Friedrich Hund: Reminiscences of Robert S. Mulliken, Göttingen 1988

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Friedrich Hund: Reminiscences of Robert S. Mulliken, Göttingen 1988
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Friedrich Hund: Erinnerungen an Robert S. Mulliken, Göttingen 1988
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Video ; F, 12 min

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Aufgenommen für die Inaugurationsveranstaltung der Robert S. Mulliken-Lectures der University of Chicago, 1988. Parallelität der Biographien von Mulliken und Hund, frühe Forschungen. Persönliche und wissenschaftliche Kontakte seit Mullikens Göttingen-Aufenthalt 1927. Frühe Tage der Molekularspektroskopie: experimentelle Ansätze bei Mulliken, theoretische bei Hund. Begegnung in den USA 1929. "Einigung über die Bezeichnung der Molekelzustände." Erinnerung an ein Treffen in Heidelberg 1930. (Mit persönlichen Fotos.)
The inaugurative Robert S. Mulliken-Lecture at the University of Chicago, 1988. Parallels between the biographies of Mulliken and Hund, early research, personal and scientific contact since Mullikens Göttingen days in 1927. The early days of molecular spectroscopy: Mullikens experimental approach and Hunds theoretical. Their meeting in the USA in 1929 and their "Agreement about representing molecular states". Reminiscences of their meeting in Heidelberg 1930. (With personal photos.)
Keywords Hund, Friedrich Mulliken, Robert S. Molekular-Spektroskopie Molecular Spectroscopy Mulliken, Robert S. Hund, Friedrich
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Ladies and Gentlemen, you have invited me to be present at the
inauguration of Robert Mulliken's memorial lectures and to contribute some
reminiscences of the early days of molecular spectroscopy and my association with Mulliken. I appreciate the honour you have accorded to me with your invitation and I would like to express my deep gratitude to you, but I am too old to attend in person. Therefore allow me at least in this way to pass on some recollections of my
colleague and friend Robert Mulliken. Mulliken and I were born
in the same year, 1896. During the war we both had to do some scientific work connected with it; so we had fortunately no occasion to shoot on one another. Mulliken got his PhD in 1921, I my doctoral degree in 1922. Both theses had nothing to do with molecular spectra.
I began to lecture in 1925, he became assistant
professor in 1926.
I connot remember Mulliken's short visit to Göttingen in 1925, but we certainly noticed one another in the following year. I had had some success in understanding the complex atomic spectra with the moment of momenta vector model of Russel and Saunders; I was proud of it and I hoped then to understand the electronic terms of molecular spectra in the same manner. In a paper I discussed the coupling of rotation with electron motion in diatomic molecules, using as examples the rich empirical results Mulliken had obtained. I especially noticed his comparing the spectra of isoelectric molecules. After my paper, Mulliken understood his results better and published new analyses.
The Schrödinger equation appeared in 1926, and now it was rather trivial to interpolate the electronic quantum
states of a diatomic molecule between the limiting case of two separated atoms and the other limiting case where the positive electric charges of the two nuclei were united to one. I wrote a paper on it and in the summer term of 1927 I gave a lecture course on molecular structure at Göttingen.
Mulliken, then for some months at Göttingen, heard at least a part of it. We had lively discussions, we became good
friends, and Mulliken joined me on a hiking trip in the Black Forest. I fear he had to put up with some unusual ways of cooking meals and several uncomfortable nights' lodging.
With our varied approaches to the molecules, Mulliken's wide experimental skill and overview and my perhaps somewhat clearer understanding of the theory, each of us was a good complement to the other.
From then on our paths ran parallel.
Mulliken was quick to comprehend the possibilities that quantum mechanics opened up, the meaning of orbits and quantum numbers. He systematized and explored molecular
spectra by assigning quantum numbers to the individual electrons. I independently did the same, and
in summer 1928 I had just handed a paper to the editor when I received
a copy or a proof sheet of Mulliken's paper. I immediately saw that in some respects Mulliken's interpretation was more convincing. So I requested back my manuscript and wrote a
new one which combined Mulliken's and my reflections.
Of course, Mulliken has the priority in systematizing the simple molecular spectra by assigning quantum numbers to the individual electrons. Mulliken's concept of the "promoted electrons" and my schedule for correlating the electronic states with the two limiting cases were the germ of what was later called the correlation diagram.
In the following year, 1929, I gave lectures at Harvard on molecular structure, John Slater among the audience gave valuable comments. Mulliken and I then met in Chicago.
A picture shows us in front of the Ryerson Physical Laboratory: in the first row you see Heisenberg, Dirac, Gale and Hund; in the second A. H. Compton, Monk, Eckart, Mulliken and Hoyt.
At that time it became urgent to establish a unitary nomenclature for molecules (symbols for orbitals, terms, symmetry etc.). Mulliken and I discussed the possibilities in the long train trip between Chicago and New York. Then other colleagues agreed.
A picture I took during the meeting of the American Physical Society in Washington (in 1929) shows Morse, Crawford, Mulliken and Dennison and has a note: "Einigung über die Bezeichnung der Molekelzustände - agreement on the nomenclature for molecular states."
We had not invented quantum chemistry. Heitler and London began this discipline in 1927. Both, Mulliken and I, had some difficulty in understanding them. Within the framework of molecular orbitals it was Gerhard Herzberg who explained chemical binding in a simple and convincing manner, with bonding and antibonding electrons where an antibonding electron could counterbalance a bonding electron. Of course Mulliken's early concept of promoted electrons came near to the idea.
Chemists got interested in the physical explanation of chemical binding and so Mulliken and I were invited to the meeting of the Bunsen Gesellschaft, German Association for Physical Chemistry, at Heidelberg in 1930 and we had to give lectures. Mulliken perhaps was much more impressed by the evening festivity in the Heidelberger Schloß close to the famous Heidelberger Faß, a tremendous cask of wine, where Walter Weizel contributed to the amusement. Mrs. Mulliken designated later on the evening as the wildest party of her life.
But back to the molecules: all I have said, for Mulliken, all this was only the beginning of a lifelong research in molecular structure. You know it, the big volume of of selected papers edited by Ramsay and Hinze bears witness to it. Now, at the end of my speech, I hope the memorial lectures will contribute to keep alive Mulliken's great merits. To Gerhard Herzberg who probably is present here special regards and compliments may be given.