In 1959, Herschbach joined the University of California at Berkeley, where he was appointed an Assistant Professor of Chemistry and became an Associate Professor in 1961. At Berkeley, he and graduate students George Kwei and James Norris constructed a cross-beam instrument large enough for reactive scattering experiments involve alkali and various molecular partners. His interest in studying elementary chemical processes in molecular-beam reactive collisions challenged an often-accepted belief that "collisions do not occur in crossed molecular beams". The results of his studies of K + CH3I were the first to provide a detailed view of an elementary collision, demonstrating a direct rebound process in which the KI product recoiled from an incoming K atom beam. Subsequent studies of K + Br2 resulted in the discovery that the hot-wire surface ionization detector they were using was potentially contaminated by previous use, and had to be pre-treated to obtain reliable results. Changes to the instrumentation yielded reliable results, including the observation that the K + Br2 reaction involved a stripping reaction, in which the KBr product scattered forward from the incident K atom beam. As the research continued, it became possible to correlate the electronic structure of reactants and products with the reaction dynamics.