Obituary: Gerhard Herzberg

GERHARD HERZBERG was the pre-eminent molecular spectroscopist of his generation.

His career spanned the half- century following the revolution of physics through the advent of wave- mechanical quantum-theory in the 1920s, a development he witnessed directly during his formative years as a student. It was also a period that saw the essentially complete reduction of macroscopic physical chemistry, and hence in principle of the whole of chemistry, to microscopic chemical physics, for which wave-mechanics provides a particularly essential foundation. Much of the experimental basis of this reduction came through spectroscopy.

Herzberg was born in Hamburg on Christmas Day 1904 into a middle-class family having little academic background. Interest in astronomy, atomic and molecular physics was aroused by his teachers in secondary school and he went on to study technical physics at the Technische Hochschule in Darmstadt. During his pre-doctoral studies he quickly revealed exceptional talents, particularly in spectroscopy, and a post-doctoral period at Gottingen and Bristol (1928-30) brought him into contact with many of the great names in the new physics. There he also met and married Luise Oettinger, a student of physics at Frankfurt, who became herself a highly respected spectroscopist.

His first independent post was as Privatdozent in spectroscopy at Darmstadt. But, informed in 1934 that, married to a wife of Jewish origins, he could not continue to teach students, he had to contemplate emigration. One of the leading molecular physicists at the age of 30, he realised he would soon be without a job as well as without a country. Openings were few but the intervention of a postgraduate student, John Spinks, from the University of Saskatchewan, led to an offer by its President, W.C. Murray, of a Research Professorship there.

As Herzberg recalled,

From Chicago . . . we travelled by train via Minneapolis, Winnipeg and Regina and finally to Saskatoon. On travelling between Winnipeg and Saskatoon, we became increasingly worried because the train seemed to stop at places that had only a few houses, and we were wondering what Saskatoon would be like. We were pleasantly surprised.

So began 10 highly productive and happy years and a lasting attachment to Canada.

Facilities at Saskatoon were however limited and the teaching load considerable. Herzberg therefore accepted an invitation in 1945 to join the staff of the University of Chicago's Yerkes Observatory. The attraction lay in the emphasis on research and in a closer association with his first love, astronomy. Life in the new surroundings was, however, not without problems. Rescue came soon, in an invitation to return to Canada as Director of the Division of Physics of the National Research Council in Ottawa, in 1948. There he was to remain for the rest of his life and to create his greatest achievement, the laboratories of its spectroscopy section - among the world's "Grand Labs" in physics.

Traditionally, NRC's role had been to provide federal support for applied research in infrastructural areas such as building, telecommunications, roads, agriculture and fisheries. It also maintained the national standards laboratory. Largely through the foresight of E.W.R. Steacie, Director of its Division of Chemistry, NRC's post-war remit was widened to include areas of "pure" scientific research, to strengthen what we would now call Canada's "science base" in a way that its universities could then not yet do.

The Division of Physics was therefore assured the long-term resources needed to equip and operate the new laboratories. Herzberg followed Steacie in a departure from what might have been the conventional model of staffing in a governmental institution. The number of permanent staff was small, typically around half a dozen. The list of appointments made during his tenure of the Directorship, between 1948 and 1969, gives only 12 names. They were chosen to lead the different but still independent specialities within the broad church that spectroscopy had become, each largely free to pursue his own direction.

The main body of scientific workers lay in a stream of post-doctoral research fellows, appointed and funded by NRC for periods up to two years and usually assigned to work with a member of the permanent staff - much as postgraduates work in universities. The success of such arrangements depends on the ability to attract good people, and of this there was never any question. The name of Herzberg, "GH" as he was now affectionately known, was an irresistible draw world-wide. Although his personal research, undiminished in vigour and productivity, had become a minor element in the total output, his authority, guidance and enthusiastic support continued to provide a dominant coherence to all that went on. His personal example set the standards, especially in the publications that appeared under the laboratory's name.

Some 80 post-docs passed through in the years 1948-69. Many from abroad remained in Canada, fulfilling one of the founders' hopes. Many went on to distinguished careers in universities, the public service or in science-based industries. Besides Herzberg himself, seven former post-docs or staff-members became Fellows of the Royal Society. To the lists must be added the many spectroscopists of all levels of seniority and from all over the world who were welcomed as visitors for durations of weeks or months, to learn or to use the laboratories' equipment so generously put at their disposal. Many analyses in distant labs were made on spectra taken in Ottawa. Finally, Herzberg's success would not have been possible without the support of two people who joined him at the outset: Alex Douglas, his former graduate student at Saskatoon, and Jack Shoosmith, his technical assistant, formerly at King's College London.

Spectroscopy is a technique. It analyses the radiation absorbed and emitted by atoms and molecules. It is through their spectra that these reveal themselves to us: their electronic and geometric structures, their internal dynamics, the strengths of their bonds and their chemical interconversions. Spectroscopy gives us our main, and often only, means of access to those physical domains of space and time, way outside the bounds of direct human experience, in which atoms and molecules operate. Therein lies its fascination. Molecular structures take us to small distances - millionths of a millimetre. Molecular astrophysics takes us into the vast distance of outer space. And primary chemical reactions take us into regimes of brief instants in which a microsecond can be a long, long time.

Herzberg's explorations of all of these domains were numerous, diverse and profound. He was above all an experimentalist, drawing on theory as needed but taking it largely as developed by others.

His interests fall into several classes. One lifelong interest lay in the study of the electronic structures of diatomic molecules. Their numerous electronic states occur in sets whose patterns have led to the recognition of an electronic shell-structure of chemical bonding, a wholly quantum- mechanical, non-classical phenomenon generating the laws of valency familiar to all secondary-school students of chemistry, in a way analogous to the shell- structure of atoms that generates the chemist's periodic table of the elements.

The concept of antibonding electrons first introduced as such by Herzberg is now also a part of A-level chemistry. Atoms of all the elements will intercombine to form diatomic molecules in at least one bound electronic state, which has to live only long enough to emit a spectrum, a fraction of a microsecond. Typical sources are electric discharges through gases and thus, as an example, the known bound states of diatomic helium, the quintessentially inert monatomic element, long outnumbered those of diatomic hydrogen, the prototype of chemical binding.

Herzberg's most seminal contribution lies however in the monumental review of the field, in his book Spectra of Diatomic Molecules, the second edition of which (1950) is definitive and timeless.

In astrophysical spectroscopy, Herzberg's most abiding interest, the problem is often to identify an unknown molecular species responsible for a known stellar or interstellar spectrum, by reproducing the stellar spectrum in a terrestrial source. Successes include the detection of CH+ in interstellar space and triatomic carbon, C3, in the tails of comets. The most heroic, however, was perhaps the identification of molecular hydrogen in the atmospheres of Jupiter, Neptune and Uranus by its weak absorption-lines in the infra-red, reproduced in the laboratory in an absorption-cell some 20m long, fitted with internal mirrors giving with 250 traversals an absorbing-path of 5km through cooled, compressed gaseous hydrogen.

The third major interest lay in attempts to extend the study of electronic structure to molecules of more than two atoms. An immediate problem is that most small, chemically stable polyatomic molecules have closed-shell configurations and are colourless. Their excited electronic states lie at high energies and are so unstable that they do not live long enough to emit radiation. Their absorption-spectra are often also diffuse. Open- shell molecules, in contrast, are usually coloured in the visible or near ultraviolet and their absorption-spectra are sharp. The largest accessible class of such molecules are the chemist's free radicals, long postulated as short-lived, highly reactive intermediates in many chemical processes but barely characterised in structural detail.

The breakthrough came with the invention of flash-photolysis, first exploited by Norrish and Porter in Cambridge in the late 1940s and then developed independently by Herzberg and D.A. Ramsay who joined him from the Chemistry Division at NRC in 1949. A gas of stable molecules such as ammonia, NH3, is subjected to an intense flash of ultraviolet light for a period of microseconds. It decomposes into the radical NH2 and atomic hydrogen, H. After a short delay, a second short flash of white light probes the reaction-vessel before the radicals have had time to decay by reactive collisions and the emergent probe light carries their absorption spectrum. The spectrum of NH2 was the first to be thus fully analysed.

Many others followed, including those of HCO, HNO, BH2 and N3, and it seems ironic that more is known about excited states of free radicals than of stable molecules. But the crowning triumph was in obtaining the spectra of the methyl radical, CH3 (1956) and, finally, after 17 years of effort, of the methylene radical, CH2 (1959), subsequently in both of its electronic isomers, triplet and singlet. The spectra were taken by Jack Shoosmith.

For these explorations of free radicals, Herzberg's probably most notable achievements, he was awarded the Nobel Prize for Chemistry in 1971.

Herzberg's life was rich and influential. What endures? In the short term, the respect and affectionate memories in the minds of all who knew him - colleagues, collaborators, students for his integrity, modesty, intellectual stimulation, generosity and warm friendship. In the medium term, some of his spectroscopic triumphs and the traditions in the lab he founded, perhaps. But spectroscopy has moved on, and the development of new technologies such as lasers, combined with the immensely powerful computational facilities we now have, unknown in Herzberg's days, have changed the directions of enquiry.

The foundations remain, however, and GH's lasting monument will be his trilogy of monographs, Molecular Spectra and Molecular Structure, I-III, published between 1939 and 1966. The cry "Look it up in Herzberg" will resound in laboratories as long as molecular spectroscopy is practised.

The field of spectroscopy has always seemed to me to be much freer from the competitive pressures that tend to afflict many other areas of research, writes Sir Harold Kroto. I think this is because two founding fathers of the field were real gentlemen and their general attitude had a massive influence both intellectually and ethically on the way others in the field worked. One was E. Bright Wilson of Harvard, who died in 1989, and the other was Gerhard Herzberg.

The exciting and open atmosphere that pervaded molecular spectroscopic research was due in no small measure to their influence on their numerous students and collaborators.

As a young chemistry student who had, almost overnight, fallen in love with spectroscopy, I suddenly became acquainted with real spectroscopy when I found a book in a local bookshop. It was an amazing book, beautifully bound in a yellowy-golden cloth-bound cover. The title was emblazoned in gold letters on the spine, The Spectra of Diatomic Molecules, by Gerhard Herzberg. It was the first in what was to become the famous series Molecular Spectra and Molecular Structure.

I understood almost nothing of the text but it was illustrated with beautiful reproductions of spectra. They showed elegant patterns of lines that tell us that molecules could count accurately; and this captured my imagination. These patterns were the key to understanding molecules: their atomic composition, their structures and many other properties, all to be divined by careful analysis of their colours.

In spectroscopy was to be found the most perfect and particularly the most direct pictorial and quantitative expression of quantum-theory. This perhaps indicates why I and many of my spectroscopic colleagues are so enamoured of the field. I am sure that it was this perfection to which we became addicted, and perhaps the greatest addict was GH himself - the father of high-resolution molecular spectroscopy.

In 1964, when I received my PhD, an invitation to come to Ottawa - the Mecca of molecular spectroscopy - was irresistible. GH had by then assembled an all-star team of senior scientists: Alec Douglas, Cec Costain, Don Ramsay, Boris Stoicheff - all outstanding in their own right. I had met GH once before at a conference, in 1963, but in Ottawa I quickly came to know him on a more personal level. He always seemed larger than life, with a strong, distinctive but friendly personality, even though he was physically quite small.

GH's features were striking, his voice deep and vibrant and his perfect spoken English appeared, to me, to be a combination of German precision in delivery but without accent. He was always approachable and keen to discuss all manner of problems. Furthermore, he exuded an enthusiasm that immediately made one feel a valued member of the laboratory. The ease with which I was able to communicate with the other senior scientists and my peers was a key part of my education and came from the example he set.

Post-docs enjoyed significant freedom to pursue independent research. In such cases their results would be published independently although the post-docs might be working on other projects with supervisors. But GH personally read all manuscripts produced by the group and went through them meticulously: they had to be up to standard - his standard!

One day I made some quite unexpected discoveries of new spectroscopic features belonging to a species that GH had previously studied himself. I saw that he was just as excited as I was over the new findings. He was particularly kind to young scientists.

In later years I visited NRC fairly regularly and found that as GH turned successively 70, then 80 and then even 90, he never lost the almost boyish passion for trying to unravel the puzzles that molecules leave buried in the complex spectroscopic patterns they create when they interact with light.

Perhaps no incident sums up GH's personality in my memory better than the time I went to see him after he had read through the manuscript of my first successful independent investigation. It was a study of which I was most proud and he praised my work and the written account. However, he felt I should give somewhat more credit to two others who had observed the same features previously but had not been able to assign them correctly. GH pointed out that the previous workers could not in fact have correctly assigned the features because they had only a small "pocket" spectrometer whereas I had the best equipment in the world at my disposal. It was a lesson in magnanimity that I have never forgotten.

Gerhard Herzberg, molecular physicist: born Hamburg, Germany 25 December 1904; Privatdozent, Technische Hochschule, Darmstadt 1930-35; Research Professor, University of Saskatchewan 1935-45; FRSC 1939; Professor of Spectroscopy, Yerkes Observatory of the University of Chicago 1945- 48; Director, Division of Physics, National Research Council of Canada 1949-69, Distinguished Research Scientist 1969-95 (Emeritus); FRS 1951; CC (Canada) 1968, PC (Canada) 1992; Nobel Prize for Chemistry 1971; married 1929 Luise Oettinger (died 1971; one son, one daughter), 1972 Monika Tenthoff; died Ottawa, Ontario 3 March 1999.