The development of instantaneous photography in the 19th century brought new understanding of human and animal movements that were too rapid to observe with the naked eye. Suddenly, it was possible to see with certainty that all four hooves of a galloping horse left the ground at once. Similarly, a central theme of modern physical chemistry is the observation of chemical reactions at the molecular level and in real time. So-called molecular movies allow researchers to view the intermediate steps of a reaction, not just its results (see Physics Today, August 2003, page 19, and April 2011, page 13).
In the conventional view of chemical reactions, atoms follow straight, predictable paths, and one knows exactly where to find them. In a photodissociation reaction, in which a molecule breaks apart upon excitation with light, that simple picture gives rise to two basic types of reaction pathway: Either a...
