THE J O U R N A L OF
PHYSICAL CHEMISTRY Registered in US.Patent Office
0 Copyright, 1980, by t h e American Chemical Society
VOLUME 84, NUMBER 17
AUGUST 21, 1980
Reminiscences of My Scientific Rapport with R. S. Mulliked J. H. Van Vleck Department of Physics, Harvard University, Cambrldge, Massachusetts 02 138 (Received:August 22, 1979)
Robert Mulliken was born in Newburyport, Mass., June 7,1896. There is now a street there named Mulliken Way in his honor. His father was Professor of Organic Chemistry at MIT, and he learned chemical terminology at an early age by helping in proofreading. After graduating from MIT in 1917 he entered the armed services as a private, and at the end of the war he emerged a private first class. His rank was thus a more exalted one than mine, as I started and ended as an apprentice seaman third class, a position so lowly it has subsequently been abolished. Also, Mulliken had a more active career, for he was in the chemical warfare services, and reports that he worked in a room about the size of a living room filled with hoods containing every known kind of poison gas, cholorine, mustard gas, cyanide, sneeze gas, tear gas, you name it. The only water I saw in my naval career on State Street in Madison was that which I used to wash the barracks. I first met Mulliken, I believe, when we were both students in a course on “Quantum Theories (sic) and the Theory of Atomic Structure” given by Professor Millikan in the summer of 1920. I was already a graduate student a t Narvard, but attended Chicago summer school because a t the time it was practically the only university offering graduate courses in physics during the summer quarter. Mulliken had (alreadyhad a year of graduate study at The University of Chicago. He had selected it for his graduate work because he wanted to concentrate on isotopes. He informs us that he wrote a high school thesis on “The Electron, What It Is and What It Does”. He had such intellectual curiosity that he wondered about similar questions as applied to the nucleus. At the time about all there was in nuclear chemistry was the study of isotopes, and Chicago’s Professor Harkins was the leading authority of the country on this subject. At first Harkins assigned him a topic dealing with surface tension, but Mulliken finally was able to have the assignment changed to the ‘Much of the biographical information in the present paper is taken from an address, “Molecular Scientists and Molecular Structure” given by Mulliken at the Sanibel Island Conference of 1964 sponsored by Lowdin and subsequently pub1ished.l The writer has also benefitted by reading an unpublished manuscript (available in the Library of American Institute of Physics) on Mulliken’s early days on the faculty at Chicago prepared by S. Bloomenthal, his first doctoral student there. 0022-3654/80/2084-209 1$01.OO/O
separation of the isotopes of mercury, the subject of his doctoral thesis. After he received his Ph.D. in 1921 he was awarded a fellowship of the National Research Council, and continued studying isotopes at Chicago. Then in 1923 occurred the great discontinuity in his scientific life. In his vitae in “Who’s Who in America”, Mulliken lists his fields as follows: research on separation of isotopes, 1920-1922; research on molecular spectra and molecular structure, 1923- (the dash is most appropriate). At the same time that he changed his research interests he also shifted where he worked, as in 1923 he went to Harvard, where he spent approximately 3 years. Naturally I hoped I could say that he decided to move to Harvard because he thought that he could get better training and education, but this wish belies the facts. In 1923 Mulliken was told that he could have a renewal of his NRC fellowship only if he changed his research institution and field. So he was foiled in his desire to stay on in Chicago. In an endeavor to meet the NRC requirements he suggested going to Cambridge, England to work on @-rayspectroscopy with Rutherford, but this request was not granted because the board felt he lacked background in this field, and also it tended to discourage subsidizing foreign study at the time. So Mulliken finally settled on working in molecular spectra at Harvard. In his autobiographical memoirsLhe mentions how he was void of any previous experience in 0 1980 American Chemical Society
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the new field, and how he had the help of Saunders, on the experimental side, and of Kemble on theory. Being myself something of a rugged individualist, it rather irks me to say that bureaucracy was right and Mulliken was wrong, but I believe that if Mulliken had joined the crew of experimental nuclear physicists at Cambridge, his contribution to science would not have been as unique and outstanding as it has been, although I am sure that he would have made substantial contributions in this field which soon became overpopular. Had Mulliken remained at Chicago, he could have had Gale to help him on the experimental side of his new field of spectroscopy, but except perhaps for Epstein at Cal Tech there was no one in the country who was nearly as well qualified as was Kemble to direct research and give training in quantum theory in the early part of the 1920’s, the last days of the old quantum theory and the dawn of quantum mechanics. I cannot overemphasize Kemble’s role in training young theoretical physicists in quantum theoretical physics. For one thing, the lecture course which Kemble gave provided a systematic presentation of the mathematical basis of the old quantum theory, in sharp contrast to Millikan’s course which Mulliken and I attended in 1920, as already mentioned, and which was a rather disconnected survey of topics in quantum theory as viewed by an experimentalist, with many of the lectures given by the students themselves, none,too familiar with the subject. In her doctor’s thesis entitled, “Quantum Physics in America, 1920-1935” (to be published in book form by Arno Press, New York), Dr. K, Sopka, a historian of physics, includes a sort of academic geneological chart showing Kemble’s academic descendants, i.e., his own Ph.D’s, those who took their Ph.D.’s under them, and so on until 1935. This impressive chart includes only Kemble’s legitimate scientific children, Le., those who officially took their Ph.D.’s under his direction. However, it does not include two illegitimate children whom he inducted into quantum theory. These men were Slater and (a little later chronologically) Mulliken. They do not qualify for legitimacy because Slater wrote an experimental thesis under Bridgman, and Mulliken was a postdoc. If these two men and their academic progeny were included, the chart would have to be at least three times as big. If you read Mulliken’s vitae in “Who’s Who” there is a gap of 1year between the termination of his NRC fellowship at Harvard in 1925 and the commencement of his term as assistant professor at New York University in 1926. It took some research (letter of inquiry to him written by Mrs. Sopka) to solve this mystery. Actually it was an important year. He had saved up enough money on his fellowship to travel in Europe in the summer of 1925. In his memoirs1 he recites the physicists he met on that trip and one cannot help noticing that they were mainly experimentalists, in contrast to the many theorists he visited on a later European trip in 1927. Kemble secured for Mulliken some sort of research grant from Harvard for him to stay there during the academic year 1925-1926, during which time among other things he worked with F. H. Crawford on the Zeeman effect of molecules, and he had the benefit of further contact with Kemble. His assistant professorship at New York University started in 1926,just a few months after the advent of the true quantum mechanics, and I doubt if it had been completely assimulated at Harvard while he was still there. You perhaps have heard about the little group at Columbia in 1926 that held an informal seminar on quantum mechanics once a week or so, with Kronig, Rabi, Quinby, and Wang among the group. Mulliken does not appear to have been a member
Van Vleck
of this seminar despite the fact that in those days it cost only 54: for the subway ride from Washington Square to Columbia and the probability of being mugged en route was less than now. Perhaps Mulliken had heard Debye’s dictum that a good theoretical physicst should wander into the desert for 1year or so. No one would call Manhattan Island exactly a desert, but as far as I can see Mulliken was more or less isolated at Washington Square from other theorists. Nevertheless, he succeeded in forming by himself a wonderful understanding of the results of quantum mechanics even though he was not a mathematical physicist. It is remarkable how soon Mulliken became established as a leading figure in molecular spectra. In a report on physical optics written in late 1926 Birge2 stated, “Mulliken has developed a complete systematization of band spectra” (in terms of electron configuration). After declining offers from John Hopkins and Minnesota he finally moved from New York to the University of Chicago in 1928. It was primarily Arthur Compton and Gale who spotted his outstanding contributions and who were responsible for the offer. The then president of Chicago, Max Mason, though violently opposed to quantum theory in the days when he was a physicist, fortunately endorsed the appointment. I will not attempt to describe Mulliken’s career at Chicago except for one thing. I have read somewhere that a delay in getting a grating was responsible for Mulliken shifting from experiment to theory in his early days at Chicago. Here again the bureaucrats, the purchasing agent, or whoever was responsible for the delay are to be congratulated. Perhaps it was instrumental in his writing his classic articles on the interpretation of molecular spectra which were two of the articles in the first four volumes of the Reviews of Modern Physics, then called the Physical Review Supplement. These first four volumes are real classics, like the 11th edition of the “Encyclopedia Britannica”. Besides Mulliken’s contributions there are articles by Arthur Compton on the Compton effect and related phenomena, Karl Compton on gaseous discharge, Kemble on quantum mechanics, Birge on the best values of atomic constants, Dennison on polyatomic molecules, Eckart on group theory, and Fermi on the quantum theory of radiation. I now turn, after this review of Mulliken’s early career, to my announced title of my scientific rapport with Mulliken. In looking at my publications over the years, I find that six of my papers end with notes of thanks to Mulliken, considerably more than for any one else. I do not mean that there are not other physicists who have influenced my career in a major way. In particular I owe to Kemble my early training in quantum theory and it was two Dutch physicists, Kramers and Gorter, who aroused my interest in respectively crystalline electric fields and parmagnetic relaxation. However, I have had more interactions with Mulliken on the details of specific problems than with any one else. Most of the rest of what I will say centers around the six of my papers which end with thanks to Mulliken, thereby giving some indication of the type of problems in which he has had an interest, and how wide this interest is. My scientific rapport with Mulliken seems to have begun, or at any rate first made headway, in 1926 where he was in New York and I in Minneapolis. We must have seen each other at meetings at the American Physical Society, for I appreciated that he was an authority on molecular spectra, and he knew that I was specializing in understanding the mathematical framework of the then
The Journal of Physical Chemistry, Vol. 84,
Scientific Rapport with Mulliken
new quantum mechanics, particularly as applied to molecules. We exchanged results in advance of publication, then harder than now since there were no Xerox machines, with the corresponding preprint mania. There are two common paramagnetic gases, oxygen and nitric oxide, The magnetic susceptibility of oxygen does not pose any great problem, as it is in a 32state, and so has almost exactly the value corresponding to a spin of 1 only weakly bound to the molecule. On the other hand, accounting for the magnetic behavior of nitric oxide requires much more detailed quantum mechanics. Pauli had tried to explain it in the days of the old quantum theory in almost his first paper, but understandably without any real success. I started thinking about the problem it posed in the fall of 1926. As early as 1925, Mulliken had concluded that the ground level of the NO molecule was 211 (in present day notation), and this result was in the lite r a t ~ r e .However, ~ only from correspondence with Mulliken I learned of the fact that the separation of the doublet components which is caused by spin-orbit interaction was about 122 cm-l. I proceeded with the quantum mechanical calculation and found that the results agreed with experiment at room temperatures, the only temperature for which susceptibility measurements were then available. I wrote the results to Mulliken. They showed that the ground doublet was regular rather than inverted, something about which he was not sure at the time. I decided to submit a 10minute paperq for the meeting of the American Physical Society held in New York in February, 1927. This was a comparatively small meeting, and I was too late to meet the deadline for the big annual meeting which had been held a month and a half earlier in conjunction with the AAAS in PhiZadelphia and which I had attended. So I persuaded Mulliken to read my paper for me at the New York meeting. This, incidentally, was the first 10-min paper I contributed to the Physical Society in which I used the real quantum mechanics rather than the old quantum theory. The spectroscopically determined 122-cm-l separation of the ground level of NO was first mentioned in print in a letter by Jenkins, Barton, and Mulliken in the January 22,1927 issue of Nature. One of the concluding remarks of this communication was “From the data now available, it will be possible to calculate ... the magnetic susceptibility of gaseous nitric oxide”. In fact, by the time their paper appeared, I had already done so. When I first made my calculations on NO, quantum mechanics was so new and I wazi so inexperienced in it, that I did not know whether to believe them. There were data only at room temperatures There should theoretically be deviations from Curie’s law, and hence a definitive test of the theory would be provided if the predicted deviations could be confirmed experimentally. Unknown to me, measurements of the susceptibility down to progressively lower temperatures were made at three different laboratories in different parts ot‘the world, and the theory was indeed confirmed, much to my gratification. The calculation of the susceptibility of a molecule which has a 211state with components separated by 122 cm-l is obviously a rather specialized one, but I cannot overemphasize the influence which this early computation of mine and the number 122 had on my career in magnetism. I t taught me the importance of including the second-order Zeeman effect, and I learned the limiting conditions under which Curie’s law is valid, also that the all important thing for the susceptibility is the size of the energy intervals compared to kT. The formalism which I developed furnished the backbone for three papers which I published I
TABLE I ‘P ’P case (b)
,P case (a) 3Pcase (b) 3P case (a) ‘Dor2D
‘s
(p-type doubling)
A u = ( c l - C z ) i U t 1) AuIl2= Avjlz= (CI - cz)iko’Z t 1) A v I l 2 = a(j t AY,,, = bo’’ - 1 / 4 ) ( j t 3/z) 0 Avo = Aul = A v z = (CI - ci)jko’k 1) A v o = fi A u , f (C, - CZbO + 11, A v z 0 Au- 0 A V = a’(.ik ‘12)
-
-
+
No. 17, 1980 2093
A1H CH,*OH HgH, CdH, ZnH,
BO,NO c , , He,
N* CH
in 1927-1928 in Physical Review, and a substantial part of the material for my book on electric and magnetic susceptibilities. My next piece of scientific rapport with Mulliken was on the subject of A doubling. When a molecule has electronic angular momentum, either orbital or spin, ita states are doubly degenerate in a stationary molecule since the sence of rotation about the axis of figure is immaterial. However, this degeneracy is lifted by the molecular rotation, giving rise to a decomposition which is called H y p e doubling. For example, each of the components 2&/21 21z3 2 of the 211 doublet of NO is in reality a pair of near y coincident energy levels whose decomposition increases with the amount of rotation. The existence of A doubling was first predicted qualitatively by Hund, and Kronig calculated quantum mechanically the amount of the splitting to be expected for molecules in singlet states. I had a hard time understanding Kronig’s wave-mechanical analysis, especially the recursion formulas he used for generalized Jacobi polynomials. Also I realized that except for singlet states it was essential to include the spin. I finally was able to derive the formulas for A doubling inclusive of spin. I wrote Mulliken my results and asked him how they agreed with experiment. I still remember the language of his letter in reply, although unfortunately I have not preserved it. He said that my predictions fitted experiment “like a glove”. Consequently, we submitted a 10-min paper at one of the meetings of the American Physical Society. It was read by title since the meeting was in California. The abstract5 of this paper is the one publication in which we were coauthors and we were then at our maximum geographical separation, he in New York and I at the University of Minnesota. By the time we decided to publish more extensively, he had moved to Chicago and I to Madison, Wisconsin. Despite this proprinquity, we decided it was easier to publish our results in two separate continguous paper^.^^^ The left side of Table I, reproduced from my 1929 article, shows my prediction of the dependence of the separation Av of the A-doublet components on the rotational quantum number, and the right lists the molecules which in the following paper Mulliken showed agreed with experiment. In modern notation, the letters S, P, D would be replaced by E,II, A and j , j , respectively by J,K. The subscripts 1/2, 3 / 2 refer to the components of the 211 state having respectively 1Ql = ‘/2, and IQl = 3/2 (Q denotes the combined spin and orbital angular momentum about the molecular axis). Mulliken’s paper was, incidentally, number VI11 (sic) in a series of articles in Physical Review which Mulliken entitled, “Electronic States and Band Spectrum Structure in Diatomic molecule^'^, but with the subheading, “Some Empirical Relations in a-Type Doubling” (now called A-type doubling). In 1930 Mulliken and I both held Guggenheim fellowships. Mulliken had been married for only a short time, so that the trip to Europe was practically a honeymoon
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for him, while I had been married almost 3 years. We both spent considerable time in Leipzig. Mulliken’s knowledge of German was much better than mine, so that he could speak extemporaneously. On the other hand, before going to Leipzig, I had, in Zurich, hired a then graduate student (Staub) to translate the one talk which I gave at colloquia in various German-speaking universities. I committed it to memory, and after my lecture at Leipzig, Heisenberg told me, “I had no idea you knew German so well-you spoke even more grammatically than Mulliken”. However, when I finished the same lecture in Munich, Sommerfeld appropriately remarked, “you seemed to do all right in the talk itself, but seemed very confused in the discussion that followed”. When I returned to Wisconsin in 1932, I had a very able postdoctoral student from Scotland, Robert Schlapp, and I suggested that he make some calculations on the relative intensities of the various components of the red atmospheric absorption band. Since its lines are intersystem combinations, the calculations8are more complicated than for an ordinary band. It was known that the lower state for these bands is 3Zg. Schlapp found that good agreement with experiment could be obtained if he assumed that the lines were caused by electric dipole transitions, and that the upper state is ‘2;. However, Mulliken insisted that the upper state should be lZg+ on the basis of how states are arranged when one uses the well-known Mulliken-type diagram that links the behavior for infinitely separated and united atoms. This conclusion was also confirmed by related measurements by Herzberg on the location of the l A state. On the other hand, for a while I was violently opposed to the lZg+ interpretation, as the upper and lower states would have the same parity and so the radiation would have to be of the quadrupolar rather than dipolar type provided it arises from an electric moment. Then the calculations of Schlapp would not work. In particular there would be some unobserved transitions of the type A J = f2. After Schlapp had published his results and returned to Edinburgh, it finally dawned on me that the transition could be caused by a magnetic rather than electric dipole m ~ m e n t .The ~ upper and lower states would then have the same parity. The calculations of Schlapp on the relative intensities of the different components lines of the band then were still applicable, as all that is required is that the moment tranform like a vector, i.e., a dipole rather than like a second rank tensor such as a quadrupole moment. So the first observation of magnetic resonance was made by Frauenhaufer in 1814 when he detected the red atmospheric absorption bands, shown by Ergaroff in 1851 to be due to oxygen. Incidentally, the absolute as well as relative intensities agree with the magnetic dipole interpretation. If the band were caused by an electric dipole moment, the red atmospheric bands would be 100000 times stronger than they actually are, and the whole biological development of our planet might have been different. Theoretical estimates of the quadrupolar intensities are about as large as for the magnetic dipole, thereby explaining why the components AJ = f 2 have never been observed. Besides the three articles which I have cited, the other threelo which terminate with thanks to Mulliken are a rather schematic calculation of the binding energy of the methane molecule, one on the quenching of iodine fluorescence by a magnetic field, and one on isotope shifts in molecular spectra, which I wrote shortly after moving to Harvard in 1934. I will not go into the technical details of these papers, but without so doing it is clear that anything connected with isotope effects in molecules ties in
Van Vleck
with Mulliken’s original interest in this subject. In 1935 I also wrote a paperll entitled “The Group Relation between the Mulliken and Slater-Pauling Theories of Valence”. In the early 1930’s Mulliken had become interested in polyatomic molecules, and I helped translate the different types of symmetry which he observed into the language of group theory. This paper showed how a given symmetry type is preserved as one passes between the limiting cases of uncorrelated molecular orbitals and localized electron pair bonds. About 4 years after the appearance of my paper, another article with essentially identical subject matter appeared in the same journal.12 So even in those early days of much sparser literature it was difficult for authors, editors, and referees to keep up with what was going on. In 1939 Gilbert King13 and I wrote a paper on the dipole-dipole resonance forces coupling identical atoms at large distances when one of the atoms is in an excited states and the other in its ground level. Because of the resonance effect associated with deexcitation of one atom and excitation of the other, the potential energy coupling two well-separated atoms should vary with the inverse cube of the distance of separation rather than as the inverse sixth power characteristic of the ordinary nonresonant case. We calculated the value of the coefficient C in the potential C / r 3when the normal and excited states are respectively of the S and P type. In 1960, Mulliken wrote me that King and I had the sign of C wrong for the triplet states of the molecule formed by the two atoms, although correct for the singlet case.14 There are two remarks I would like to make in this connection. One is that it makes one rather unhappy that an error in a paper can go for over 20 years undetected, and one wonders who reads one’s papers, at least critically. The other is that whereas Mulliken considers himself more of an interpretive than a highly mathematical physicist, at the same time he has a remarkable understanding of the import of quantum mechanics, and ability to ferret out the meaning of a mathematical calculation. A letter from Dean Crewe suggests that my talk should discuss the recent outgrowths of Mulliken’s earlier research in molecular spectroscopy. It is almost superfluous to mention that Mulliken has laid much of the foundation for modern molecular spectroscopy and that as time progresses it is increasingly recognized that the method of molecular orbitals provides a more intuitive and flexible description of valence phenomena than does that of localized electron pair bonds. I would like to focus my attention particularly on the astrophysical impact of some of Mulliken’s early work. The isotope shifts in molecular spectra are obviously of interest when they are observed in the sky, for they shed light on the cosmological questions connected with the abundance of different isotopes.15 A very powerful new tool of the astrophysicist is radioastronomy, id., the microwave scanning of regions exterior to the earth. Here the small spacings between the two components of a A doublet, such as Mulliken and I collaborated on in 1928, turn out to be very useful, though at the time of rather academic interest. The nice thing about A doublets is that their width depends on J, and so by proper selection of J one can find a wavelength that can be picked up by the microwave antenna, which usually can span only a limited frequency interval. Also isotopes effects are more easily detected in microwave than in optical spectra, as small changes in atomic weight produce a greater percentage change in frequency. Two of the molecules included in the 1929 table reproduced earlier in the present paper were
J. Phys. Chem. 1980, 84, 2095-2102
OH and CH. A A doublet of both these molecules has been observed by the radioastronomers. This doublet for the ‘II3/2 state of OH with J = 3/2 comes at a wavelength of 18 cm, and was observed in Cassiopea A and later in some 50 interstellar sources in the gallatic plane.16 When I was in Sweden last December I was proudly shown the radioastronomy equipment where the A doublet of CH in the 2111/2state with J = 1/2 was first observed in 1973.17 It has R wavelength of about 9 cm. It was also detected almost simultaneously by the astronomers working at Green Bank.ls This same doublet was also foundlS in the comet Kohoutek in 1974, but this is a more difficult experiment, and apparently the line is observable only during certain phases of the comet’s trajectory.’O Penzius and Wilson were recently made Nobel-laureates for their discovery by means of radioastronomy that the temperature of interstellar space is about 2.7-3.0 K. The measurement was made at a wavelength of 7.35 cm. However, it should not be forgotten that in 1941 McKellar21obtained a value of 2.3 K for the temperature associated with the distribution of CN molecules between their two lowest rotational states in interstellar space. This method using relative intensities in optical spectra is, of course, not as direct as the determination of a background temperature by radioastronomy, but certainly Herzberg was too cautious in saying in 1950 that McKellar’s result had “only a very restricted meaning”.” The important thing is that it relates to a completely different wavelength, viz. 0.263 cm, than can be detected by radioastronomy unless one has a laboratory above the earth’s atmosphere. The near agreement of the temperature obtained at completely different wavelengths by very different methods gives one confidence in the validity of the Planck radiation formula in empty space, thus furnishing evidence for the big bang theory of the universe.
2005
Herzberg in the concluding paragraph of his wonderful book2’ states, “the study of molecules in interstellar space is a veryyoung field”. This was in 1950. Today it ie still a young field, but a very virile one. For the part of the universe other thar, interstellar space, i.e., stars, comets, and planets, including our own, molecular spectra is in its prime and still going strong.
References and Notes (1) (2) (3) (4) (5)
R. S. Mulllken, J. Chem. Phys., 43, S2 (1965). R. T. Blrge, J. Opt. Soc. Am., 14, 103 (1927). R. S. Mulllken, Phys. Rev., 28, 561 (1925).
J. H. Van Vleck, Phys. Rev., 29, 6 f 3 (1927). J. H. Van Vleck and R. S. Mulliken, Phys. Rev., 32, 327 (1928). (6)J. H. Van Vleck, Phys. Rev., 33,467 (1929). (7) R. S. Mulllken, Phys. Rev., 33, 507 (1929). (8) R. Schlapp, Phys. Rev., 39, 806 (1929); 51, 342 (1937). (9) J. H. Van Vleck, Astrophys. J., 80, 161 (1934). (10) J. H. Van Vleck, J. Chem. Phys., 1, 177 (1933); Phys. Rev., 40, 544 (1932); J . Chem. Phys., 4, 327 (1936). (1 1) J. H. Van Vleck, J. Chem. Phys., 3,803 (1935). (12) 0. E. Kimball, J. Chem. Phys., 8, 188 (1940). (13) 0. W. King and J. H. Van Vleck, Phys. Rev., 55, 1165 (1939). (14) R. S. Mulliken, Phys. Rev., 120, 1674 (1960). (15) Cf., for instance, the review article by M. Bertojo, M. F. Chul, and C. H. Townes, Science, 184, 619 (1974). (16) S. Welnreb, M. L. Meeks, J. C. Carter, A. H. Bartlett, and A. E. E. Rogers, Nature (London), 208, 440 (1965). (17) 0. E. H. Rydbeck, J. Ellder, and W. M. Irvlne, Nature(London),248, 466 (1973). (18) B. E. Turner and B. Zuckerman, Astrophys. J., L59 (1974). (19) J. H. Black, E. J. Chalsson, J. A. Ball, H. Penfleld, and A. E. Lllley, Astrophys. J., 191, L45 (1974). (20) 0. E. H. Rydbeck, P. D. Godfrey et ai., Icarus, 23, 595 (1974). (21) A. McKellar, Publ. Dom. Astrophys. Obs., Victorla, 13.C., 7, 25 (1941). The relation between the results of McKellar and of Panrlus and Wilson was first noted by G. B. Field and J. L. Hltchcock, Phys. Rev. Lett., 16,817 (1966). Subsequent to the original measurements of P and W microwave measurements of the background radlatlon have been extended to cover a range of wavelength from 0.33 to 73.5 cm. (22) 0. Herzberg, “Spectra of Diatomic Molecules”, Van NostrandRelnhdd, New York, 1950, pp 496-7.
Spectroscopic Studies Based on the Pioneering Work of R. S. Mulliken 0. Herrberg National Research Council of Canada, Ottawa, Canada (Received: September 10, 1979)
Mulliken’s work on Rydberg series has greatly stimulated the work on the Rydberg series of Hz and N2carried out at NRC Ottawa. The changing coupling conditions with increasing n are reflected in the changing structure of Rydberg bands. The lowest Rydberg state of N2,a”,Z:‘ predicted by Mulliken was observed by absorption in flash discharges. King and Van Vleck predicted van der Waals maxima in certain excited states of H2and Mulliken in 1960 sharpened the understanding of these maxima. We have observed several levels in the C lIIUstate above the dissociation limit which confirm these predictions. A new group of diffuse emission bands of H2near 1500 A agrees with diffuse structures predicted by Dalgarno and Stephens in the continuous part of the Lyman system (B-X). Independently, Mulliken accounted for certain diffuse bands of I2 as such structured continua. Thg spectra of HeNe+ and HeAr+ recently analyzed represent excellent examples of large A doubling and spin doubling in 211 states first discussed by Van Vleck and Mulliken and Christy. A brief discussion of HzO+and NH3+(the latter quite fragmentary) concludes the discussion of work based on Mulliken’s seminal ideas.
The year 1928 in which Robert S. Mulliken joined the faculty of the University of Chicago was a vintage year in molecular spectroscopy. Not only was this the year in which the Raman effect was discovered but also in this year appeared, among other important papers, the famous 0022-3654/80/2084-2095$01.00/0
paper by Wigner and Witmerl on the correlation of diatomic molecular states with those of the separated atoms, the well-known paper by Hill and Van Vleck’ on the spin splitting in 211 states, the important paper by Condon3on the wave mechanical interpretation of thedFranck-Condon 0 1980 American Chemical Society
