Decay of bound states due to coupling with free particle states is a general phenomenon occurring at energy scales from MeV in nuclear physics to peV in ultracold atomic gases. Such a coupling gives rise to Fano-Feshbach resonances (FFR) that have become key to understanding and controlling interactions. In the first part of the talk, we show that bound states, despite being resonantly coupled to a scattering state, become protected from decay whenever the relative phase is a multiple of π. We demonstrate that for spin-orbit excited molecular ions this results in predissociation lifetimes spanning four orders of magnitude.
While FFRs are well known in multichannel systems, in the second part of the talk we show that probing the angular dependence of the cross section allows us to unveil asymmetric Fano profiles also in a single channel shape resonance. Using a model description for partial wave interference, we disentangle the resonant and background contributions and extract the relative phase responsible for the characteristic Fano-like profiles from experimental measurements of elastic collisions between metastable helium and deuterium molecules.