Resonances in Bimolecular Chemical Reactions

The existence of reactive resonances has long been predicted based on theoretical quantum reaction dynamics. Although many aspects of the theoretical modeling of such resonances are understood, it has remained an open question as to how resonance phenomena may be observed in a molecular beam experiment. Many of the simple ideas for resonance signatures are based on the idealization of the isolated narrow resonance, which generally does not apply to reactive resonances. Due to the occurrence of impact parameter averaging, the resonant scattering becomes smeared over broad ranges of collision energy making it difficult to distinguish between direct and complex scattering mechanisms. In this article, recent advances in the study of short-lived reactive resonances are reviewed. The imprint of resonances upon dynamical observables is discussed. Three case studies are presented for simple triatomic systems that are believed to exhibit resonance phenomena: F+HD -> D+HF, H+HD -> D+H-2, and F+HCl -> Cl+HF. It is seen that reactive resonances do strongly influence collision observables, but in a different way for each case. At this stage, there does not appear to be a unique resonance signature that can be applied to all reactions.