High-resolution spectroscopy of long-range molecular states of 85Rb2

We present analysis of low-$n$ long-range molecular Rydberg states in $^{85}$Rb$_2$, based on high-resolution spectra. The weakly bound states are accessed by bound-bound transitions from high-$v$ levels of the $a , ^3 Sigma _u^+$ state, which are prepared by photoassociation of laser-cooled atoms. Single-photon transitions to target states near the $5s + 7p$ asymptote are excited by a frequency-doubled pulse-amplified CW laser with a narrow linewidth, under 200 MHz. The long-range portion of the bonding potential is dominated by the elastic scattering interaction of the Rydberg electron of a perturbed $7p$ atom and a nearby ground-state atom, in much the same manner as trilobite states. We use time of flight to selectively measure molecular ions, which are formed via autoionization. This technique gives a two orders-of-magnitude improvement in linewidth over our previous work, reported in Ref. [1]. We also present calculations of a proposed scheme for STIRAP transfer from the current $v''=35$ level of the $a , ^3 Sigma _u^+$ state to the $v''=39$ level. The long-range states accessible to us are defined in large part by the Franck-Condon factors, which are dominated by the outer lobe of the wavefunction. Thus, choosing a $v''$ sets $R$, and determines the Franck-Condon window. The proposed $v'' = 39$ level has a classical outer turning point at $sim 72 , a_0$, and will provide access to higher-$n$ states with longer-range wells. This work is supported by the NSF and AFOSR.\ noindent [1] M. A. Bellos textit{et al.}, Phys. Rev. Lett. textbf{111}, 053001 (2013