Chemical changes femtochemistry

J.C. Williamson, J. Cao, H. Ihee, H. Frey and A.H. Zewail (1997) Nature, vol. 386, p. 159 - Clocking transient chemical changes by ultrafast electron diffraction . A.H. Zewail (2000) Angewandte Chemie International Edition, vol. 39, p. 2586 - Femtochemistry atomic-scale dynamics of the chemical bond using ultrafast lasers . 

In a classical Bohr orbit, the electron makes a complete journey in 0.15 fs. In reactions, the chemical transformation involves the separation of nuclei at velocities much slower than that of the electron. For a velocity 105 cm/s and a distance change of 10 8 cm (1 A), the time scale is 100 fs. This is a key concept in the ability of femtochemistry to expose the elementary motions as they actually occur. The classical picture has been verified by quantum calculations. Furthermore, as the deBroglie wavelength is on the atomic scale, we can speak of the coherent motion of a single-molecule trajectory and not of an ensemble-averaged phenomenon. Unlike kinetics, studies of dynamics require such coherence, a concept we have been involved with for some time. 

The fastest that anything happens in a chemical reaction is on a time scale of approximately 1 femtosecond (1 fs = 10 15 s). That is the time it takes for a bond to stretch or bend, and perhaps break. If we could follow atoms on that time scale, we could make a movie of the changes in molecules as they take part in a chemical reaction. The new field of femtochemistry, the study of very fast chemical processes, is bringing us closer to realizing that dream. Lasers can emit very short but intense pulses of electromagnetic radiation, so they can be used to study processes on very short time scales. 

Important classes of chemical reactions in the ground electronic state have equal parity for the in- and out-going channels, e.g., proton transfer and hydride transfer [47, 48], To achieve finite rates, such processes require accessible electronic states with correct parity that play the role of transition structures. These latter acquire here the quality of true molecular species which, due to quantum mechanical couplings with asymptotic channel systems, will be endowed with finite life times. The elementary interconversion step in a chemical reaction is not a nuclear rearrangement associated with a smooth change in electronic structure, it is aFranck-Condon electronic process with timescales in the (sub)femto-second range characteristic of femtochemistry [49],... 

Femtochemistry following Zewail s work has become a Buzz word in experimental physical chemistry research today. It has changed our view of chemical reactions and we can ask detailed questions which could not be asked before. 

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