**Hermann Arthur Jahn**'s parents were Friedrich Wilhelm Hermann Jahn and Marion May Curtiss. Friedrich Jahn was born in Germany but emigrated to England in 1890 where he married Marion May Curtiss (born 4 May 1876 in Gainsbrough) in Gainsborough, Lincolnshire, in October 1897. Their children were Hermann Edgar Jahn (born 1900), Roland Alfred Jahn (born 1905), Hermann Arthur Jahn (the subject of this biography), and Elfrida Marion Jahn (born 1910). Hermann Jahn was born in Colchester but spent his youth in Lincoln. He attended the City School, Lincoln, founded 1896 in Monks Road (the premises are now used by Lincoln College). This school became the Municipal Technical Day School when the city became a Local Education Authority in 1903.

Sometimes when writing these biographies we discover information which we do not quite understand. One such relates to Marion May Jahn and we record it here. On 13 August 1915 the Home Office issued her a Naturalisation Certificate. She is described as a resident of Lincoln being re-admitted from Germany. We can find no information relating to any other members of the family travelling back to Lincoln from Germany at this time.

After leaving the City School in 1925, Jahn entered University College, London where he read chemistry. He was awarded a B.Sc. in chemistry in 1928 and then a Master's degree for his thesis *Viscosity of mixtures of gases of identical molecular weight*. Excited by the recent remarkable results in quantum theory, he then went to Leipzig where his doctoral studies were supervised by Werner Heisenberg and, after he was appointed to Leipzig in 1931, by Bartel van der Waerden. Jahn received his doctorate for his thesis *Rotation und Schwingung des Methanmoleküls* (The rotation and oscillation of the methane molecule). He received his doctorate on 14 February 1935 and was appointed to a position in the Davy-Faraday Laboratory of the Royal Institution in Albermarle Street, London. There he continued with the research that he had done for his doctoral thesis [2]:-

The two Royal Society of London papers referred to in this quote are (with Edward Teller)His Ph.D. work on the vibrations of the methane molecule was published in the 'Annalen der Physik'(1935), was extended to the acetylene molecule(1936), the absorption spectrum of methane(1937)and culminated in two Royal Society papers(the first with Edward Teller)on the stability of polyatomic molecules in degenerate electronic states(1937/38). This was the Jahn-Teller effect dating from ... round about the year1935when Teller was also in London. This work was immediately recognised as being very important, and Jahn's name along with Teller's became enshrined in the literature.

*Stability of Polyatomic Molecules in Degenerate Electronic States. I. Orbital Degeneracy*, and the single authored paper

*Stability of polyatomic molecules in degenerate electronic states. II. Spin degeneracy*. In the first of these the authors note:-

They begin their Introduction by writing:-This research was carried out when the authors were working in the Sir William Ramsay Laboratories of Inorganic and Physical Chemistry, University College, London.

The proof of their results is based on using group theory. Similarly, the second of these papers, with Jahn as the only author, uses irreducible representations of symmetric groups in the proof of the results. He changed topics around 1940 and began studying scattering of X-rays.In the following we investigate the conditions under which a polyatomic molecule can have a stable equilibrium configuration when its electronic state has orbital degeneracy, i.e. degeneracy not arising from the spin. We shall show that stability and degeneracy are not possible simultaneously unless the molecule is a linear one, i.e. unless all the nuclei in the equilibrium configuration lie on a straight line. We shall see also that the instability is only slight if the degeneracy is due solely to electrons having no great influence on the binding of the molecule.

During the Second World War, Jahn worked at the Royal Aircraft Establishment where the work was primarily on aircraft engine problems. Jahn, who was an expert on vibrations in molecules, now worked on vibrational problems in aircraft. In particular he looked at how to avoid flutter in wings and aerofoils of aircraft. He wrote a number of reports (jointly with Gurney Harry Lionel Buxton (1876-1962)) such as *Comparative calculations of the critical speed for flexural-torsion flutter of a typical cantilever wing* (1942), and *Note on the possibility of antisymmetric elevator flutter on the Typhoon* (1943). Also with George Temple he wrote *Flutter at supersonic speeds*.

Jahn was employed at the Royal Aircraft Establishment from 1941 to 1946. During these years he married Lily Schüler (1910-2000). She had been born in Frankfurt in Germany into a Jewish family and had trained as a primary school teacher. However, when the Nazis came to power in 1933, since she was Jewish, she was dismissed from her teaching post. She went to Leipzig where she taught part-time in a Jewish elementary school. She met Jahn when he was studying for his doctorate in Leipzig. She escaped from Germany and immigrated to England where she worked first as a housemaid. She then got a teaching position in Wembley and, in 1943, she married Jahn. They had two children, Michael (born 1944) and Margaret (born 1946).

Jahn then spent the two years 1946-48 working in the Department of Mathematical Physics in the University of Birmingham. While there he published *Improvement of an approximate set of latent roots and modal columns of a matrix by methods akin to those of classical perturbation theory* (1948). He gave the following abstract:-

In 1947 Jahn considered working for the Atomic Energy Research Establishment situated near Harwell. This had only recently been established (January 1946) on the site of RAF Harwell, south of Oxford, when scientists moved into the accommodation that the RAF had used up to that time [2]:-A method is described for simultaneously improving all the latent roots and modal columns of a given matrix, starting from a given complete set of approximate modal columns. It is considered that the method will be useful as a final step in any iteration process of determining these quantities. The method is illustrated by a numerical example. The modification needed when two or more of the latent roots are coincident, or nearly so, is very briefly indicated. The fundamental formulae are akin to those of classical perturbation theory, the corresponding formulae of which, for the special case of a Lagrange frequency equation, are given for convenience in the Appendix.

[The Jahn-Fuchs connection was that Klaus Fuchs had worked for Rudolf Peierls (1907-1995) during World War II on the development of atomic weapons, both in the UK and in the United States. Peierls was head of the Department of Mathematical Physics in the University of Birmingham where Jahn was working at the time he went to be interviewed by Fuchs. The 'irony' referred to in the above quote is that in January 1950 Fuchs confessed to supplying information from the American, British and Canadian Manhattan Project to the Soviet Union. Jahn did not take a job at the Atomic Energy Research Establishment but, in 1948, was appointed to the first Chair in Applied Mathematics at University College, Southampton.Jahn]was interviewed by Klaus Fuchs, Head of Theoretical Physics at Harwell, in the hope of possibly obtaining a position there in1947. But Jahn was a realist. "I suppose I may not be able to get a post here", he remarked, "because of my German ancestry". "Why not?" said Fuchs, "Look at me. I have access to the classified material, and I have German ancestry." The irony of the conversation transpired only later.

At Southampton one of Jahn's first research students was James Philip Elliott (1929-2008). Elliott had been an undergraduate at University College, Southampton, graduating with a mathematics degree in 1949. He then went on to study for a Ph.D. at Southampton advised by Jahn. He graduated with his doctorate in 1951. Jahn and Elliott explored the application of Racah's tensor operator techniques to nuclear structure and applied group theoretic techniques to the understanding of atomic nuclei. Let us note that Elliott continued to work in this area for the rest of his career and he was elected a Fellow of the Royal Society of London in 1980. He was awarded both the Rutherford Medal and the Rutherford Prize by the Institute of Physics in 1994, and also awarded the Lise Meitner Prize of the European Physical Society in 2002. Jahn also continued to work in this areas of the rest of his career [2]:-

Peter Theodore Landsberg, who was Jahn's successor to his chair of Applied Mathematics in Southampton, writes [4]:-He contributed to about10papers in this area in the decade from1950to1960. In these he stressed the importance of applying group theoretic techniques to nuclear structure problems, discovered some important symmetry properties of the coupling coefficients of angular momentum theory, and did more than anyone else to keep alive the connection between the symmetric group and the continuous groups which is made explicit through the use of Young operators[named after Alfred Young]. In addition he initiated the use of computers in the calculation and tabulation of coupling coefficients in rational form. In a different area he advocated the application of Morpurgo differential equations to nuclear physics problems many years before such a programme was developed in France and the U.S.S.R. Rather more algebraic topics dominated the last years of his life. In an effort to improve the available algorithms for constructing symmetrised state functions he continued to do work on the symmetric group until the time of his terminal illness.

Throughout his time at Southampton he gave inestimable help to students and colleagues alike, and taught always the virtues of patient persevering research. He and his wife, Lily, entertained generously and thus provided a focus for a rapidly growing Mathematics Department of what became the University of Southampton in the1950s. Jahn[was]a first-rate mathematical physicist who made significant contributions to the theory of vibrations in molecules, solids and nuclei, and whose work is widely appreciated. Behind it all was a meticulously careful but humorous and kind scientist. ... Jahn, who died ... after a nine-months illness, bravely borne, was modest to a fault and hence not easy to know. But it was worth a little effort to penetrate this reserve, for one would soon be rewarded.

**Article by:** *J J O'Connor* and *E F Robertson*