Quantum effects in cold collisions

In cold collisions, where only a few partial waves contribute to the scattering dynamics, the wave nature of matter emerges, becoming observable in e.g. diffraction patterns, interference effects, or scattering resonances. The latter help build an intuitive understanding of the collision process, due to the spatial localization of the resonance wavefunctions. For resonances that are localized behind the centrifugal barrier and thus in the reaction region, 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. Resonances of both types are found in the example of metastable helium colliding with deuterium molecules [1].

In addition to building intuition, scattering resonances can be exploited to magnify quantum effects in cold collisions. Probing, for example, the resonances with molecules that are rotationally state-selected allows for disentangling the isotropic and anisotropic contributions to the inter-particle interaction [2]. Angle-resolved measurements of the quantum scattering resonances may reveal asymmetric resonance lineshapes, characteristic for Fano interference of a quasi-bound state with a continuum of states [3]. A similar interference effect can also be observed in the product channel of cold reactive collisions populating Fano-Feshbach resonances. These resonances may be protected against decay despite resonant coupling to a scattering continuum if a phase condition is fulfilled. For rare gas diatomic ions, the corresponding phase dependence results in predissociation lifetimes spanning four orders of magnitude, found to be in good agreement with experimental measurements [4].