energy curves for two-atom systems. He discovered a method for determining the disassociation of diatomic molecules from their band spectra.

This came to be known as the Franck-Condon principle. It has provided the key to the understanding of a wide range of phenomena in molecular physics the intensity distribution in band spectra, predissociation, photo dissociation and pressure broadening of spectral lines.

Atomic bomb project

During World War II, he joined the metallurgical division of the atomic bomb project at Chicago. After the surrender of Germany, he along with other scientists became seriously concerned about the consequences of using the new weapon. His views were released in the Franck Report six days before the New Mexico atom bomb test (10 July 1945). The post-war years marked the beginning of his friendship with Niels Bohr, for whom he had profound admiration. The obituary note on Bohr that Franck wrote in 1963 is a moving testimony to their friendship. He was made an honorary citizen of Goettingen, where he died (May 21, 1964) while on a tour to visit his old friends. (The Dictionary of Scientific Biography)

R.Parthasarathy

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