  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 $2.5 \times 10^{-5}$ 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.
