Spectra and Autoionization Lifetimes of Long-Range Rydberg Molecular States of $^{85}$Rb$_2$

We present high-resolution autoionization data and modeling of the $7p$ long-range Rydberg molecular states in $^{85}$Rb$_2$. Our process excites a photoassociation resonance in the $1 \, (0_g^-)$ state which decays to $v''=35$ and 36 long-range levels of the $a \, ^3 \Sigma _u^+$ state and to the continuum. These bound molecules are then excited via a single UV photon to target states below the $5s + 7p$ asymptote by a frequency-doubled pulse-amplified CW laser with narrow linewidth, $\sim150$ MHz. The long-range portion of the bonding potential is formed by the scattering interaction of the Rydberg electron of a perturbed $7p$ atom scattering from a nearby ground-state atom. We use time-of-flight to selectively measure molecular ions, which are formed via autoionization. Using a hyperfine model of the $a \, ^3 \Sigma _u^+$ and its coupling to the $X \, ^1\Sigma_g^+$ state\footnote{E. Tiemann, private communication (2015)}, we are able to place an upper limit on the autoionization linewidth of 450 MHz, corresponding to a lifetime $\geq 3.5 \times 10^{-10}$ s. Excited-state hyperfine structure suggests a still-lower linewidth (and thus longer lifetime), but its contribution is not yet fully understood. This work is supported by NSF and AFOSR.