  The first direct experimental observation of an electric quadrupole
($\textit{E}$2) transition between bound states of an atomic negative ion has
been made. The transition was observed in the negative ion of bismuth by
resonant (1+1) photodetachment from Bi$^-$ $^3\textit{P}_2$ via excitation of
the Bi$^-$ $^3\textit{P}_0$ fine structure state. The $\textit{E}$2 transition
properties were independently calculated using a hybrid theoretical approach to
account for the strong multi-level electron interactions and relativistic
effects. The experimental and theoretical results are in excellent agreement,
providing valuable new insights into this complex system and forbidden
transitions in negative ions.
