Sydney Leach. 11 April 1924—24 December 2019

Abstract

Sydney Leach will be remembered as an outstanding and inspirational scientist, an irreplaceable friend to many—artists and musicians as well as academic colleagues. He encouraged and influenced numerous scientists as a mentor. After graduating from King's College London and carrying out war work at Farnborough, he spent all his scientific life based in Paris, working principally at Orsay and, in his later years, at the Observatoire de Paris–Meudon. Sydney was a major influence in establishing chemical physics in France after World War II, founding the highly influential Laboratoire de Photophysique Moléculaire (LPPM) at Orsay, where much of his pioneering work was performed. The ‘Sydney lab’ lives on in the newly created Institut des Sciences Moléculaires d'Orsay. Early experiments often took place at the synchrotron source (ACO, Super-ACO), just a few hundred yards from LPPM. He was a pioneering advocate of synchrotron radiation, and a driving force for its use in spectroscopy and photodynamics, along with free-electron lasers, supersonic jets, coincidence spectroscopy and matrix isolation—techniques that were applied and refined over decades and used to explore fundamental processes such as photoionization, vibronic coupling and radiationless transitions. Sydney's seminal studies of polyatomic molecular ions led him towards fresh horizons in planetary atmospheric and space science. His work opened new vistas in cometary spectroscopy, polycyclic aromatic hydrocarbons, fullerenes and their possible roles in the chemistry of the interstellar medium and, finally, biologically relevant species, helping to instigate the newly developing subject of astrobiology—a perfect example of his sustained prescience in the world of science.