Evolution of dipole-dipole dynamics in cold ammonia collisions

Cold polar molecules offer fascinating prospects for ultracold chemistry and quantum physics. However, their inherent collision properties remain largely unknown, as it seems fundamentally impossible to reach sufficiently low energies by merging two beams of molecules with substantial dipole moments. Here we report measurements of state-to-state cross-sections for collisions between ammonia isotopologues at energies between 0.3 and 100 cm^-1 using an advanced beam-merging protocol. We experimentally observed a local maximum in the cross-sections, which indicates that the dipole moments can switch off at low collision energies. Scattering calculations reproduced this maximum in good agreement and explained the observed scaling with the molecule's parity splitting energies. Measurements of the correlated energy transfer in both collision partners yielded direct evidence of the suppression of the dipole-dipole interaction at low energies. Our results demonstrate how collisions between an important class of polar molecules evolve from high temperatures towards the ultracold regime in a counterintuitive way, and offer distinctive opportunities to control cold molecular collisions with external fields.