Femtosecond quantum control of molecular bond formation

Abstract

Ultrafast lasers are versatile tools used in many scientific areas, from welding to eye surgery. They are also used to coherently manipulate light-matter interactions such as chemical reactions, but so far control experiments have concentrated on cleavage or rearrangement of existing molecular bonds. Here we demonstrate the synthesis of several molecular species starting from small reactant molecules in laser-induced catalytic surface reactions, and even the increase of the relative reaction efficiency by feedbackoptimized laser pulses. We show that the control mechanism is nontrivial and sensitive to the relative proportion of the reactants.The control experiments open up a pathway towards photocatalysis and are relevant for research in physics, chemistry, and biology where light-induced bond formation is important.femtochemistry | surface science E ver since their invention, lasers were considered the ideal tool for microscopic control over chemical bonds, and several seminal coherent control approaches have been developed (1-3). A very successful method to this task is femtosecond quantum control, where selectivity over photoinduced reactions is achieved by exploiting the coherence properties and ultrashort time scales of femtosecond laser radiation (4-6). Combined with learning algorithms processing experimental feedback to adaptively find optimized pulses best suited for solving the control task (7), chemical reactions can even be controlled without a priori knowledge about the reaction mechanisms. This scheme has been successfully applied to dissociative reactions in the gas phase, first on organometallic compounds (8) and later on many other systems. The method is not limited to gas phase experiments, as fluorescence optimizations of molecules in the liquid phase have shown (9-12). Recently, also more complex control tasks have been realized, like the energy flow in large biomolecules (13) or the quantum yield in a photoisomerization reaction (14-16). Femtosecond lasers have also been introduced to the field of photoassociation from atoms in cold traps, in both theory (17,18) and first experiments (19,20). However, the selective laser manipulation of bond-forming reactions starting from small reactant molecules that may furthermore exhibit competing bond-forming reaction channels has not been shown yet.In this contribution, we present the realization of femtosecond laser-assisted catalytic reactions of carbon monoxide and hydrogen or deuterium at a metal surface and further demonstrate that the relative reaction efficiency can be increased by the benefits of femtosecond laser pulses tailored especially for a desired reaction outcome. These experiments represent a first step and a reaction path toward laser-induced catalysis of molecular systems.Femtosecond laser sources have been employed by laser scientists to explore processes on metal surfaces as soon as they were available. Other types of lasers have been used earlier for this purpose, but starting from the first demonstration of intact desorption of ...

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