Determining the nature of quantum resonances by probing elastic and reactive 
scattering in cold collisions

Abstract

Scattering resonances play a central role in collision processes in physics and 
chemistry. They help build an intuitive understanding of the collision dynamics 
due to the spatial localization of the scattering wavefunctions. For resonances 
that are localized in the reaction region, located at short separation behind the 
centrifugal barrier, sharp peaks in the reaction rates are the characteristic 
signature, observed recently with state-of-the-art experiments in low-energy 
collisions. If, however, the localization occurs outside of the reaction region, 
mostly the elastic scattering is modified. This may occur due to above-barrier 
resonances, the quantum analogue of classical orbiting. By probing both elastic 
and inelastic scattering of metastable helium with deuterium molecules in 
merged-beam experiments, we differentiate between the nature of quantum 
resonances - tunnelling resonances versus above-barrier resonances - and 
corroborate our findings by calculating the corresponding scattering 
wavefunctions.