A mechanism for fast electronic energy transfer in spin-forbidden reactions

The quenching behavior of neutral metastable species during collisions with a neutral partner is investigated. Usually, radiationless energy transfer can be described by invoking the radiation field as an intermediary between interacting molecules. However, for metastables, the dominant radiation transition moments are suppressed, most often by spin selection rules between the excited and ground energy states. The fast reaction rates for the quenching of the metastables suggest a different intermediary for such cases. Motivated by this phenomena, a two-electron exchange process is proposed as the quenching mechanism. During a collision, it is envisioned that the excited outer shell electron of the metastable effectively undergoes a change of molecular center and is replaced by an electron from the collision partner. The theoretical framework for this process is developed from first principles of quantum field theory. The electron exchange term is obtained explicitly, using the minimal-coupling method.

An application of the theory was computed for the energy transfer from the Ar metastable to N₂. This reaction has been studied in depth experimentally, and has a mean cross section of 9 A², with a range of measurements from 2.7 A² to 18 A². The computed theoretical cross section using the two-electron exchange interaction in the first Born approximation is 1.7 A², which is in good agreement with the data considering the range of measured values and the difficulty involved in interpreting the experiments.