Ralph Kronig On the displacement of the energy levels of atomic systems in strong radiation fields (med G. Ooms og P.J. Rijnierse) & On the congealing of a moving fluid in a narrow cold channel (med W.H.A. Grasso)
Abstract
Ralph Kronig (1904-1995) was a German-American theoretical physicist working in the Netherlands through most of his career. He was born in Dresden of American parents, and studied at Columbia University in New York. After his PhD in 1927 he returned to Europe. The first couple of years he visited several important scientific centers, as Bohr’s institute in Copenhagen. In Zürich he was assistant to Wolfgang Pauli. In the Netherlands Kronig worked as lecturer at the University of Groningen from 1931 until he in 1936 was appointed professor of physics at the Delft University of Technology. He stayed at Delft until retirement in 1969, and for several years he served also as Rector of the university.
The years 1925-26, when Kronig started his scientific career, were revolutionary years in physics, due to the development of quantum mechanics. Kronig soon became a well-known expert in quantum theory.
In 1925 the anomalous Zeeman effect, the behaviour of atoms in a magnetic field, was a mystery. The experimental results showed that the atoms had more energy states than expected. As a young PhD student at Columbia Ralph Kronig was the first who understood that this could be explained by assuming that the electrons in the atom had spin, an internal degree of rotation. The orientation of the spin vector relative to the magnetic field produced the additional degrees of freedom. Unfortunately Kronig explained this to Wolfgang Pauli. Pauli ridiculed the idea of spin, and faced with his criticism the young Kronig decided not to publish his theory. Half a year later the Dutch physicists George Uhlenbeck and Samuel Goudsmit published the spin theory. However, that Kronig was first is a historical fact, also acknowledged by Uhlenbeck and Goudsmit. On this background the discovery of the spin was never honoured with a Nobel Prize. Kronig did not, however, hold a grudge against Pauli for his intervention. In fact, Pauli and Kronig remained close friends.
In 1926 Kronig published a theory that contained quantitative relations between the index of diffraction and the attenuation of X-rays in materials. These relations are, however, very general, since they are based on not much more than the physical principle of causality. Independently H. A. Kramers published in 1927 the same connections, the Kramers-Kronig relations. These relations, in extended form, have been very popular in elementary particle physics, and they are still useful in several fields of physics.
The Kronig-Penney model in solid state physics is a simple quantummechanical system, where a particle moves in one-dimensional potential consisting of a periodic array of rectangular potential barriers. Kronig solved this idealized picture of an electron in a crystal lattice in cooperation with a young British physicist William G. Penney. The popularity of the Kronig-Penney model is mainly due to the fact that the solution exhibits, simply and explicitly, the existence of energy bands in solids.
