Chapter 13 Quantum Control by Adaptive Femtosecond Pulse Shaping Application to CH BrCl in the Gas Phase and [Ru(dpb) ] in the Solution Phase Downloaded by CORNELL UNIV on July 25, 2016 | http://pubs.acs.org Publication Date: June 21, 2002 | doi: 10.1021/bk-2002-0821.ch013
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Niels H. Damrauer and Gustav Gerber* Physikalisehes Institut, Universität Würzburg, Am Hubland, D-97074 Würzburg, Germany
Femtosecond adaptive pulse shaping is used to achieve bond -selectivephotochemistry for C H B r C l in the gas-phase as well as to control charge-transfer excitation of the [Ru(dpb) ] complex in the solution phase (where dpb = 4,4'-diphenyl-2,2'bipyridine). The technique exploits phase shaping of ultrafast broadband laser pulses within a 128-parameter search space. Because of the enormity of the variational space, a learning loop is implemented to find optimum pulse shapes. Successful applications of this technique to new systems in both the gas and solution phase emphasizes its generality and utility for controlling the outcome of light-matter interactions. 2
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One of the central themes in modern chemistry is the struggle to control the outcome of reactions with the goal of creating useful and interesting products. As such, the modern synthetic chemist must have a broad knowledge of functionality and reaction conditions in order to create or break a variety of chemical bonds. Thermodynamic control is exercised with careful choice and
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© 2002 American Chemical Society
Bandrauk et al.; Laser Control and Manipulation of Molecules ACS Symposium Series; American Chemical Society: Washington, DC, 2002.
Downloaded by CORNELL UNIV on July 25, 2016 | http://pubs.acs.org Publication Date: June 21, 2002 | doi: 10.1021/bk-2002-0821.ch013
191 design o f starting materials followed by variation o f external conditions such as temperature, pressure, and concentration. Kinetic control is implemented using catalysts designed to reduce barriers to desired product channels. W i t h respect to these types o f control, light can be thought o f as having the potential to be very useful. In principle, excited-state potential energy surfaces could be exploited to change free-energy relationships between starting materials and products thus exercising thermodynamic control. Kinetic control, on the other hand, would be achievable i f excess energy could be placed within molecular systems using tunable sources i n order to overcome specific reaction barriers to product formation. Nonetheless, one has only to open a common journal or textbook to realize the basic shortcoming o f photochemistry at achieving a prominent role i n the methodology o f synthetic control. W h i l e there are well understood and used photochemical routes to product formation, the numbers pale in comparison to the volumes o f literature associated with more traditional synthetic methods. The responsibility for this shortcoming lies in the complexity of the excited-state electronic structure o f many molecular reactants and the speed at which energy is statistically distributed within the molecule and to its surroundings following photo-excitation. U s i n g typical laboratory photochemical variables such as intensity and wavelength, product yields are difficult to predict and even more so to control. In the previous fifteen years, it has been shown that variations o f spectraltemporal characteristics o f applied coherent light (beyond simple variation o f wavelength or intensity) can be used to exert control over the outcome o f lightmatter interactions by exploiting available interference phenomena.(1 -3) Initial experiments subjected small molecules in the gas-phase to changes i n a single control variable o f the incident coherent light field. One example involved manipulation o f the relative phase between two c w laser fields interacting with HC\(4) according to the Brumer-Shapiro control methodology.(5) A second example involved manipulation o f the relative timing between two ultrashort laser pulses interacting with Na (
