Short-range photoassociation from the inner wall of the lowest triplet potential of⁸⁵Rb₂

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.

Selective control of photoassociation of alkaline earth dimers: A theoretical study

“…Using the Franck–Condon filtering technique, vibrational coherence is generated according to the maximized Franck‐Condon overlap . The selected optimal target states have provided a feasible route of photoassociation for alkaline earth dimers.…”

“…For a given initial vibrational state, there is a limited range of collision energies, which allows colliding pair to reach the Franck–Condon window for photoassociation located at a short internuclear distance . The “Franck–Condon window” is the maximized Franck–Condon overlap, where the transition probability of photoassociation is much larger; then we can predict the optimal target state of photoassociation . In other words, the purpose of the Franck–Condon filtering is mainly purification for the transition in order to allow the generation of molecular coherence .…”

“…Alkaline earth dimers are well suited for photoassociation studies for two main reasons: (1) because of the van der Waals force, the molecular ground electronic states are shallow, which well supports a few bound states and (2) the electronically excited states with deep potential wells are conveniently accessible into a photoassociation process via optical transitions . It has been nominated as photoassociation candidates having a large Franck–Condon overlap and much longer lifetime of vibrational levels in the excited state . The interatomic distance of A 1 ∑ + u in alkaline earth dimers, although the potential well of A' 1 Π u is deeper than the A 1 ∑ + u , is more close to the ground state .…”

“…[20] The "Franck-Condon window" is the maximized Franck-Condon overlap, where the transition probability of photoassociation is much larger; then we can predict the optimal target state of photoassociation. [23] In other words, the purpose of the Franck-Condon filtering is mainly purification for the transition in order to allow the generation of molecular coherence. [22] Therefore, it is an important, first step for photoassociation that calculate the Franck-Condon factors (FCFs).…”

“…[20] It has been nominated as photoassociation candidates having a large Franck-Condon overlap and much longer lifetime of vibrational levels in the excited state. [23,24] The interatomic distance of A 1 Σ + u in alkaline earth dimers, although the potential well of A' 1 Π u is deeper than the A 1 Σ + u , is more close to the ground state. [25][26][27] In Be 2 , Mg 2 , and Ca 2 , the electronic transition dipole moment (TDM) of A 1 Σ + u X 1 Σ + g is larger than A' 1 Π u X 1 Σ + g , [21,26,27] that is to say, A 1 Σ + u is the most likely target state for selective control of photoassociation.…”