Observation of Deeply-Bound ${}^{85}$Rb${}_2$ Vibrational Levels Using Feshbach Optimized Photoassociation

We demonstrate Feshbach optimized photoassociation (FOPA) into the $0_{g}^{-} (5$S${1/2}+5$P${1/2}$)statein$^{85}$Rb$_{2}$. FOPA uses the enhancement of the amplitude of the initial atomic scattering wave function due to a Feshbach resonance to increase the molecular formation rate from photoassociation. We observe three vibrational levels, $v=$127, 140, and 150, with previously unmeasured binding energies of 256, 154, and 96 cm${}^{-1}$. We measure the frequency, central magnetic field position, and magnetic field width of each Feshbach resonance. Our findings experimentally confirm that this technique can measure vibrational levels lower than those accessible to traditional photoassociative spectroscopy.