  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
${\rm > 5.2\times10^{11} cm^{-3}}$.
