Dalgarno, Alexander (1928–2015), mathematician, physicist and astrophysicist, was born, along with his twin sister Pamela, on 5 January 1928 in Wood Green, London, at the home of their parents William Dalgarno and Margaret (née Murray). Alexander and Pamela had an older brother, Murray, and sister, Margaret; another brother died before they were born. Their parents both hailed from Aberdeen, marrying there before moving to London where William became an insurance executive. In 1936, as the family prospered, they settled in Winchmore Hill in Enfield. Alexander attended primary school in nearby Palmers Green. There he passed the ‘eleven plus’ examination aged nine. After two further compulsory years at primary he entered Southgate Grammar School where he excelled at cricket and soccer (he was offered a trial by Tottenham Hotspur Football Club) and greatly enjoyed solving mathematical puzzles and logical problems.
In 1945 Dalgarno entered University College London (UCL) to study mathematics. The first member of his family to attend university, he obtained a first-class Bachelor of Science (B.Sc.) degree in 1947. Degree stipulations required he attend a further year of lectures, so he undertook a self-guided ‘advanced subjects’ course in physics, for which he was awarded a distinction in 1948. A chance meeting that summer with Australian physicist Harrie Massey led to Dalgarno being awarded a postgraduate fellowship in physics. Dalgarno, researching quantum mechanics to examine collisions in helium gas, was awarded a doctorate in 1951 for his thesis ‘Collisions of normal and metastable helium atoms’. He then applied for and was offered a job at the Atomic Energy Research Establishment at Harwell, Oxfordshire, which he declined due to the paltry salary.
One of Massey’s former students, David Bates (qv), was appointed professor and head of the department of applied mathematics at Queen’s University Belfast (QUB) in 1951. Upon Massey’s recommendation Bates appointed Dalgarno as a temporary assistant lecturer in the department the same year. Dalgarno thrived in Belfast and was successively appointed lecturer (1952), reader (1956) and professor (1961). He initially lived in college accommodation where he became close friends with the poet Philip Larkin (then a librarian at QUB), with whom he shared an interest in jazz and the theatre. Larkin was best man in 1957 when Dalgarno married Barbara Kane in Belfast, and was also godfather to their first child, Fergus. Dalgarno and his wife had three further children together (Penelope, Piers and Rebecca), before later divorcing.
Spurred by Bates, in the early 1950s Dalgarno examined the quantum nature of aeronomy (the processes governing the response of the upper atmosphere to the sun). Together they examined the sources of dayglow and nightglow (the ‘aurorae’), revising downwards, by several hundred kilometres, the observationally inferred altitudes these processes occurred at. Calculating the properties of gases in the atmosphere was challenging, especially as the nature of reactions between them were unknown. Dalgarno’s research, confirmed by experiments undertaken later that decade on rockets, demonstrated that atomic and molecular physics were a critical component of aeronomy.
During summer 1954, while visiting Massachusetts Institute of Technology (MIT) as a Fulbright scholar, he encountered an electronic digital computer for the first time. Dalgarno’s research into atomic collisions relied on complex calculations, up to that point laboriously undertaken on error-prone mechanical calculators. He returned to Belfast determined to engage with electronic computing.
Dalgarno, with Brian Armstrong, organised a conference at QUB in September 1956 on airglow and the aurorae, and the university became a recognised centre for aeronomy and associated atomic and molecular physics research. Dalgarno supervised a stable of postgraduate students exploring approaches to calculating atomic and molecular properties. They deployed perturbation theory and variational methods (combining statistical approaches with empirical data) to ascertain atomic and molecular properties using sum rules. Fruitful work with his postgraduate student, John Lewis, developed an elegant and powerful sum rule technique (highly applicable to the analysis of complex systems) that went on to be widely used in physics and mathematics (Lewis would later head the school of theoretical physics at the Dublin Institute of Advanced Studies (DIAS)). This approach advanced Dalgarno’s research on long-range interactions, which underpinned his later work at Harvard on scattering calculations.
In 1961 Bates convinced the Northern Irish government and the US Office of Naval Research (on behalf of the Advanced Research Projects Agency, established in 1958 by President Dwight Eisenhower in response to the Soviet Union’s launch of the Sputnik satellite), to fund the acquisition of a ‘Digital Electric Universal Computing Engine’ (DEUCE). Dalgarno, who played an important role in negotiations leading to the acquisition, was the first director of the QUB computer lab (1960–65). There he exploited electronic computing techniques to examine and model complex atomic and molecular processes. In the early 1960s Dalgarno, with others, developed the S-matrix theory of molecular rotational excitation, which spurred major advances in molecular physics and theoretical chemistry.
He enjoyed the combination of observation and experimentation utilised to investigate the processes underpinning aeronomy. Measuring the rates of the underlying processes, especially the behaviour of hydrogen (the most abundant element and molecule in the universe), drew him to astrophysics. Dalgarno’s work began to focus on the emerging field of atomic, molecular and optical (AMO) physics. From the late 1950s rocket technology allowed direct measurement of, and experiments to be undertaken in, the upper atmosphere. Dalgarno developed links with the Geophysics Research Directorate of the US Air Force Cambridge Research Center, visiting Massachusetts over several summers. There he contributed to the preliminary design and planning of experiments to be undertaken in the upper atmosphere, engaging with leading researchers in the field. A 1960 paper he co-authored with Arnold Arthurs, who he met at the Center, remains (in 2023) Dalgarno’s most cited paper (‘The theory of scattering by a rigid rotator’, Proceedings of the Royal Society (of London) A, vol. 256, issue 1287, 19 July 1960).
Dalgarno took a sabbatical (1962–3) to serve as chief scientist at the Geophysics Corporation of America, which had been established by former US Air Force scientists and members of the Harvard astronomy department. Talks he gave on aeronomy that year across the US were well attended by engaged and knowledgeable scientists, which impressed Dalgarno. QUB then lacked any notable expertise in astrophysics, and Dalgarno was keen to join an institution that did. From 1966–7 he served on the board of the school of theoretical physics at DIAS.
Dalgarno’s contacts in Massachusetts led to the offer of a joint appointment as professor in the department of astronomy at Harvard University, and as a physicist at the linked Smithsonian Astrophysical Observatory, which he took up in July 1967, bringing several of his post-doctoral students with him from Belfast. At Harvard Dalgarno chaired the department of astronomy (1971–6) and was acting director of the Harvard College Observatory (1971–3). In 1977 he was appointed Philips professor of astronomy. His work and reputation attracted research students from Harvard’s departments of chemistry, physics and astronomy, as well as from leading American and international universities. He was integral to the 1988 establishment of the Institute of Theoretical Atomic and Molecular Physics at Harvard and was its first director (1988–93). He is regarded as a key progenitor of molecular astrophysics.
He fulfilled several prominent roles at the American Physical Society, the US National Academy of Sciences, and elsewhere, serving on numerous committees; he was also a visiting lecturer at many universities and research institutes. Dalgarno was warmly regarded by his students and had a seemingly endless supply of engaging research questions with which to task them. He had a phenomenal memory, easily recalling the complete citation of any published research he had read. Dalgarno was also an effective editor (helming the prestigious Astrophysical Journal Letters from 1974 to 2002) and a superb writer, rapidly producing elegant and concise papers. Dalgarno wrote or co-authored over 1000 papers, 799 of which were refereed. As early adopters of computational methods and computing technology, physicists were at the forefront of the development of structured approaches to citation and bibliometrics. Dalgarno, in his various professional roles across institutions and research bodies, and as editor of a leading journal, contributed to these advances.
Dalgarno utilised mathematical thinking to investigate aeronomy, in turn engaging with astronomy and becoming a leading figure in astrophysics. His research focus evolved from the Earth’s upper atmosphere to the atmospheres of Mercury, Venus, Mars and Jupiter, as well as various comets. He combined experimental and theoretical approaches to atomic and molecular phenomena which drew on mathematics, physics and chemistry and, in turn, made distinguished contributions to those domains and to astrophysics. He was fondly recalled by his many students and colleagues for being both receptive to questions and always amenable to discussing research problems.
His many significant awards and honours include the gold medal of the Royal Astronomical Society (1986; fellow 1977), the Royal Society of Chemistry Spiers medal (1992), the American Geophysical Union’s Fleming medal (1995), the Royal Society Hughes medal (2002; fellow 1972) and the Benjamin Franklin medal (2013). In 1980 he was awarded an honorary doctor of science (D.Sc.) by QUB, and in 1989 he was made an honorary member of the Royal Irish Academy. In 1998 asteroid ‘6941’ was named after him by the International Astronomical Union. Papers presented at a major 2008 event marking his professional achievements were published as Proceedings of the Dalgarno celebratory symposium: contributions to atomic, molecular, and optical physics, astrophysics, and atmospheric physics (2009).
Dalgarno remained active late in life, playing squash and tennis into his eighties. He died on 9 April 2015 in Cambridge, Massachusetts, after enduring Parkinson’s disease for some time. He was survived by his four children and Fern Creelan, his partner of thirty years.