Loss of Ultracold ^{87}Rb^{133}Cs Molecules via Optical Excitation of Long-Lived Two-Body Collision Complexes.

We show that the lifetime of ultracold ground-state ^{87}Rb^{133}Cs molecules in an optical trap is limited by fast optical excitation of long-lived two-body collision complexes. We partially suppress this loss mechanism by applying square-wave modulation to the trap intensity, such that the molecules spend 75% of each modulation cycle in the dark. By varying the modulation frequency, we show that the lifetime of the collision complex is 0.53±0.06  ms in the dark. We find that the rate of optical excitation of the collision complex is 3_{-2}^{+4}×10^{3}  W^{-1} cm^{2} s^{-1} for λ=1550  nm, leading to a lifetime of <100  ns for typical trap intensities. These results explain the two-body loss observed in experiments on nonreactive bialkali molecules.