Loss of ultracold RbCs 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 $0.53\pm 0.06$ ms in the dark. We find that the rate of optical excitation of the collision complex is $3_{-2}^{+4}\times {10}^3$ W${}^{-1}$ cm${}^2$ s${}^{-1}$ for $\lambda =1550$ 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.