In Memoriam: Ahmed Hassan Zewail (1946–2016)

It is with great sorrow that we heard of the passing away of Ahmed H. Zewail, the father of Femtochemistry and the pioneer of ultrafast electron-based Science, on August 2, 2016. This represents a tremendous loss not only for the scientific community but also for humanity, as he was a real Statesman. In 1987, Ahmed Zewail published a short communication in the Journal of Chemical Physics that sparked off a revolution in Science. Shortly afterwards, two articles were published in the same journal, presenting a more complete account, both technical and scientific, of the “real-time” probing of a chemical reaction, in this case, the photodissociation of the ICN molecule, with femtosecond resolution. This was the beginning of breathtaking series of discoveries, which represented the birth of a new branch of Chemistry, Femtochemistry, for which he was awarded the Nobel Prize for Chemistry in 1999. Ahmed Zewail was born in 1946 in Damanhur, Delta of the Nile, Egypt. He obtained his Master’s degree from the University of Alexandria in 1969. Inspired by the style of one of his mentors, he decided to move to the United States for his Ph.D. studies. He did his thesis under the supervision of the late Robin Hochstrasser at the University of Pennsylvania and obtained the doctoral degree in 1973. Thereafter, Zewail became a postdoctoral research assistant for two years at the University of California, Berkeley, in the group of Charles B. Harris. During that time he started to work with the newly developed picosecond lasers, the springboard to the new research that he initiated at Caltech in 1976 as an assistant professor. In Caltech, he developed his concepts about coherence, and in a series of papers—which earned him tenure after two years—he pioneered studies of coherence and dephasing with lasers in the case of both isolated or the condensed phase molecules. This work served as a seed to the birth of Femtochemistry. Indeed, even on the short picosecond time scale, molecular states already reside in eigenstates (the static limit) and there is only one evolution possibly observable: the change of population with the time of that state. Hence, with picosecond resolution, one was still concerned with kinetics, not dynamics. The advent of the femtosecond laser technology in the mid-1980s, thanks to the works of C. V. Shank and his co-workers, opened the door to the above mentioned breakthrough in 1987–1988 in which he demonstrated the “realtime” visualization of nuclear motion in molecular systems. Ahmed Zewail’s genius was his ability to convey the power and insight of the new discoveries by a judicious choice of the systems to study, going from simple ones to systems of ever greater complexity. With this powerful tool in hand, Zewail and co-workers could observe the dynamical processes of bondbreaking, bond making, and of the cornerstone of reactivity, the transition state, with atomic scale resolution. S. Forsen, a Swedish researcher of Lund University, likened the situation, before the advent of Femtochemistry, to an audience seeing only the very beginning and the very end of Hamlet “the main characters are introduced, then the curtain falls for a change of scenery, and, as it rises again, we see on the stage a considerable number of bodies and a few survivors. Not an easy task for the unexperienced to unravel what actually took place.” But with Ahmed Zewail’s achievements, the stage was set to see the whole action!