Observation of the Nuclear Magnetic Octupole Moment ofCs133

We describe our high-resolution measurements of the ${}^{133}\mathrm{C}\mathrm{s}$ $6p{}^{2}P_{3/2}$ 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 $\le 2\mathrm{k}\mathrm{H}\mathrm{z}$, 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 $a=50.28827(23)\mathrm{M}\mathrm{H}\mathrm{z}$, $b=-0.4934(17)\mathrm{M}\mathrm{H}\mathrm{z}$, and $c=0.56(7)\mathrm{k}\mathrm{H}\mathrm{z}$. 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 $Q=-3.55(4)\mathrm{m}\mathrm{b}$ and nuclear magnetic octupole moment $\Omega =0.82(10)\mathrm{b}\times {\mu }_N$.