Detecting high density ultracold molecules using atom-molecule collision

Utilizing single-photon photoassociation, we have achieved ultracold rubidium molecules with a high number density that provides a new efficient approach toward molecular quantum degeneracy. A new detection mechanism for ultracold molecule utilizing the inelastic atom-molecule collision is demonstrated. The resonant coupling effect on the formation of the ${\rm X^1\Sigma^+_g}$ ground state ${\rm ^{85}Rb_2}$ allows for a sufficient number of more deeply bound ultracold molecules, which induced an additional trap loss and heating of the co-existing atoms owing to the inelastic atom-molecule collision. Therefore, after photoassociation process, the ultracold molecules can be investigated using the absorption image of the ultracold rubidium atoms mixed with the molecules in a crossed optical dipole trap. The existence of the ultracold molecules was then verified, and the amount of the accumulated molecules was measured. This method is to detect the final produced ultracold molecules, and hence distinct from the conventional trap loss experiments, which is used to study the association resonance. It is composed of measurements of the time evolution of atomic cloud and a decay model, by which the number density of the ultracold ${\rm ^{85}Rb_2}$ molecules in the optical trap was estimated to be ${\rm > 5.2\times10^{11} cm^{-3}}$.