Collisions of Spin-polarized YO Molecules for Single Partial Waves

Efficient sub-Doppler laser cooling and optical trapping of YO molecules offer new opportunities to study collisional dynamics in the quantum regime. Confined in a crossed optical dipole trap, we achieve the highest phase-space density of 2.5 × 10^-5 for a bulk laser-cooled molecular sample. This sets the stage to study YO-YO collisions in the microkelvin temperature regime, and reveal state-dependent, single-partial-wave two-body collisional loss rates. We determine the partial-wave contributions to specific rotational states (first excited N=1 and ground N=0) following two strategies. First, we measure the change of the collision rate in a spin mixture of N=1 by tuning the kinetic energy with respect to the p- and d-wave centrifugal barriers. Second, we compare loss rates between a spin mixture and a spin-polarized state in N=0. Using quantum defect theory with a partially absorbing boundary condition at short range, we show that the dependence on temperature for N=1 can be reproduced in the presence of a d-wave or f-wave resonance, and the dependence on a spin mixture for N=0 with a p-wave resonance.