Femtosecond Activation of Reactions and the Concept of Nonergodic Molecules

The description of chemical reaction dynamics often assumes that vibrational modes are well coupled (ergodic) and redistribute energy rapidly with respect to the course of the reaction. To experimentally probe nonergodic, nonstatistical behavior, studies of a series of reactions induced by femtosecond activation for molecules of varying size but having the same reaction coordinates [CH$_2$ - (CH$_2$)$_{n-2}$ - C = O$^\dagger → products, with n = 4, 5, 6, and 10] were performed. Comparison of the experimental results with theoretical electronic structure and rate calculations showed a two to four orders of magnitude difference, indicating that the basic assumption of statistical energy redistribution is invalid. These results suggest that chemical selectivity can be achieved with femtosecond activation even at very high energies.