Triplet-singlet conversion in ultracold Cs₂ and production of ground state molecules

We propose a process to convert ultracold metastable Cs₂ molecules in their lowest triplet state into (singlet) ground state molecules in their lowest vibrational levels. Molecules are first pumped into an excited triplet state, and the triplet-singlet conversion is facilitated by a two-step spontaneous decay through the coupled A¹Σ_u⁺ sim b ³Π_u states. Using spectroscopic data and accurate quantum chemistry calculations for Cs₂ potential curves and transition dipole moments, we show that this process has a high rate and competes favorably with the single-photon decay back to the lowest triplet state. In addition, we demonstrate that this conversion process represents a loss channel for vibrational cooling of metastable triplet molecules, preventing an efficient optical pumping cycle down to low vibrational levels.