Enhancement of the formation of ultracold $^{85}$Rb$_2$ molecules due to resonant coupling

TL;DR

This paper demonstrates that resonant electronic state coupling significantly enhances the formation of ultracold $^{85}$Rb$_2$ molecules via photoassociation, leading to increased populations of specific vibrational ground states.

Contribution

It introduces a new calculation method accounting for resonant spin-orbit coupling effects, explaining the enhanced molecule formation observed.

Findings

Vibrational levels $v''$=112-116 are more populated than expected.

Ground-state molecule population shows oscillatory behavior with laser tuning.

Resonant spin-orbit coupling enhances formation of deeply bound molecules.

Abstract

We have studied the effect of resonant electronic state coupling on the formation of ultracold ground-state $^{85}$Rb$_2$. Ultracold Rb$_2$ molecules are formed by photoassociation (PA) to a coupled pair of $0_u^+$ states, $0_u^+(P_{1/2})$ and $0_u^+(P_{3/2})$, in the region below the $5S+5P_{1/2}$ limit. Subsequent radiative decay produces high vibrational levels of the ground state, $X ^1\Sigma_g^+$. The population distribution of these $X$ state vibrational levels is monitored by resonance-enhanced two-photon ionization through the $2 ^1\Sigma_u^+$ state. We find that the populations of vibrational levels $v''$=112$-$116 are far larger than can be accounted for by the Franck-Condon factors for $0_u^+(P_{1/2}) \to X ^1\Sigma_g^+$ transitions with the $0_u^+(P_{1/2})$ state treated as a single channel. Further, the ground-state molecule population exhibits oscillatory behavior as the PA laser is tuned through a succession of $0_u^+$ state vibrational levels. Both of these effects are explained by a new calculation of transition amplitudes that includes the resonant character of the spin-orbit coupling of the two $0_u^+$ states. The resulting enhancement of more deeply bound ground-state molecule formation will be useful for future experiments on ultracold molecules.