Ian William Murison Smith. 15 June 1937—8 November 2016

Ian Smith was one of the world-wide leading researchers into reaction kinetics, energy transfer and molecular dynamics in gas phase systems. He was able to span all of these aspects of collisional behaviour, and to form connections and insights that allowed him to make advances in all of them, advances which have stood the tests of time. His graduate work at the University of Cambridge was followed by academic positions in Cambridge and in Birmingham. He provided new insights into molecular energy transfer by systematically measuring how vibrational frequencies and multipole moments influenced the magnitude of the energy transfer cross sections. In reaction kinetics his detailed work on the OH radical has led to an understanding of the importance of its reactions not only in applied areas such as atmospheric and combustion chemistry, but also in fundamental dynamical studies of radical recombination and the effect of reagent energies on reactive processes. Of particular significance has been his development of both cryogenic and nozzle expansion methods of studying collisional processes at temperatures down to 10 K, with the discovery that reactions without activation barriers can get faster as the temperature decreases. The impact of this work upon astrochemistry has been profound, and has stimulated theoretical explanations of the effect. He was one of the first pioneers in the UK of the use of laser techniques to study collisional processes, using laser-induced fluorescence as a sensitive detection tool, UV laser photolysis to form reactive species and laser pumping to create excited state populations in order to observe their effect upon reaction pathways and vibrational relaxation. Ian was devoted to his family—his wife Sue, their four children and 11 grandchildren. His research collaborators will always remember his scholarship, his unending suggestions of interesting problems to study, his deep integrity and humanity. His legacy lives on with his personal and academic families.