31 October 1986 Robert Sanderson Mulliken, 7 June 1896
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- Professor Saunders provided me with the necessary equipment... and also gave me some of his
- This [Max Born’s] quantum condition worked pretty well in many respects for the understanding
- £ as electronic angular momentum along the direction of the resultant of this with the nuclear
- In the time since I had first met Hund in 1925, he had finished his famous book Linienspektren undPeriodisches System derElemente
- [Z. Physik. 36, 657 (1926)]. (I never discovered why it was that Hund, like Sommerfeld, always
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31 October 1986 Robert Sanderson Mulliken, 7 June 1896 - H. C. Longuet-Higgins, F. R. S. , 327-354, published 1 March 1990
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R O B E R T SANDERSON M ULLIKEN 7 June 1896—31 O ctober 1986 Elected For.Mem.R.S. 1967 BY H.C. L o n g u e t -H ig g in s , F.R.S. P r e l u d e R O B ER T SANDERSON M ULLIKEN was born on 7 June 1896 in Newburyport, Rhode Island. His father, Samuel Parsons Mulliken, was a Professor of Organic Chemistry at the M assachusetts Institute of Technology, to which he would travel a distance of some 30 miles every day on the Boston and Maine Railroad. Samuel’s income was modest, and the family had little contact with the more influential members of the community. “This may account”, says Robert in his autobiography (B 250)*, “for a deep-seated inferiority complex which I have had, especially toward people in authority, but not towards prominent scientists”. H e recalls helping his father proof read his four-volume tre a tise^ method for the identification o f pure organic compounds'. “It is natural”, says Robert loyally, “that I became interested in science although I was strongly tem pted to move in other directions, such as philosophy or other humanistic fields”. His mother Katherine W. Mulliken (a distant cousin of his father) formed a stronger bond with her son. She was a member of the Unitarian Church, and frequently took R obert along with her. She failed to persuade him to learn the piano, but succeeded in imbuing him with a sense of justice and of the beauty of nature. “I have sometimes experienced very strong feelings of intimacy with nature”, he writes (B 250) “but not of beauty in its laws”. His upbringing was strict and conventional and one gains the impression of an extremely well-behaved but rather shy child. He writes with lurking admiration about a red-haired classmate at the Jackman Grammar School, who became famous for his unconventional exploits and is quoted (B 250) as saying “my worst trouble is that I got my lessons done too quick”. Bossy Gillis, as he was called, rose rapidly in the community and was eventually elected mayor of Newburyport. Half a century later Robert Mulliken was invited back to Newburyport by another mayor, to have a street named after him. * Numbers in this form refer to entries in the bibliography and list of references. 329
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330 Biographical Memoirs T h e e a r l y y e a r s Mulliken gives a vivid account of his early years as a scientist in an article entitled “Molecular Scientists and Molecular Science: Some Reminiscences” (B 204), publish ed the year before he was awarded the Nobel Prize in Chemistry. His interest in the structure of m atter had developed at an early age. In 1913, the year in which Niels Bohr put forward his quantum theory of the atom, Robert presented, at his high school graduation ceremony, an essay (B 211) entitled “Electrons - what they are and what they do”. At M.I.T. R obert discovered that science could be a hard task master. In quantitative chemical analysis, which he hated, he learned the virtues of patience and discipline, and soon came to appreciate the need for self-discipline in any field of endeavour. His B Sc. in chemistry at M.I.T. in 1917 involved the tedious but demanding task of preparing some organic chlorides, for purposes unspecified. These materials “had delightful odors, among which there were very interesting resemblances and differen ces” (B 204) - a typically Mullikenesque account of the matter. After the entry of the U.S.A. into World War I in 1917, R obert joined the U.S. Bureau of Mines as a Junior Chemical Engineer. His first job was in a laboratory for research on poison gases, at American University in Washington, D.C., presided over by Lt. James B. Conant of the Chemical W arfare Service. “The laboratory was about the size of a large living room but was equipped with enough hoods to contain stocks of every kind of poison gas known to man: several kinds of tear gas, of sneeze gas, and of mustard gas, also liquid hydrogen cyanide and chlorine” (B 204). A budding romance with “a very lovely girl” was cut short by an accident in the laboratory, when Robert was working with mustard gas; he had to spend six months in hospital recovering from a severe burn, which required a skin graft. A fter a further brief period of war service, studying the scattering of light by particle suspensions such as cigarette smoke, he found himself back in hospital with 1918 influenza. Emerging from hospital as a World War I veteran, he was honourably discharged from the Chemical W arfare Service as a Private, First Class. In the
fal1 of 1919, after a brief period of employment with the New Jersey Zinc Company in Palmerton, Pennsylvania, Robert went to the University of Chicago to study for a Ph.D. with Professor W.D. Harkins. “I wanted to work on something really fundamental, namely atomic nuclei, in which at that time hardly anyone except Rutherford in England, and Harkins in this country, seemed to be interested. How ever, while at Chicago in 1919— 22 I spent much time in the library reading with great interest the papers of Irving Langmuir, and from them going back to the earlier papers of G.N. Lewis on chemical bonding and valence. After some work on surface tension, another of Harkins’ interests, I did my Ph.D. thesis under his guidance on the partial separation of mercury isotopes by irreversible evaporation” (B 204). By 1922 he had built the first ever isotope factory, stocked with bottles of mercury in which the density had been changed, up or down, by many parts per million, and had recovered his self-esteem as an experimental scientist. “Since then”, he writes, with a rare objectivity, on February 5, 2018 http://rsbm.royalsocietypublishing.org/ Downloaded from Robert Sanderson Mulliken 331
“I have become neither a proper experimentalist nor a proper theorist, but a middle man betw een experiment and theory - and between chemistry and physics” (B 204). This rem ark is a vital clue to Mulliken’s contribution to science. In the 1920s the theoretical physicists were forging a new quantum mechanics while their experimental colleagues puzzled over the intricacies of molecular spectra. As a chemical physicist Mulliken appreciated not only the need to base the quantum theory firmly on experiment but also the importance of using it for interpreting the band spectra of diatomic and polyatomic molecules. This process of interpretation occupied most of his scientific life. A fter H arkin’s death in 1951 R obert Mulliken undertook the preparation of a Biographical Memoir of his old teacher for the National Academy of Sciences; the memoir (B 239) was eventually published in 1975. H a r v a r d In 1921 R obert was awarded a fellowship by the National Research Council, to enable him to develop some ideas of his own. “My first proposal” he writes in the Reminiscences (B 204) “was to go to Cambridge to work with R utherford on a project in /Lray spectroscopy, but the fellowship board did not approve. So I found another promising area of investigation in the isotope effect in band spectra”. At the resolution then obtainable, the emission spectra of molecules appeared to consist of bands, quite unlike the lines in the spectra of atoms. “I found some reproduced photographs in a published article by Wilfred Jevons on what was supposed to be the BN spectrum. Jevons had not said anything about there being bands of two isotopes, but it seemed clear theoretically that there ought to be, and I soon found in Jevons’ published photographs a number of extra bandheads in the positions to be expected for the less abundant isotope of boron” (B 204). It was a student friend, Norman Hillberry, who had suggested to R obert Mulliken that he combine his interest in isotopes with the then hot topic of band spectra and look for isotope effects in molecular electronic spectra. Many years later [in Molecular orbitals in chemistry, physics and biology: a tribute to R.S. Mulliken (ed. P.-O. Lowdin and B. Pullman), p.3. New York: Academic Press (1964), Charles Coulson recalls some advice he received about band spectra from R.H. Fowler at Cambridge: “D on’t take that up unless you are prepared to spend all your life at it. It’s a full-time job, and if you want to be free to dabble in other topics as they arise, keep away from band spectra”. R obert Mulliken somehow managed to do both. So in 1923 Mulliken went as a National Research Council post-doctoral fellow to the Jefferson Physical Laboratory at Harvard, where F.A. Saunders (of Russell- Saunders coupling) and Theodore Lyman (Pioneer of vacuum spectroscopy) were pushing back the frontiers of experimental spectroscopy. Professor Saunders provided me with the necessary equipment... and also gave me some of his old plates showing strange-looking bands which he suggested I might like to try to explain - an invitation which I immediately accepted. Another wonderful piece of good fortune for me was that Jevons had obtained his boron nitride spectrum by putting boron trichloride into active on February 5, 2018 http://rsbm.royalsocietypublishing.org/ Downloaded from 332 Biographical Memoirs nitrogen. This led me to learn that many other beautiful spectra could be obtained by putting other substances into active nitrogen, as had been shown by FJ. Strutt, later Lord Rayleigh. (B 204). (Much later Mulliken was lucky enough to observe a display of ball lightning, and noticed that its colour was remarkably similar to that of active nitrogen.) Robert concluded, however, from careful measurements of the isotope effect, that Jevons’ “boron nitride” spectrum was really due not to BN but to BO. A fter an exchange of letters in the columns of Nature, in which Jevons said (B 204) that o f course the spectrum was that of BN, because it was obtained by putting BC 3 into active nitrogen, Mulliken finally wrote to Jevons, at the suggestion of R.T. Birge, suggesting that they meet to talk things over. Eventually, in the summer of 1925, “I visited Jevons who explained that his letter to Nature was not really his own idea, but that the departm ent head put him up to it; so we got this m atter straightened out”. The identity of the said departm ent head is not recorded. There was more to the BO story than the above account might suggest. In order to obtain really good agreement between theory and experiment it was necessary to suppose, not only that the spectrum was due to BO rather than BN, but also that the lowest energy level of a quantum vibrator has half a quantum of vibrational energy rather than no energy at all. Mulliken pointed this out in another letter to Nature (B 13); a year or two later the concept of zero-point energy emerged from the new quantum mechanics. Besides Saunders and Lyman the faculty at Harvard included E.C. Kemble the molecular theorist and P.W. Bridgman the high pressure physicist. R obert’s contem poraries included J.R. Oppenheimer and J.H. van Vleck, as well as John Slater, Sam Allison and Louis Slichter with whom he shared living quarters at various times. “My first real understanding of Robert Mulliken”, writes Slater in tribute to R.S. Mulliken, “came when, after a sleepless night in which he had been kept awake by the flashes [of trolley cars in the street], he said, £What beautiful copper arcs!’ H e had been looking at them with his pocket replica grating, which he always had with him”. While conducting his own research in the Jefferson Laboratory, Mulliken read what he could of the spectroscopic literature. The leading book on the subject was Som- merfeld’s Atombauund Spektrallinien, which included an up-to-date chapter on band spectra. “Doubtless stimulated by Sommerfeld’s book, I made efforts to bring new order into the data on band spectra, using the old quantum theory, where things centred round the phase integral /p d q as discussed in Max Born’s iiber Atom m echanik” (B 204). Mulliken acquired his own copy of Sommerfeld’s book in 1925, just before the new quantum theory rose like a phoenix from the ashes of the Old. His ability to profit from such an investment testifies to his early mastery of scientific German, part of the essential equipment of any serious physicist. The theoretical physicists were still groping for principles that might account for the properties of atoms and molecules, but Robert Mulliken was happy to use existing formulae for summarizing the spectroscopic data.
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puzzling things about quarter-integral and half-integral quantum numbers. In this connection the Kramers-Pauli molecular rotational energy formula of 1923 g nlhc = [ ( j 1- in which a could be understood as electronic angular momentum along the internuclear axis and £ as electronic angular momentum along the direction of the resultant of this with the nuclear rotational angular momentum, seemed to be useful, but not entirely satisfactory (B 204). Physical theory had, apparently, disintegrated into a num ber of separate fragments, each encapsulating a few grains of truth, but with little connection between them. Bohr had enunciated his A u fb a u p rin zipf or the periodic system of the atoms in 1922-23, and Sommerfeld had given a beautiful survey of the subject in his but electron spin had yet to be discovered, and the Exclusion Principle was still years away. The complexities of band spectra presented a challenge to the taxonomist in Robert Mulliken. “My efforts towards a better understanding and classification of the struc tures of bands led naturally to attempts also to understand molecular electronic states as more or less like those of atoms. At first it looked as if the lower energy levels of BeF, Co + , CN, MgF, AlO, SiN, etc. [all having 9 valence electrons], were analogous to those of the sodium atom [with 1 valence electron], those of CO, NO + , N2; SiO [with 10] to those of Mg [with 2] and those of NO [with 11] to those of A 1 [with 3]” (B 204). The isoelectronic principle promised to yield important insights in molecular spectroscopy as well as in molecular structure, and was to dominate much of Mulliken’s later work. O ther spectroscopists, including Mecke, Birge, Sponer and Hulthen, had glimpsed some of these regularities, but it was not until the advent of the new quantum theory that H und and Mulliken could account for them in a principled fashion.
During the tenure of his National Research Fellowship R obert managed to save enough money to travel, and confidently invited himself to visit many of the leading spectroscopists in Europe. In the summer of 1925 he and Sam Allison crossed the Atlantic together by ship. In London Jevons kindly introduced him to a number of British scientists, including A. Fowler at Imperial College. Lord Rayleigh invited him to breakfast, and in Cambridge he and F.W. Aston discussed their common interest in isotopes. H e recalls that Aston also gave him a demonstration of rock-climbing along one wall of his college: “though not hazardously - on a ledge only one foot above the ground” (B 204). After a brief detour through France, where he discussed with F. Baldet the comet-tail bands of CO +, Robert made for Germany. In Bonn he introduced himself to H. Kaiser and R. Mecke; in Munster, to A. Kratzer; and in Gottingen, to James Franck and H ertha Sponer. “In Max Born’s Institute of Theoretical Physics I was especially interested in talking with Hund, who was then Born’s assistant” (B 204). This was the beginning of a firm and lasting scientific friendship, with wide-ranging implications for the future of molecular physics. on February 5, 2018 http://rsbm.royalsocietypublishing.org/ Downloaded from 334 Biographical Memoirs After a visit to Berlin, where he called on the great atomic spectroscopist Paschen at the Technische Hochschule, Robert travelled to Denmark to meet Niels Bohr. Bohr was at his seaside home in the country, but came to the railroad station on his bicycle to meet Mulliken; they walked back to the house, to be greeted by Bohr’s charming and remarkable wife and their five stalwart sons. Mulliken asked Bohr his opinion about the Rydberg states of molecules - those in which one electron has been excited into a particularly large orbit - but Bohr’s reply is not recorded. A visit to Lund to meet Erik Hulthen, and the glorious summer ended with a trip to the Dolomites in the company of Sam Allison and Hermann Mark, and M ark’s wife Mimi who found it very tiring. Robert Mulliken, so self-effacing in ordinary company, had firmly established himself as a member of the international scientific community. N e w Y o r k U n i v e r s i t y In the fall of 1926, again at the suggestion of Norman Hillberry, R obert moved from Harvard to take up an appointment as Assistant Professor of Physics at the Washing ton Square branch of New York University. He took the opportunity to widen his knowledge of physics and to initiate some more experimental work on diatomic spectra, as well as writing up his joint work at Harvard on the nitric oxide bands. But the most memorable event of the period for Robert was his second visit to Europe, in the summer of 1927. The spin of the electron had now been discovered, Pauli had proposed his Exclusion Principle and Heisenberg had formulated the new quantum mechanics. On a less ethereal plane, Lindberg had just completed his first solo flight across the Atlantic, and Paris was still rocking with the celebrations when Robert arrived there on his way to spend the summer in Gottingen. His recollections of that period are of particular historical interest, and are worth quoting
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