Controlling chemical reactions with lasers

A long-standing goal of chemistry is to control the rate and distribution of products in chemical reactions, with an eye toward minimizing undesirable side reactions and improving the efficiency of industrial processes. We already have at our disposal a number of successful strategies for achieving this goal. For example, it is possible to synthesize a catalyst that accelerates a specific type of reaction but not others, so a desired product may be formed more quickly than an undesired product. However, this strategy is not very general, as a new catalyst needs to be developed for every reaction type of interest. A more ambitious and potentially more powerful strategy consists of using lasers to prepare specific states of reactant molecules that lead to a specific activated complex and hence a specific product, perhaps not even the major product isolated under ordinary laboratory conditions. Here we examine two ways in which the outcome of a chemical reaction can be affected by laser irradiation. Some reactions may be controlled by exciting the reactants to different vibrational states. Consider the gas-phase reaction between H and HOD. It has been observed that H2 and OD are the preferred products when thermally equilibrated H atoms react with vibrationally excited HOD molecules prepared by laser irradiation at a wave length that excites the H-OD stretching mode from the v = 0 to the v = 4 energy level. On the other hand, when the same stretching mode is excited to the v = 5 energy level, HD and OH are the preferred products. This control strategy is commonly referred to as mode-selective chemistry and has been used to alter product distributions in a number of bimolecular reactions. However, the technique is limited to those cases in which energy can be deposited and remains localized in the desired vibrational mode of the reactant for a time that is much longer than the reaction time. This is difficult to

achieve in large molecules, in which intramolecular vibrational relaxation redistri butes energy among the many vibrational modes within a few picoseconds. A strategy that seeks to avoid the problem of vibrational relaxation uses ultrafast lasers and is related closely to the techniques used for the spectroscopic detection of transition states. Consider the reaction I2 + Xe → XeI* + I, which occurs via a harpoon mechanism with a transition state denoted as [Xe+···I−···I]. The reaction can be initi ated by exciting I2 to an electronic state at least 52 460 cm−1above the ground state and then followed by measuring the time dependence of the chemiluminescence of XeI*. To exert control over the yield of the product, a pair of femtosecond pulses can be used to induce the reaction. The first pulse excites the I2 molecule to a low energy and unreactive electronic state. We already know that excitation by a femtosecond pulse generates a wave packet that can be treated as a particle travelling across the potential energy surface. In this case, the wave packet does not have enough energy to react, but excitation by another laser pulse with the proper wavelength can provide the necessary additional energy. It follows that activated complexes with different geometries can be prepared by varying the time delay between the two pulses, as the partially localized wave packet will be at different locations along the potential energy surface as it evolves after being formed by the first pulse. Because the reaction occurs via the harpoon mechanism, the product yield is expected to be optimal if the second pulse is applied when the wave packet is at a point where the Xe···I₂ distance is just right for electron transfer from Xe to I₂ to occur. This type of control of the I₂ + Xe reaction has been demonstrated.

So far, the control techniques we have discussed have only been applied to reactions between relatively small molecules, with simple and well-understood potential energy surfaces. Extension of these techniques to the controlled synthesis of materials in routine laboratory work will require much more sophisticated knowledge of how laser pulses may be combined to stimulate a specific molecular response in a complex system.

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مواضيع ذات صلة

Surface composition

Surface growth

Experimental results

Electron tunnelling

Spectroscopic observation of the activated complex

Quantum mechanical scattering theory

Classical trajectories

Attractive and repulsive surfaces

The direction of attack and separation

Potential energy surfaces

State-to-state dynamics

Experimental probes of reactive collisions