Non-adiabatic quantum dynamics of cold and ultracold chemical reactions

Electronically non-adiabatic processes are usually associated with high energy molecular collisions and photodissociation. In contrast, for many of the cold/ultracold molecular systems of current experimental interest, an excited electronic state is energetically accessible even for collision temperatures approaching absolute zero. For these systems, a non-adiabatic fully quantum mechanical treatment is required that includes the two coupled electronic states. A recently developed non-adiabatic quantum dynamics methodology is discussed which is ideally suited for treating these systems. The methodology is applied to several ultracold chemical reactions under active experimental investigation, such as Li + LiNa → Li₂ + Na. A novel quantum interference mechanism which is unique to ultracold collisions is shown to effectively control the reaction outcome due to the nearly maximal constructive or destructive interference (a quantum molecular switch!). Non-adiabatic effects are shown to reverse the nature of the interference leading to the opposite (correct) theoretical prediction of the ultracold rate coefficients. The highly non-linear nature and sensitivity of the quantum interference might be exploited by experimentalists via the application of external fields and/or the selection of a particular initial quantum state. Possible technological applications include quantum control, probing molecular interactions, sensing, and precision measurements.

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