Loss of ultracold RbCs molecules via optical excitation of long-lived two-body collision complexes

TL;DR

This study reveals that ultracold RbCs molecules' lifetime is limited by optical excitation of collision complexes, and demonstrates a method to partially suppress this loss using trap modulation.

Contribution

It introduces a trap modulation technique to reduce optical excitation of collision complexes, improving understanding of two-body loss mechanisms in ultracold molecules.

Findings

Collision complex lifetime is approximately 0.53 ms in the dark.

Optical excitation rate of the complex is about 3×10^3 W^{-1}cm^2s^{-1}.

Modulation reduces loss, explaining previous observations in nonreactive molecules.

Abstract

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 $0.53\pm0.06$ ms in the dark. We find that the rate of optical excitation of the collision complex is $3^{+4}_{-2}\times10^{3}$ 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.