Weak-Field Coherent Control of Ultrafast Molecule Making

The  coherent control of binary chemical photoreactions has been a long-standing goal of the field. The operating principle is using shaped femtosecond pulses to actively drive a reaction along its full desired path from the initial state of the reactants to the final state of the desired products. Its realization at high temperatures, which are typical of chemical reactions, will enable a novel type of photochemistry. Such a scheme starts with coherently controlled ultrafast making of a molecular intermediate from the pair of free colliding reactants via a photo-excitation at short internuclear distances to target electronically-excited molecular states. It proceeds with further photo-control of the generated intermediate, eventually leading to its dissociation into the intended products.

 

So far, despite its importance, coherent control of ultrafast molecule making was successfully demonstrated only in a few works, all of which are in the strong-field regime. Here, we experimentally demonstrate for the first time coherent control of ultrafast molecule making in the weak-field regime. It is achieved under thermally hot conditions, which are extremely challenging due to the initial incoherent population of a vast number of scattering eigenstates. We use weak linearly chirped femtosecond pulses to coherently control the formation of KAr complexes from thermally hot pairs of colliding K and Ar atoms via a resonance-mediated two-photon transition. Our ab initio studies are in excellent agreement with the experiments and explain them. The control mechanism is identified to combine Franck-Condon filtering and phase-dependent excitation of the selected excitation channels. The former purifies the vast thermal ensemble of excitation channels and selects a sub-ensemble that is susceptible to coherent control, while the latter realizes their control by manipulating intra-channel interferences. The demonstrated control of the KAr system serves as a basic model for controlling triatomic systems, with an atom and a diatomic molecule as the reactants, that are non-reactive in their ground electronic state and all their excited electronic states are weakly-bound or unbound.

 

With the weak-field feasibility, the toolbox for coherent control of ultrafast molecule making can now handle many new excitation scenarios that their control is superior or only possible with weak fields. This opens up new feasible routes for the coherent control of binary chemical photoreactions.