Excitation of the 5$p$-8$p$ Electric Quadrupole Transition in Cold Rb Atoms

We have observed the electric-dipole (E1) forbidden, but electric-quadrupole (E2) allowed, 5$p$-8$p$ transition in a sample of ultracold $^{87}$Rb atoms. The 5$p_{3/2}$ level is populated by a diode laser at 780 nm, while the 5$p_{3/2}$ to 8$p_{1/2,3/2}$ transition is driven with a dye laser at 587 nm. We have performed the experiment both with cw dye laser excitation and with pulsed amplification ($\sim $7 ns) of this cw light. In both cases, the 8$p$ atoms are detected by pulsed photoionization. With cw excitation, we can easily resolve the 5$p$ and 8$p$ hyperfine structures. By comparing transitions from the 5$p_{3/2}$, \textit{F$^{\prime}$}=0 level to 8$p_{1/2}$ levels with \textit{F$^{\prime \prime}$}=1 and 2, we can set a limit on the weak magnetic-dipole (M1) contribution to the transition strength.\footnote{S.B. Bayram, et al., Phys. Rev. A 62, 012503 (2000).} This work is supported by NSF.