Professor John Callomon: Scientist who made major contributions in geology and spectroscopy

John Callomon was a professor at University College, London who was distinguished in two quite different fields: in high-resolution molecular spectroscopy, and geological dating through the study of Jurassic ammonites within stratigraphical palaeontology.

In spectroscopy he led in both experimental work and analysis and theory. He constructed, with Graham Chandler, a four-metre vacuum Czerny-Turner spectrograph, one of the last and best of the large high-resolution spectrometers in the world and known affectionately as the "yellow submarine".

He also observed and analysed the gas-phase electronic spectra of a variety of small molecules; he was the lead author of a 1966 landmark paper on the spectrum of benzene which has been cited more than 400 times. He was particularly interested in and contributed to our understanding of the relaxation of molecules from electronic excited states, a challenging field on the borderlines of molecular physics and chemical reactivity which is relevant to our understanding of the origins of human life – a subject of great interest to him.

In geology he pioneered the discovery of sexual dimorphism in ammonites (the distinction between male and female ammonites, and the fact that the females are always larger). He became a world leader in the use of ammonites as geological clocks, and illuminated our stratigraphical knowledge of the middle and upper Jurassic rocks all around the world. His work with Tove Birkelund on east Greenland, in particular, was rewarded with the award of the prestigious Steno Medal by the Danish Geological Society. This work is illustrated by many publications, such as his paper "The ammonite succession in the middle Jurassic of East Greenland" (Bulletin of the Geological Society of Denmark, 1992), which resulted in significant contributions to the geological mapping of Greenland.

He once described how the two of them were delivered by helicopter to their camping site at Hurry Inlet in East Greenland with two packing cases of tents and supplies for a visit of some weeks; when the same helicopter came to collect them the supplies had been consumed but the packing cases were filled with rocks for the return journey. The associated collection of ammonites, meticulously curated by Callomon, is now in the Geological Museum in Copenhagen, where he worked on and off for a number of years. The rest of his collections are to go to the Natural History Museum at Oxford. The observation of sexual dimorphism in ammonites by John in England and by Henryk Makowski in Poland in 1963, has come to be regarded as the most significant development in this subject in the last 50 years.

His interests covered all fields of science. The clarity of his writing, and his mastery of the English language, combined with an ability to simplify and present his subject with his own unique turn of phrase and use of simile, resulted in his contributions that always brought a smile of appreciation and understanding.

John Hannes Callomon was born in Berlin on 7 April 1928, his father an electrical engineer. He was an only child, shy and retiring. With the rise of the Third Reich and the developing political situation, his parents accepted the offer of a job in England with GEC in Birmingham, in 1937, and there the family was befriended by Horace and Julie Sanders of Erdington. Horace was a metallurgical engineer and an enthusiastic geologist, and he conveyed his enthusiasms to John, of whom he said that "he soaked up knowledge like a sponge". It was Horace, who is now 100, who inspired Callomon's lifelong fascination with his sciences, stimulated by their many explorations on bicycles of the West Midlands and its geology.

Callomon won a scholarship to King Edward's Grammar School, Birmingham, and an open scholarship to St John's College, Oxford, where he graduated with a first in chemistry in 1950, followed by a DPhil in infrared spectroscopy in 1953. His interests in palaeontology were also developing, stimulated by the publication of Arkell's book on The Geology of Oxford in 1947.

Oxford was followed by two years in Ottawa Gerhard Herzberg working in the future Nobel Prize-winner's spectroscopy laboratory, one of the world's greatest. Then he returned to a Fellowship at University College, where he joined the academic staff, and spent the rest of his career, becoming a personal Professor in 1981.

Callomon travelled widely in pursuit of both geology and chemistry, and lived life to the full. His publications in geology exceed those in spectroscopy. He continued to make active contributions long into his retirement, often appearing at research seminars, always knowledgeable, appreciative of a good wine, and always a loyal supporter of St John's. He invariably brought sanity and clarity of expression, with a scholarly touch that few could match. Although he would often deflate the overblown expressions of others, he always did it in a gentle and helpful style. He was a friend to many, and he had few critics.

John Hannes Callomon, spectroscopist, geologist and paleontologist: born Berlin 7 April 1928; Professor of Chemistry, University College, London; married 1953 Esther (three sons) died 1 April 2010.