Real-Time Clocking of Energy Redistribution in Molecules Falling Apart

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In this project in the Physical Chemistry program of the Chemistry Division, Professor Mostafa El-Sayed of the Georgia Institute of Technology will pursue a series of investigations in transition state spectroscopy. Specifically, the internal vibrational energy redistribution (IVR) in photodissociating molecules will be studied in real time using a pump-probe method. Two femtosecond lasers are to be used, one to cause photodissociation and the other, suitably time delayed with respect to the first, to resonantly ionize one of the photofragments whose recoil velocity distribution is determined as function of the pump-probe time delay. The spatial distribution of the recoil velocities of the photofragments will also be determined. This distribution depends on the internal energy distribution over the various degrees of freedom of the parent and photofragment species. The experiments will be done with unsaturated organic and metal iodide molecules. One of the current problem areas in chemistry is the nature of the transition state. Inherent in the field of chemistry is the simple idea that during a chemical reaction the reactants are converted to products by breaking reactant bonds and forming new bonds. During the reaction new species are formed which are neither reactants nor products. These species are transient species and, due to their role in the transition from reactants to products, are said to occur in the `transition region`, and their specific nuclear configurations are termed `transition states`. One of the most important issues in chemical reaction dynamics is to elucidate the relation between the properties of the transition state and the characteristics of the reaction product. The duration, or life time, of a transition state, and thus the duration of a chemical reaction rocess, varies from one trillionth of one second (one picosecond) to one housandth of a trillionth of one second (one quadrillionth, or one femtosecond). Direct, detailed molecular level information about the transition state of molecular systems can only be obtained with sophisticated techniques that employ lasers with ultra short pulse duration (femtosecond lasers). The research proposed by Professor El-Sayed uses novel laser techniques to obtain needed data for the understanding of the transition state in chemical reactions.