The passing of Gerhard Herzberg on 3 March 1999 at the age of 94 makes this an appropriate time to look back on the outstanding contributions of three Canadians to the early study of molecules in space. I was privileged to know two of them, Gerhard Herzberg and Alex Douglas, personally during the many years I spent at the Sussex Drive laboratories of the National Research Council (NRC) in Ottawa. I never was able to meet Andrew McKellar who died in 1960, but I have been fascinated by the story of his work since I first learned of it shortly after Penzias and Wilson (1965) directly measured the 3K cosmic microwave background (CMB) signature of the Big Bang which McKellar had pre-discovered 24 years earlier.
The detailed story of Gerhard Herzberg's early years is reasonably well known (Stoicheff 1972, Mulliken 1979, Herzberg 1985). Upon finishing secondary school in Hamburg just after WW I, Herzberg explored the possibility of pursuing an education that would enable him to become a professional astronomer. However, the Director of the Hamburg Observatory advised that there was no point in even thinking of a career in astronomy unless one had private means of support. [According to Takeshi Oka (1985), "All spectroscopists should thank the Director of the Hamburg Observatory"!] No matter. Before the age of 30 Herzberg became one of the leading molecular physicists in the world and his career was thriving in Darmstadt. However, in 1933 the Nazis under Adolph Hitler took control of Germany at a time when much of the world was in the midst of the Great Depression. When Herzberg decided in 1935 to leave Germany (his wife Luise, also a spectroscopist, was Jewish), the only definite offer he had was from the (far-sighted) University of Saskatchewan in Saskatoon. GH, as everyone called him, could not have realized then that he was about to spend ten fruitful years teaching at an institution that was to produce disproportionally large numbers of successful physicists and chemists (Costain 1985). Among them was Alex Douglas, Herzberg's most successful student and later his closest colleague. [Unfortunately, Douglas died prematurely in 1981 at the age of 65 after a very productive career, mostly spent at NRC in Ottawa (Herzberg 1982).]
In 1937 P. Swings and L. Rosenfeld in Belgium conjectured that several unidentified absorption lines of interstellar origin might be due to simple diatomic molecules and, more specifically, suggested that the 4300.3 A line was due to CH. These suggestions were proven by Andrew McKellar (1940, 1941b), then a 30-year-old astronomer working at DAO, who conclusively identified CH and CN in the absorption spectrum of the bright star zeta Ophiuchi. [ I will come back to the work of Andrew McKellar and his pre-discovery of the CMB in the next section.]
Three sharp interstellar lines which remained unidentified suggested a vibrational progression of a diatomic molecule not known at the time (McKellar 1941a). P. Swings called attention to these lines at an informal conference on interstellar molecules held in the early summer of 1941 at Yerkes Observatory. Swings hypothesized that the three lines were due to a photoionized diatomic molecule containing C, N, O, or H atoms. Edward Teller (see footnote #1) and Gerhard Herzberg (then working at the Univ. of Saskatchewan) suggested, by comparison to BH which is isoelectronic wth CH+, that CH+ was the most likely candidate. Within a week or so after Herzberg returned to Saskatoon, his student Alex Douglas and GH had obtained a spectrum of CH+. They used their new grating spectrograph (see footnote #2) to examine the ro-vibrational structure in the spectrum of CH+ and found that the R(0) lines matched the observed interstellar lines exactly (Douglas and Herzberg 1941, 1942).
We need to be reminded how difficult it once was to detect molecular hydrogen in the atmospheres of the giant planets and in the interstellar medium. This is because H2 is a homonuclear molecule with a singlet-Sigma ground state, and hence exhibits neither an electric-dipole nor a magnetic-dipole rotation-vibration spectrum. During his years in Saskatoon, GH suggested the possibility of detecting the extremely weak electric-quadrupole infrared rotation-vibration spectrum of H2 in absorption against the bright continuum emission of the giant planets (Herzberg 1938). He also suggested the possibility of detecting HD through its weak electric-dipole spectrum. It was not until after WW II, at the end of his three years at Yerkes Observatory, that Herzberg actually accomplished his goal of measuring in the laboratory the 2-0 and 3-0 bands of the quadrupole (absorption) spectrum of H2 (Herzberg 1949, 1950a). He used a cell that was 75 ft long with an optical system capable of as many as 250 traversals. Filling the cell with H2 at 10 atmospheres yielded the gas column of 55 km atm that he needed for the experiment. Later, the absorption spectra of other astrophysically important molecules were measured in the same way. S. Chandrasekhar has written in this connection that "the CH2 bands in the spectrum of Venus were reproduced with remarkable fidelity" (Stoicheff 1972). Later, after GH had returned to Canada to join the NRC in Ottawa, he measured the 3-0 and 4-0 bands of the rotation-vibration spectrum of HD (Herzberg 1950b); ten years later he collaborated with PDF R. Durie to obtain the fundamental (1-0) band of HD (Durie and Herzberg 1960).
Another early achievement by Herzberg in Ottawa (made before the sharp quadrupole absorption lines of H2 were detected astronomically in the spectra of the giant planets) was his identification of molecular hydrogen in the atmospheres of Uranus and Neptune by means of the pressure-induced quadrupole spectra of H2 (Herzberg 1951, 1952). GH used a column of H2 at high pressure and low temperature to reproduce in the lab an unidentified feature at 8270 A that had been observed by G. Kuiper in the spectra of Uranus and Neptune with the pressure-induced 3-0 S(0) line of H2.
The Herzbergs at home with friends in Ottawa South. From left to right: R. Craig, Mrs. Shoosmith, Agnes Herzberg, Mrs. Luise Herzberg, Paul Herzberg, GH, Jack Shoosmith, F. Geiger.
Photo by Dr. Hin Lew in 1951 at about the time Herzberg identified molecular hydrogen in the spectra of Uranus and Neptune. .
I recall very well how much personal pleasure it gave GH when he learned that quadrupole emission spectra of H2 are produced by strong collisional shocks and UV excitation in a wide variety of interstellar, circumstellar, and extragalactic environments (cf. Geballe 1994). This phenomenon was first observed from shocked H2 in the Orion molecular cloud (Gautier et al. 1976). More than a decade later I recall showing GH an H2 emission spectrum of Orion (peak 1) which I had been given by Tom Geballe at UKIRT. It included pure rotational lines as high as S(17), arising from J=19 (Brand et al. 1988). GH was, at first, astonished and almost disbelieving, but his reservations soon turned to great satisfaction.
In a similar connection, at the Festschrift honouring Gerhard Herzberg on the occasion of his 90th birthday, Geballe (1994) pointed out to GH a sobering example of 45 years of technical progress. When Lacy et al. (1994) first detected a ro-vibrational line of interstellar H2 in absorption, against the young stellar object NGC 2024/IRS2, the column of H2 that was responsible was ~ 3.5 x 1022 cm-2, approximately 4000 times less than GH used in first measuring the quadrupole spectrum of H2 in the laboratory (Herzberg 1949)!
The citation of the Nobel Prize in Chemistry which was awarded to Gerhard Herzberg on 2 November 1971 reads "for your contributions to the knowledge of electronic structure and geometry of molecules, particularly free radicals" (my italics). Professor Stig Claesson of the Swedish Royal Academy of Sciences, who presented the Nobel Prize to Herzberg, singled out the free radicals methylene, CH2, and methyl, CH3, in his ceremonial address (Claesson 1972). These radicals had long been postulated as intermediates in chemical reactions, and they are building blocks for larger organic molecules. Naturally, their electronic and geometric structures are of great interest.
The story of GH's discovery of the spectra of CH2 is a romance that spans 17 years. On the basis of Herzberg's well-considered assumption that CH2 was the probable carrier of the strong but unidentified 4050 A group of cometary lines first discovered by Huggins (1882), he used an interrupted discharge in methane to obtain a laboratory spectrum that seemed to prove him correct (Herzberg 1942 a,b). However, a decade later Alex Douglas showed that the carrier of the 4050 A group is, in fact, C3 (Douglas 1951, Clusius and Douglas 1954). [The C3 radical was definitively detected outside the Solar System in the circumstellar envelope of the extreme carbon star IRC +10214 (CW Leo) by Hinkle, Keady, and Bernath (1988).] Not until 1959 did Herzberg and his lab assistant, Jack Shoosmith, obtain the correct spectrum of methylene at 1415 A (Herzberg and Shoosmith 1959, Herzberg 1961).
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GH practising classical mechanics at an Ottawa bowling alley. Photo by Dr. Hin Lew in 1958, shortly before Herzberg and Shoosmith obtained their famous spectrum of CH2. |
Takeshi Oka, one of Herzberg's closest colleagues after the mid-1960's, remembers GH saying that as soon as he saw Shoosmith's wet photographic plate he was convinced that the spectrum of CH2 had been obtained (Oka 1985). Herzberg initially believed that CH2 was a linear molecule in its (triplet-Sigma) ground state. Subsequently, Herzberg and Johns (1971) realized that the vacuum-UV spectra of CH2, CHD, and CD2 could be rationalized by assuming that ground-state CH2 was bent at an angle of about 136o. This hypothesis was eventually confirmed in detail by Sears et al. (1982, 1984) who also calculated accurate frequencies for the rotational lines in the ground state to enable CH2 to be searched for in space. It took another 13 years before methylene was finally confirmed as an interstellar molecule (Hollis et al. 1995).
The spectrum of the methyl radical, CH3, was actually measured in the NRC laboratories three years before CH2 was finally found, and with much less difficulty (Herzberg and Shoosmith 1956; see also Herzberg 1961). Jack Shoosmith had arrived at NRC at almost the same time as Herzberg and worked as his technical assistant until Shoosmith retired in 1969. [GH also "retired" in that year, but only from his management responsibilities. He remained at NRC as an active staff member and continued his research for another 25 years.] Shoosmith was listed as the co-author with GH of the papers reporting the first laboratory measurements of CH3 and CH2. It was not until recently that Bézard et al.(1998) detected the methyl radical outside the Earth's atmosphere, in the upper atmosphere of Saturn, using the Short-Wavelength Spectrometer of the ISO satellite.
The role of the postdoctorate fellows (PDFs) is often cited as a major reason for the great success of Herzberg and Douglas' spectroscopy laboratory at NRC (Stoicheff 1972, Douglas 1972). In the mid-1960s three young PDFs arrived. Takeshi Oka was from Japan, Jim Watson was from Great Britain, and Harry Kroto was from England. All would later play important roles in interstellar molecule research after HIA was formed in 1975.
I have already mentioned (in § 2.) the crucial role played by McKellar (1940, 1941b) in identifying the presence of CH and CN in the diffuse interstellar medium. As important as this work was at the time, it is now overshadowed by his measurement of the rotational excitation temperature of the cyanogen radical (CN) in the gas between the stars. The first hint that McKellar appreciated the possible significance of the CN absorption-line data for yielding an extremely low "effective" temperature of the interstellar gas can be found in McKellar (1940). A short while later, after he himself had identified CN lines in spectra taken at Mt. Wilson Observatory, McKellar (1941a) wrote: "Also from [Walter S.] Adams' results on the interstellar CN lines, it can be calculated that the `Rotational' temperature of interstellar space is about 2 K." In a lengthy paper published the same year in the Publications of the Dominion Astrophysical Observatory, McKellar (1941b) made his arguments more quantitative. Therein he determined a "rotational" temperature of 2.3 K for the gas of interstellar space where CN absorption takes place. Nothing more on this subject seems to have been written by McKellar from March, 1941 to the end of his life in May, 1960.
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Left to right: J.A. Pearce (DAO Director), A.D. Robertson (Yarrows Ltd.),
and A. McKellar with the newly aluminized 72" mirror in
January 1943. Photo provided by Bob McKellar. |
Just after WW II George Gamow and his colleagues Ralph Alpher and Robert Herman began to analyze the conditions under which deuterium would be formed in a hot early Universe (Alpher, Bethe, and Gamow 1948, Gamow 1948). These papers do not actually predict the radiation temperature of the Universe at the current epoch. This was first done explicitly by Alpher and Herman (1948, 1949) who estimated that the temperature would be greater or equal to about 5 K . [Various other estimates were made by these and other workers later.]
Neither McKellar nor any other astronomer-spectroscopist seems to have been aware of these predictions. Herzberg, in the 1950 second edition of his classic monograph on the spectra of diatomic molecules, mentions the findings of McKellar (1941b) in passing. He wrote (Herzberg 1950c): "From the intensity ratio of the lines with K=0 and K=1 a rotational temperature of 2.3 K follows, which has of course only a very restricted meaning." To be honest, GH was only paraphrasing McKellar's own expressed reservations about the physical significance of his results (see, e.g., McKellar 1941b, p 267). Nevertheless, we might well wonder how much this remark by the greatest astronomical spectroscopist of his time might have influenced McKellar (and others). [Oddly, the revised 1989 third edition of Herzberg's book leaves his assessment of the CN temperature unchanged (see p.497 of the 3rd edition). I. Dabrowski (priv. comm. 1999) regards this as merely an oversight on GH's part.]
As for the radioastronomers and cosmologists who ultimately succeeded in detecting the CMB directly, there is quite a lot of information about what they knew (see, e.g., Thaddeus 1972 and references therein, Weinberg 1977 and references therein). It is pretty clear by now that none of them knew about, or remembered, McKellar's work.
After the direct detection by Penzias and Wilson (1965) of the CMB at a temperature of 3.5 ± 1.0 K, papers appeared by Field and Hitchcock (1966) and by Thaddeus and Clauser (1966) which recalled McKellar's earlier temperature estimates from CN absorption-line intensity ratios. Thaddeus and Clauser (1966) and Thaddeus (1972) are particularly useful in demonstrating why the rotational excitation temperature of CN works well as a thermometer for the CMB. In fact, Thaddeus (1972) pointed out that the weighted mean of the CN absorption-line excitation temperatures available to him at the time (2.78 ± 0.10 K) "may well be the most accurate determination of the background intensity existing at any wavelegth". This turned out to be a very good prediction. The best COBE estimate (Mather et al. 1999) of TCMB is 2.725 ± 0.002 K (95% confidence).
It is unfortunate that Andrew McKellar died in 1960 [see Beals (1960) for an obituary]. Five years later Penzias and Wilson, in a classic example of serendipity, directly detected the CMB and thereby made cosmology into a science. In 1978 they were awarded the Nobel Prize in Physics. As for McKellar's place in the history of this epochal discovery, I believe the scientific world is beginning to take more notice (cf. Bernstein 1993). I will not attempt to summarize or comment on the lessons that Bernstein has drawn from the McKellar "epilogue", as he called it. Suffice it to say that they are consistent with the views of the several people I have consulted who best knew Andrew McKellar and his work.
I wish to thank Izabel Dabrowski (Steacie Inst., NRC) for her help and useful suggestions, especially in matters related to Gerhard Herzberg's life, work, and personal opinions. I am grateful to A.R.W. (Bob) McKellar (Steacie Inst., NRC) for information related to his father's personal life and scientific career. I also thank Eric LeBlanc and Sharon Hanna of the DAO library for locating the needed reference material.
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Paul A. Feldman
<paul.feldman@hia.nrc.ca>
Paul is a radio astronomer at NRC's Herzberg Institute of Astrophysics in Victoria. His education was in theoretical physics at M.I.T. and Stanford University. After postdocs at Cambridge (IOTA) and Queen's Univ. (Kingston), he taught briefly at York Univ. where Chris Purton infected him with the radioastronomy "bug". He has devoted himself to feeding this illness for the past 25 years, first at DRAO, then in Ottawa, and most recently at DAO. His current research interests are in star formation, astrochemistry, and the submm photometry of asteroids. He is not to be confused with the "other" Paul (D.) Feldman (JHU) with whom he shares minor planet 3658 Feldman. His current hobbies are learning to garden in zone 8/9 and finding really good food in a small city. |
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