Short-range photoassociation from the inner wall of the lowest triplet potential of $^{85}$Rb$_2$

TL;DR

This paper investigates short-range photoassociation in ultracold $^{85}$Rb$_2$, showing that excitation near the inner wall of the lowest triplet potential can efficiently form molecules, with theoretical predictions matching experimental data.

Contribution

It demonstrates that short-range photoassociation occurs at the inner wall of the triplet potential and provides a detailed Franck-Condon analysis aligning with experimental results.

Findings

Franck-Condon factors from bound states match scattering states

Photoassociation at the inner wall is efficient for triplet states

Ground-state singlet photoassociation is weaker and below detection sensitivity

Abstract

Ultracold photoassociation is typically performed at large internuclear separations, where the scattering wavefunction amplitude is large and Franck-Condon overlap is maximized. Recently, work by this group and others on alkali-metal diatomics has shown that photoassociation can efficiently form molecules at short internuclear distance in both homonuclear and heteronuclear dimers. We propose that this short-range photoassociation is due to excitation near the wavefunction amplitude maximum at the inner wall of the lowest triplet potential. We show that Franck-Condon factors from the highest-energy bound state can almost precisely reproduce Franck-Condon factors from a low-energy scattering state, and that both calculations match experimental data from the near-zero positive-energy scattering state with reasonable accuracy. We also show that the corresponding photoassociation from the inner wall of the ground-state singlet potential at much shorter internuclear distance is weaker and undetectable under our current experimental conditions. We predict from Franck-Condon factors that the strongest of these weaker short-range photoassociation transitions are one order of magnitude below our current sensitivity.