Ralph Kronig
Encyclopedia
Ralph Kronig was a German-American physicist
(March 10, 1904 – November 16, 1995). He is noted for the discovery of particle spin
and for his theory of x-ray absorption spectroscopy
. His theories include the Kronig–Penney model, the Coster–Kronig transition and the Kramers–Kronig relation.
, Germany
. He died in Zeist
on 16 November 1995 at the age of 91.
Kronig received his primary and high-school education in Dresden and went to New York
to study at Columbia University
where he received his PhD in 1925 and subsequently became instructor (1925) and assistant professor (1927).
Early in Kronig's career he had encountered Ehrenfest
who, while visiting America in 1924, had advised the young American physicist Ralph Kronig to revisit Europe. Kronig left for that continent later in 1924 and paid visits to the important centers for theoretical-physics research in Germany and Copenhagen
. It was a time of great expansion in the development of quantum mechanics
, and that development was taking place in Europe. Kronig was privileged to be a young, brilliant physicist in that glory-day of 20th century theoretical physics, which made it possible for him to live and work among the great physicists of that era (Bohr
, Ehrenfest
, Heisenberg
, Pauli
, Kramers
).
In January 1925, when Kronig was a still a Columbia University PhD student, he first proposed electron spin after hearing Pauli in Tübingen. Werner Heisenberg
and Wolfgang Pauli
immediately hated the idea. They had just ruled out all imaginable actions from quantum mechanics. Now Kronig was proposing to set the electron rotating in space. Pauli especially ridiculed the idea of spin, saying that "it is indeed very clever but of course has nothing to do with reality". Faced with such criticism, Kronig decided not to publish his theory and the idea of electron spin had to wait for others to take the credit. Ralph Kronig, had come up with the idea of electron spin several months before Uhlenbeck
and Goudsmit
. Most textbooks credit these two Dutch physicists with the discovery.
Ralph Kronig did not hold a grudge against Pauli for this turn of events. In fact, Kronig and Pauli remained friends for many years into the future. They exchanged many ideas in physics through letters. But it remains an historic fact that Kronig had told Pauli about electron spin before Pauli had published his paper showing that two electrons can inhabit the same orbital (W. Pauli, “On the Connexion between the Completion of Electron Groups in an Atom with the Complex Structure of Spectra”, Z. Physik 31, 765ff, 1925). Months later when Uhlenbeck and Goudsmit came up with particle spin, it seemed to verify Pauli's paper. Together with Rabi
Kronig gave the first solution (1927) of the Schrödinger equation for the rigid symmetric top
.
Werner Heisenberg
in developing Quantum Mechanics
involved Kronig in his seminal ideas of the theory. In the beginning of May 1925, Heisenberg wrote three times to Ralph Kronig, with whom he had cooperated a little earlier in Copenhagen on the spectral theory of multi-electron atoms. In the second letter, dated 5 May, Heisenberg wrote down in some detailed equations expressing the transition to his matrix mechanics.
In 1927, Kronig returned to Europe for good and worked in different prominent centres of research: Copenhagen, London
, Zürich
(where for a year he was Pauli's assistant). Around 1930 he settled in the Netherlands
: first in Utrecht
, then in Groningen, first as Dirk Coster
's assistant, and from 1931 as an associate professor, and since 1939 as a full professor at the Delft University of Technology
where he stayed until his retirement in 1969. He was recognized internationally by then as a renowned theorist who corresponded with the leading characters of that time and made interesting contributions to quantum mechanics and the application of it particularly on the physics of molecules and molecular spectra, an area on which he was the expert of those days.
The Max Planck medal was awarded to Ralph Kronig in 1962.
Among Ralph Kronig's substantial correspondence are many letters to and from the 20th century's greatest physicists that should be preserved for posterity and Kronig himself published many in books.
Showing Kronig's great respect for Pauli, in one letter Ralph Kronig said regarding Pauli and the slim number of actual publications made by Pauli considering the extent of his work [translated from the German]:
"his [Pauli's] publications contain however, which is understandable due to Pauli's unusually critical attitude, only a small part of the work really carried out by him. Pauli briefs in his papers on the finished results, but not on the long, often laborious way, which had led to them, and also not on incomplete attempts. A part of his work is only carried in a satisfying way in his extensive exchange of letters."
Stumm von Bordwehr (1989) gives a detailed description of the life and accomplishments of Kronig, even recounting how his name was changed to Ralph de Laer Kronig.
The short range order data of Hanawalt (1931b) stimulated Kronig (1932) to develop a theory for molecules. This model served as the starting point for all the subsequent short range order theories but few attempted to compare it to their data. Kronig's student, H. Petersen (1932, 1933) continued this work. Peterson's equation shows many of the features of the modern theory. This theory was applied to GeCl4 by Hartree, Kronig and Petersen (1934). A description of the Herculean efforts required to perform the calculations can be found in Stumm von Bordwehr (1989).
The Kramers–Kronig relation for dispersion was derived by Kronig (1926) independently of Kramers (1927).
Physicist
A physicist is a scientist who studies or practices physics. Physicists study a wide range of physical phenomena in many branches of physics spanning all length scales: from sub-atomic particles of which all ordinary matter is made to the behavior of the material Universe as a whole...
(March 10, 1904 – November 16, 1995). He is noted for the discovery of particle spin
Spin (physics)
In quantum mechanics and particle physics, spin is a fundamental characteristic property of elementary particles, composite particles , and atomic nuclei.It is worth noting that the intrinsic property of subatomic particles called spin and discussed in this article, is related in some small ways,...
and for his theory of x-ray absorption spectroscopy
X-ray absorption fine structure
X-ray absorption fine structure is a specific structure observed in X-ray absorption spectroscopy . By analyzing the XAFS, information can be acquired on the local structure and on the unoccupied electronic states.-Spectra:...
. His theories include the Kronig–Penney model, the Coster–Kronig transition and the Kramers–Kronig relation.
Background
Ralph Kronig (later Ralph de Laer Kronig) was born on 10 March 1904 from American parents in DresdenDresden
Dresden is the capital city of the Free State of Saxony in Germany. It is situated in a valley on the River Elbe, near the Czech border. The Dresden conurbation is part of the Saxon Triangle metropolitan area....
, Germany
Germany
Germany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...
. He died in Zeist
Zeist
Zeist is a municipality and a town in the central Netherlands, located east of the city of Utrecht.-Population centres :*Austerlitz*Bosch en Duin*Den Dolder*Huis ter Heide*Zeist- The town of Zeist :...
on 16 November 1995 at the age of 91.
Kronig received his primary and high-school education in Dresden and went to New York
New York
New York is a state in the Northeastern region of the United States. It is the nation's third most populous state. New York is bordered by New Jersey and Pennsylvania to the south, and by Connecticut, Massachusetts and Vermont to the east...
to study at Columbia University
Columbia University
Columbia University in the City of New York is a private, Ivy League university in Manhattan, New York City. Columbia is the oldest institution of higher learning in the state of New York, the fifth oldest in the United States, and one of the country's nine Colonial Colleges founded before the...
where he received his PhD in 1925 and subsequently became instructor (1925) and assistant professor (1927).
Early in Kronig's career he had encountered Ehrenfest
Paul Ehrenfest
Paul Ehrenfest was an Austrian and Dutch physicist, who made major contributions to the field of statistical mechanics and its relations with quantum mechanics, including the theory of phase transition and the Ehrenfest theorem.- Biography :Paul Ehrenfest was born and grew up in Vienna in a Jewish...
who, while visiting America in 1924, had advised the young American physicist Ralph Kronig to revisit Europe. Kronig left for that continent later in 1924 and paid visits to the important centers for theoretical-physics research in Germany and Copenhagen
Copenhagen
Copenhagen is the capital and largest city of Denmark, with an urban population of 1,199,224 and a metropolitan population of 1,930,260 . With the completion of the transnational Øresund Bridge in 2000, Copenhagen has become the centre of the increasingly integrating Øresund Region...
. It was a time of great expansion in the development of quantum mechanics
Quantum mechanics
Quantum mechanics, also known as quantum physics or quantum theory, is a branch of physics providing a mathematical description of much of the dual particle-like and wave-like behavior and interactions of energy and matter. It departs from classical mechanics primarily at the atomic and subatomic...
, and that development was taking place in Europe. Kronig was privileged to be a young, brilliant physicist in that glory-day of 20th century theoretical physics, which made it possible for him to live and work among the great physicists of that era (Bohr
Niels Bohr
Niels Henrik David Bohr was a Danish physicist who made foundational contributions to understanding atomic structure and quantum mechanics, for which he received the Nobel Prize in Physics in 1922. Bohr mentored and collaborated with many of the top physicists of the century at his institute in...
, Ehrenfest
Paul Ehrenfest
Paul Ehrenfest was an Austrian and Dutch physicist, who made major contributions to the field of statistical mechanics and its relations with quantum mechanics, including the theory of phase transition and the Ehrenfest theorem.- Biography :Paul Ehrenfest was born and grew up in Vienna in a Jewish...
, Heisenberg
Werner Heisenberg
Werner Karl Heisenberg was a German theoretical physicist who made foundational contributions to quantum mechanics and is best known for asserting the uncertainty principle of quantum theory...
, Pauli
Wolfgang Pauli
Wolfgang Ernst Pauli was an Austrian theoretical physicist and one of the pioneers of quantum physics. In 1945, after being nominated by Albert Einstein, he received the Nobel Prize in Physics for his "decisive contribution through his discovery of a new law of Nature, the exclusion principle or...
, Kramers
Hendrik Anthony Kramers
Hendrik Anthony "Hans" Kramers was a Dutch physicist.-Background and education:...
).
In January 1925, when Kronig was a still a Columbia University PhD student, he first proposed electron spin after hearing Pauli in Tübingen. Werner Heisenberg
Werner Heisenberg
Werner Karl Heisenberg was a German theoretical physicist who made foundational contributions to quantum mechanics and is best known for asserting the uncertainty principle of quantum theory...
and Wolfgang Pauli
Wolfgang Pauli
Wolfgang Ernst Pauli was an Austrian theoretical physicist and one of the pioneers of quantum physics. In 1945, after being nominated by Albert Einstein, he received the Nobel Prize in Physics for his "decisive contribution through his discovery of a new law of Nature, the exclusion principle or...
immediately hated the idea. They had just ruled out all imaginable actions from quantum mechanics. Now Kronig was proposing to set the electron rotating in space. Pauli especially ridiculed the idea of spin, saying that "it is indeed very clever but of course has nothing to do with reality". Faced with such criticism, Kronig decided not to publish his theory and the idea of electron spin had to wait for others to take the credit. Ralph Kronig, had come up with the idea of electron spin several months before Uhlenbeck
George Eugene Uhlenbeck
George Eugene Uhlenbeck was a Dutch-American theoretical physicist.-Background and education:George Uhlenbeck was the son of Eugenius and Anne Beeger Uhlenbeck...
and Goudsmit
Samuel Abraham Goudsmit
Samuel Abraham Goudsmit was a Dutch-American physicist famous for jointly proposing the concept of electron spin with George Eugene Uhlenbeck in 1925.-Biography:...
. Most textbooks credit these two Dutch physicists with the discovery.
Ralph Kronig did not hold a grudge against Pauli for this turn of events. In fact, Kronig and Pauli remained friends for many years into the future. They exchanged many ideas in physics through letters. But it remains an historic fact that Kronig had told Pauli about electron spin before Pauli had published his paper showing that two electrons can inhabit the same orbital (W. Pauli, “On the Connexion between the Completion of Electron Groups in an Atom with the Complex Structure of Spectra”, Z. Physik 31, 765ff, 1925). Months later when Uhlenbeck and Goudsmit came up with particle spin, it seemed to verify Pauli's paper. Together with Rabi
Isidor Isaac Rabi
Isidor Isaac Rabi was a Galician-born American physicist and Nobel laureate recognized in 1944 for his discovery of nuclear magnetic resonance.-Early years:...
Kronig gave the first solution (1927) of the Schrödinger equation for the rigid symmetric top
Rigid rotor
The rigid rotor is a mechanical model that is used to explain rotating systems.An arbitrary rigid rotor is a 3-dimensional rigid object, such as a top. To orient such an object in space three angles are required. A special rigid rotor is the linear rotor which isa 2-dimensional object, requiring...
.
Werner Heisenberg
Werner Heisenberg
Werner Karl Heisenberg was a German theoretical physicist who made foundational contributions to quantum mechanics and is best known for asserting the uncertainty principle of quantum theory...
in developing Quantum Mechanics
Quantum mechanics
Quantum mechanics, also known as quantum physics or quantum theory, is a branch of physics providing a mathematical description of much of the dual particle-like and wave-like behavior and interactions of energy and matter. It departs from classical mechanics primarily at the atomic and subatomic...
involved Kronig in his seminal ideas of the theory. In the beginning of May 1925, Heisenberg wrote three times to Ralph Kronig, with whom he had cooperated a little earlier in Copenhagen on the spectral theory of multi-electron atoms. In the second letter, dated 5 May, Heisenberg wrote down in some detailed equations expressing the transition to his matrix mechanics.
In 1927, Kronig returned to Europe for good and worked in different prominent centres of research: Copenhagen, London
London
London is the capital city of :England and the :United Kingdom, the largest metropolitan area in the United Kingdom, and the largest urban zone in the European Union by most measures. Located on the River Thames, London has been a major settlement for two millennia, its history going back to its...
, Zürich
Zürich
Zurich is the largest city in Switzerland and the capital of the canton of Zurich. It is located in central Switzerland at the northwestern tip of Lake Zurich...
(where for a year he was Pauli's assistant). Around 1930 he settled in the Netherlands
Netherlands
The Netherlands is a constituent country of the Kingdom of the Netherlands, located mainly in North-West Europe and with several islands in the Caribbean. Mainland Netherlands borders the North Sea to the north and west, Belgium to the south, and Germany to the east, and shares maritime borders...
: first in Utrecht
Utrecht (city)
Utrecht city and municipality is the capital and most populous city of the Dutch province of Utrecht. It is located in the eastern corner of the Randstad conurbation, and is the fourth largest city of the Netherlands with a population of 312,634 on 1 Jan 2011.Utrecht's ancient city centre features...
, then in Groningen, first as Dirk Coster
Dirk Coster
Dirk Coster , was a Dutch physicist. He was a Professor of Physics and Meteorology at the University of Groningen....
's assistant, and from 1931 as an associate professor, and since 1939 as a full professor at the Delft University of Technology
Delft University of Technology
Delft University of Technology , also known as TU Delft, is the largest and oldest Dutch public technical university, located in Delft, Netherlands...
where he stayed until his retirement in 1969. He was recognized internationally by then as a renowned theorist who corresponded with the leading characters of that time and made interesting contributions to quantum mechanics and the application of it particularly on the physics of molecules and molecular spectra, an area on which he was the expert of those days.
The Max Planck medal was awarded to Ralph Kronig in 1962.
Among Ralph Kronig's substantial correspondence are many letters to and from the 20th century's greatest physicists that should be preserved for posterity and Kronig himself published many in books.
Showing Kronig's great respect for Pauli, in one letter Ralph Kronig said regarding Pauli and the slim number of actual publications made by Pauli considering the extent of his work [translated from the German]:
"his [Pauli's] publications contain however, which is understandable due to Pauli's unusually critical attitude, only a small part of the work really carried out by him. Pauli briefs in his papers on the finished results, but not on the long, often laborious way, which had led to them, and also not on incomplete attempts. A part of his work is only carried in a satisfying way in his extensive exchange of letters."
Stumm von Bordwehr (1989) gives a detailed description of the life and accomplishments of Kronig, even recounting how his name was changed to Ralph de Laer Kronig.
Scientific achievement
Ralph Kronig (1931, 1932), published the first theory of x-ray absorption fine structure which contained some of the basic concepts of the modern interpretation. The Kronig-Penney Model (1931) is a one-dimensional model of a crystal that shows how the electrons in a crystal are dispersed into allowed and forbidden bands by scattering from the extended linear array of atoms. His first theory (1931) of EXAFS was the three-dimensional equivalent of this model. The theory showed that a photo electron traversing a crystal lattice would experience permitted and forbidden zones depending on its wavelength and, that even when the effect was averaged over all directions in the lattice, a residual structure should be observed. His theory was successful in predicting many generally observed features of the fine structure, including similar structure from similar lattices, inverse r2 dependence , correct r versus T dependence and increasing energy separation of the fine structure features with energy from the edge. The equation which was re-derived in a more quantitative way in 1932 was simple to apply and interpret. Every experimenter found approximate agreement with the theory. There were always some absorption features close to that predicted by the possible lattice planes. However, the expected strong reflections (e. g. (100), (110), (111), etc. ) did not always correlate with the most intense absorption features as intuitively expected. Still, agreement was close enough to be tantalizing and everyone tested the agreement of their measured "Kronig structure" with the simple Kronig theory. In the Kronig equation, energy positions Wn correspond to the zone boundaries, i. e. not the absorption maxima or minima, but the first rise in each fine structure maximum. abg are the Miller indices, a is the lattice constant and q is the angle between the electron direction and the reciprocal lattice direction. When averaged over all directions with a non-polarized x-ray beam and a polycrystalline absorber, cos2q = 1. However, with a single crystal absorber and polarized x-rays the absorption features should be larger for specific crystal planes. This was another experimental variable that might verify the theory and many attempted to test it. Thus began the long record of publications in which Kronig structure was interpreted in terms of the simple Kronig theory. Until the 1970s fully 2% of the papers published in Phys. Rev. were devoted to x-ray absorption spectroscopy and most invoked Kronig's theory.The short range order data of Hanawalt (1931b) stimulated Kronig (1932) to develop a theory for molecules. This model served as the starting point for all the subsequent short range order theories but few attempted to compare it to their data. Kronig's student, H. Petersen (1932, 1933) continued this work. Peterson's equation shows many of the features of the modern theory. This theory was applied to GeCl4 by Hartree, Kronig and Petersen (1934). A description of the Herculean efforts required to perform the calculations can be found in Stumm von Bordwehr (1989).
The Kramers–Kronig relation for dispersion was derived by Kronig (1926) independently of Kramers (1927).
Books published by Ralph Kronig
- Correspondence with Niels Bohr, 1924–1953.
- Textbook of physics. Under the editorship of R. Kronig in collaboration with J. De Boer [and others] With biographical notes and tables by J. Korringa.
- The optical basis of the theory of valence / by R. de L. Kronig
- Band spectra and molecular structure / by R. de L. Kronig
- Oral history interview with Ralph de Laer Kronig, 1962 November 12