Sticky collisions of ultracold molecules in the dark

The influence of transient complexes on collisions between ultracold molecules in their absolute ground state has recently been a topic of intense investigation. One particularly important reason for this is the experimentally observed loss of nonreactive ground-state molecules from optical dipole traps. A recent theoretical model attempted to explain this loss via photo-excitation of transient sticky complexes. By trapping fermionic NaK molecules with blue-detuned light near a narrow molecular transition, we are able to to test this hypothesis with light intensities three orders of magnitude lower than what is typical in red-detuned dipole traps. Our results disagree by at least two orders of magnitude with the theoretical predictions, showing that crucial aspects of molecular collisions are not yet understood, and provide a benchmark for the development of new theories.