Observation of Radiative Collisional Fluorescence

We report the observation of spontaneous radiative emission during the collision of two excited Ba atoms. The emission occurs at the sum energy of the excited atoms and is the emission analog of the laser-induced inelastic collision process. We measure a collision cross section for deexcitation by spontaneous radiative emission of 2.6×${10}^{-20\mathrm{}}$ ${\mathrm{cm}}^2$.