Coherent Control of Chemical Reactions

Coherent control is based on interference effects, which become essential when an excited state can be populated on two ore more excitation paths. The population rates then depend on the phase relations between the optical waves that are inducing these excitations. Coherent control has to be performed on a very short time scale in order to win over dephasing processes. Therefore femtosecond lasers are generally demanded [15.32]. 

The second scheme of coherent control utilizes the time difference between two femtosecond laser pulses interacting with a molecule on two different transitions sharing a common level, which was illustrated in Sect. 11.4.4 by the example of the Na2 molecule. Here the phase of the wave packet produced by the first pulse in the excited state develops in time, and the controlled time lag between the first and the second pulse selects a favorable phase for further excitation or deexcitation of the molecule by the second pulse. 

In the first scheme it is not necessary to use two different lasers if a femtosecond pulse with a broad spectral range is used for excitation. The different spectral components in the pulse give rise to many different excitation paths. In order to achieve optimum population in the excited state, the relative phases of these different spectral components have to be optimized. This can be realized by the pulse-shaping techniques discussed in Sect. 11.1.9 where a feedback loop with a learning algorithm is used to maximize or minimize the wanted decay channel of the excited state [15.35,15.36]. 

One example is the dissociation of laser-excited iron pentacarbonyl Fe(CO)5, where the ratio Fe(CO)5/Fe can be varied between 0.06 to 4.8 by coherent control [15.37]. Another example is the photodissociation of C5H5Fe(CO)2Cl into selected fragments. Optimum shaped femtosecond pulses can alter the ratio C5H5COCl/FeCl from 1 to 5 [15.38], or the selected bond dissociation of acetone (CH3)2CO or acetophenone C6H5COCH3 [15.39]. More examples and a detailed description of the technique can be found in [15.40] and [15.41]. 

Coherent control means the coherent preparation of a molecular wavefunction through the absorption of coherent radiation. The dream of chemists is the controlled selection of wanted reaction channels and the suppression of unwanted channels in a photoinduced reaction. This is illustrated by Fig. 10.9, where the excitation of the triatomic molecule ABC can induce either of the reactions AB -b C or AC -b B, depending on the wavefunction in the excited state of ABC, which is controlled by the form of the excitation pulse. 

Coherent control has to be performed on a very short time scale in order to win over dephasing processes. Therefore femtosecond lasers are generally demanded [1399]. 

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Coherent Control of Chemical Reactions Cryogenic Process Engineering... 

There are a number of excellent reviews on atoms in intense laser fields [9]. Consequently, we will focus our attention on the experimental aspects of the behaviour of simple molecules in such fields. Most of the work to date has used a laser field of a single frequency, but experiments are presently underway where manipulation of dissociation pathways is being attempted by varying the phase between the fundamental and the second harmonic [10]. This simplest example of coherent control of chemical reactions will be touched on briefly at the end of the paper. 

Currently, a major theme in atomic, molecular, and optical physics is coherent control of quantum states. This theme is manifested in a number of topics such as atom interferometry, Bose-Einstein condensation and the atom laser, cavity QED, quantum confutation, quantum-state engineering, wavepacket dynamics, and coherent control of chemical reactions. 

Atomic and Molecular Collisions Coherent Control of Chemical Reactions Energy Transfer, Intramolecular Ion Kinetics and Energetics Kinetics, Chemical Molecular Beam Epitaxy, Semiconductors... 

For the last decade or so, a new method has been developed to control chemical reactions that it is based on the wave nature of atoms and molecules. The new methodology is called quantum control , or coherent control of chemical reactions, and is based on the coherent excitation of the molecule by a laser. Generally speaking, an ultra-short laser pulse creates a wave packet whose time evolution describes the molecular evolution in the superposition of excited states. Quantum control tries to modify the superposition of such an ensemble of excited states and, therefore, influences the motion of the wave... 

For many applications a specific time profile of short laser pulses is desired. One example is the coherent control of chemical reactions (see Sect. 15.2) Recently, some techniques have been developed that allow such pulse shaping and that work as follows ... 

Electron Transfer Reactions Coherent Control OF Chemical Reactions Lasers, Ultrafast Pulse ItecHNOLOGY Luminescence Molecular Beam Epitaxy, Semiconductors Nanostructured Materials, Chemistry of Noni.inf.ar Optical Processes... 

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