Real-time scattering of ultracold bi-alkali molecules: trap loss studies Svetlana Kotochigova Department of Physics of Temple University

Many bi-alkali molecules in their absolute ground state are chemically “stable” with respect to molecule-molecule collisions. Then the lifetime of molecules trapped in optical traps was expected to be several seconds limited by the surrounding vacuum. Recent experimental studies with these stable molecules in optical traps, however, observed a surprisingly short lifetime comparable to that of reactive molecules. An explanation of this phenomenon, suggested by Ref. [1], relies on the fact that the excited electronic state tetramer complexes become energetically accessible to resonant absorption of photons of the trapping lasers. This leads to uncontrolled spontaneous decay and loss of molecules from the trap. The initial theoretical treatment of losses was only based on a time-independent analyses of potential energy surfaces for the optimized geometry. The logical next step requires the time-dependent study of the collisional dynamics of molecules, where the electronic character of the collisional complex can significantly change over only a few-femtoseconds in synchrony with the motion of the nuclei. In this presentation, I describe our first efforts to model time-dependent quasi-classical scattering of two ultracold ²³Na⁸⁷Rb molecules in the presence of trapping light including both electronic ground and excited states. The quantum nature of the initial v = 0, J = 0 ro-vibrational ground state of the ultracold NaRb molecules and their external motion is simulated through quasi-classical sampling of the NaRb probability distribution. We illustrate collisional dynamics in the presence of an external trapping light with a wavelength of 1064 nm by calculating the first 10 ps of the collision for a variety of initial relative orientations of molecules.
[1] A. Christianen et al. Phys. Rev. Lett. 123:123402 (2019)