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Coherent light sources have been widely used in control schemes that exploit quantum interference effects to direct the outcome of photochemical processes. The adaptive shaping of laser pulses is a particularly powerful tool in this context: experimental output as feedback in an iterative learning loop refines the applied laser field to render it best suited to constraints set by the experimenter. This approach has been experimentally implemented to control a variety of processes, but the extent to which coherent excitation can also be used to direct the dynamics of complex molecular systems in a condensed-phase environment remains unclear. Here we report feedback-optimized coherent control over the energy-flow pathways in the light-harvesting antenna complex LH2 from Rhodopseudomonas acidophila, a photosynthetic purple bacterium. We show that phases imprinted by the light field mediate the branching ratio of energy transfer between intra- and intermolecular channels in the complex's donor acceptor system. This result illustrates that molecular complexity need not prevent coherent control, which can thus be extended to probe and affect biological functions.

The critical stage in a chemical reaction — the progression through the transition state from reagents to products — occurs in less than a picosecond (10^(−12)s). Using laser pulses of femtosecond (10^(−15)s) duration it is possible to probe the nuclear motions throughout formation and break-up of the transition state. The coherence and very short duration of these femtosecond pulses provides a means to influence the course of the reaction during this stage if the time resolution is made sufficiently short. Here we describe a demonstration of such control of a chemical reaction on the femtosecond timescale. Using two sequential coherent laser pulses, we can control the reaction of iodine molecules with xenon atoms to form the product XeI by exciting the reactants through the transition state, in a two-step process. The yield of product XeI is modulated as the delay between the pulses is varied, reflecting its dependence on the nuclear motions of the reactants.

IN chemical reactions, the dynamics of the transition from reagents to products can be described by the trajectories of particles (or rigorously, of quantum mechanical wave packets) moving on a potential-energy surface. Here we use femtosecond pulsed laser techniques to follow directly the evolution in space and time of such trajectories during the breakage of a chemical bond in the dissociation of sodium iodide. The bond breakage can be described in terms of the time evolution of a single reaction coordinate, the internuclear separation. As the velocities of the separating fragments are typically of the order of a kilometre per second, a time resolution of a few tens of femtoseconds is required to view the motions on a molecular distance scale of less than an angstrom. The resolution obtained here permits the direct visualization of the wave packet's motion and provides snapshots of the trajectories along the reaction coordinate.