Detection of Recurrent Fluorescence Photons

We have detected visible photons emitted from the thermally populated electronic excited state, namely recurrent fluorescence (RF), of C₆^- stored in an electrostatic ion storage ring. Clear evidence is provided to distinguish RF from normal fluorescence, based on the temporal profile of detected photons synchronized with the revolution of C₆^- in the ring, for which the time scale is far longer than the lifetime of the intact photoexcited state. The relaxation (cooling) process via RF is likely to be commonplace for isolated molecular systems and crucial to the stabilization of molecules in interstellar environments.