Project of the month, October 2009

Control of Chemical Reactions through Rectification of the Lorentz Force

Control of Chemical Reactions through Rectification of the Lorentz Force Lasers can be used to manipulate atomic motions, breaking bonds and controlling reactions, making it possible to explore new materials and to unravel new processes in chemistry and biology. The control of the rate of certain chemical reactions by resonant laser excitation of selected vibrational modes has served as an important branch of photochemistry. We show in this report that for strong infrared laser fields there exists a mechanism alternative to resonant excitation which governs photoinduced dynamics, namely, the rectification of the Lorentz force. Although the electric field periodically changes sign the average mechanical action of the light caused by the field induced force, the Lorentz force, is not zero. This is because the variable laser field induces a synchronous charge transfer between different parts of a molecular system which results in a rectification of this force. Such a force rectification constitutes a mechanical action of the infrared laser on charged atoms or groups of atoms and brings about a way to control the nuclear motions. We have demonstrated a mechanism to control chemical reactions by the application of strong infrared fields. This mechanism, which is not based on resonant excitation, origins in the rectification of the Lorentz force. The variable electric field induces a synchronous charge transfer leading to the rectification of the Lorentz forces acting on the atoms. The atoms move like in permanent electric field because light-induced charge follows adiabatically to the oscillations of the electromagnetic field. The light-induced charge transfer and consequently the force rectification are greatly enhanced by the Stark effect which decreases the gap between occupied and unoccupied molecular levels. This mechanism brings about a unique opportunity to produce a site selective light action on atoms or groups of atoms inside large molecules like clusters, with wide ramifiations in chemistry and biochemistry. To demonstrate the new notion of rectification of the Lorentz force and the implication of this notion for selected bond breaking, we have studied the photoinduced dynamics of some water clusters.

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