Chapter 1: Ahmed Zewail: Ultra-Scientist and Citizen

To my regret, I met Ahmed Zewail in person only a few times. We first met in Spring 1976, when he arrived at Caltech, newly appointed as an assistant professor. I was visiting that semester, on sabbatical leave from Harvard. Ahmed impressed me as very bright, cheerful, and witty, brimming with ideas and zest. About a decade later, although we had not met again, I was intrigued by his innovative papers. Particularly instructive was his exposition of the role of coherence in laser spectroscopy. Moreover, using molecular beams and ultrafast lasers, Ahmed had developed a pump-probe technique that mapped, in “real time,” trajectories of products from photoinduced reactions, exemplified by hν + ICN → I + CN. At first, the method was limited to such unimolecular reactions, initiated by photodissociation. Bimolecular reactions seemed inaccessible because means were lacking to define a time zero…