The transition state is a key concept in the field of chemistry and is important in the study of chemical kinetics and reaction dynamics. Chemical reactions in the gas phase are essentially molecular scattering processes, which are quantum mechanical in nature. Thus probing and understanding detailed quantum structure in the transition state region of chemical reactions, such as reactive resonances, is a central topic in this field. In this article, we focus on recent progress in the study of resonances in elementary bimolecular reactions using state-of-the-art transition state spectroscopy methods: high-resolution photoelectron spectroscopy and quantum state specific backward scattering spectroscopy. The experimental results are compared with high-level quantum dynamics calculations based on highly accurate potential energy surfaces. The dynamics of reactive resonances are also interpreted based on scattering wavefunctions obtained by time-dependent wavepacket calculations. Here, we review many systems that illustrate how reactive resonances can strongly influence the dynamics of elementary chemical reactions.
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