Franck, James (1882–1964)

German-born US physicist. He shared the Nobel Prize for Physics in 1925 with his co-worker Gustav Hertz for their experiments of 1914 on the energy transferred by colliding electrons to mercury atoms, showing that the transfer was governed by the rules of quantum theory.

Franck was born in Hamburg and educated at Heidelberg and Berlin. In 1920 he became professor of experimental physics at Göttingen, but emigrated to the USA in 1933 after publicly protesting against the Nazis' racial policies. He was a professor at the University of Chicago 1938–49. He participated in the wartime atomic-bomb project at Los Alamos but organized the ‘Franck petition’ in 1945, which argued that the bomb should not be used against Japanese cities. After World War II he turned his research to photosynthesis.

Investigating the collisions of electrons with noble-gas atoms, Franck 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.

Franck also studied the formation, dissociation, vibration, and rotation of molecules, and was able to calculate the dissociation energies of molecules. Edward Condon interpreted this method in terms of wave mechanics, and it has become known as the Franck–Condon principle.