Observation of the Nuclear Magnetic Octupole Moment of ¹³³Cs

We describe our high-resolution measurements of the ¹³³Cs 6p ²P_3/2 state hyperfine structure. An optically narrowed diode laser excites perpendicularly a highly collimated atomic beam. The spectra are calibrated with a stable reference diode laser using a rf locking scheme allowing us to determine the splittings with an accuracy of ≤2  kHz, an order of magnitude better than previous results. The magnetic dipole a, electric quadrupole b, and magnetic octupole c hyperfine coupling constants are determined. The values we obtained are a=50.288 27(23)   MHz, b=-0.4934(17)   MHz, and c=0.56(7)   kHz. This work represents the first observation of the magnetic octupole moment of the cesium nucleus. We carry out atomic-structure calculations and determine the nuclear electric quadrupole moment Q=-3.55(4)  mb and nuclear magnetic octupole moment Ω=0.82(10)   b×μ_N.