  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.
