High Density Loading and Collisional Loss of Laser-Cooled Molecules in an Optical Trap

We report optical trapping of laser -cooled molecules at sufficient density to observe molecule -molecule collisions for the first time in a bulk gas. SrF molecules from a red-detuned magneto -optical trap (MOT) are compressed and cooled in a blue-detuned MOT. Roughly 30% of these molecules are loaded into an optical dipole trap with peak number density n 0 3 x 10 10 cm - 3 and temperature T 40 uK. We observe twobody loss with rate coefficient beta = 2.7 1.2-0.8 x 10 - 10 cm 3 s - 1 . Achieving this density and temperature opens a path to evaporative cooling towards quantum degeneracy of laser -cooled molecules.