Observation of Electric Quadrupole Transitions to Rydberg nd States of   Ultracold Rubidium Atoms
  We report the observation of dipole-forbidden, but quadrupole-allowed, one-photon transitions to high Rydberg states in Rb. Using pulsed UV excitation of ultracold atoms in a magneto-optical trap, we excite $5s \to nd$ transitions over a range of principal quantum numbers $n=27-59$. Compared to dipole-allowed (E1) transitions from $5s \to np$, these E2 transitions are weaker by a factor of approximately 2000. We also report measurements of the anomalous $np_{3/2} : np_{1/2}$ fine-structure transition strength ratio for $n=28-75$. Both results are in agreement with theoretical predictions. 
D. Tong, S. M. Farooqi, E. G. M. van Kempen, Z. Pavlovic, J.   Stanojevic, R. C\^ot\'e, E. E. Eyler, P. L. Gould