Triplet-singlet conversion in ultracold Cs$_2$ and production of ground state molecules

TL;DR

This paper proposes a high-rate process to convert ultracold Cs$_2$ molecules from their metastable triplet state to the ground singlet state via a two-step spontaneous decay, impacting vibrational cooling strategies.

Contribution

It introduces a novel triplet-singlet conversion mechanism using spectroscopic data and quantum chemistry calculations, showing its efficiency and implications for molecular cooling.

Findings

High conversion rate competes with decay back to triplet state

Conversion acts as a loss channel for vibrational cooling

Process is facilitated by decay through coupled $A^{1}\Sigma_{u}^{+} \\sim b ^{3}\\Pi_{u}$ states

Abstract

We propose a process to convert ultracold metastable Cs$_2$ 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 $A^{1}\Sigma_{u}^{+} \sim b ^{3}\Pi_{u}$ states. Using spectroscopic data and accurate quantum chemistry calculations for Cs$_2$ 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.