State-Selective Observation of Radiative Cooling of Vibrationally Excited C 2 .

We have observed radiative cooling of vibrationally excited C ₂ ^- in the X ² Σ _g ⁺ electronic ground state via electronic transitions to near-degenerate low-lying vibrational levels of the A ² Π _u electronic excited state. Combining an ion storage technique with high-resolution detachment spectroscopy, we were able to assign rovibronic transitions to the resulting complex spectra. The time evolution of the population at specific vibrational states was measured up to 60 ms, providing the first quantitative experimental support for the long-standing theoretical predictions.