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¿nd transitions over a range of principal quantum numbers n=27–59. Compared to dipole-allowed (E1) transitions from 5s¿np, these E2 transitions are weaker by a factor of approximately 2000. We also report measurements of the anomalous np3/2:np1/2 fine-structure transition strength ratio for n=28–75. Both results are in agreement with theoretical predictions