We report the first observation of a nondipole transition in an ultracold atomic vapor. We excite the $3P\mathrm{\text{−}}4P$ electric quadrupole ($E2$) transition in ${}^{23}\mathrm{N}\mathrm{a}$ confined in a magneto-optical trap, and we demonstrate its application to high-resolution spectroscopy by making the first measurement of the hyperfine structure of the $4P_{1/2}$ level and extracting the magnetic dipole constant $\mathrm{A}=30.6\pm 0.1\mathrm{M}\mathrm{H}\mathrm{z}$. We use cw optical-optical double resonance accompanied by photoionization to probe the transition.

• Received 8 April 2003

DOI:https://doi.org/10.1103/PhysRevLett.91.213004