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. 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, the scientists differentiated between the nature of quantum resonances - tunnelling resonances versus above-barrier resonances - and corroborate their findings by calculating the corresponding scattering wavefunctions.

The research findings were recently published in the renowned journal „Nature Chemistry“.