Quadrupole Transitions to Rydberg States in an Ultracold $^{85}$Rb Gas

We report the observation of dipole-forbidden, but quadrupole-allowed, transitions to Rydberg states in $^{85}$Rb. Using pulsed UV excitation ($\lambda$ = 297 nm) of ultracold atoms in a MOT, we are able to drive 5$s \rightarrow nd$ quadrupole (E2) transitions. We show that these transitions are one-photon excitations and give evidence that they are not caused by stray-field induced Stark-mixing. These transitions are observed over the range $n$=28 to $n$=50. We will give the measured E1/E2 transition strength ratio strength at various principal quantum numbers $n$. We will also report our experimental measurements of the $np$ fine-structure transition strength ratio for principal quantum number up to $n$=70.