Determination of absolute recurrent fluorescence rate coefficients for C 6 ^-

We determined absolute rate coefficients for the recurrent fluorescence (RF) process in C₆^- anions at excitation energies above the adiabatic electron attachment energy of 4.18 eV. The experiment was performed by extracting C6^- ions from a sputter ion source and storing them in a bent electrostatic ion beam trap. After 1 s of storage, during which the anions cooled down to temperatures close to room temperature, they were excited by a short laser pulse and the neutralization rate due to vibrational autodetachment (VAD) was measured as a function of time at several wavelengths. Due to the different energy dependence of the two competing decays via the RF and the VAD process, their contributions to the measured total decay rate coefficients could be disentangled. For excitation energies 4.6 eV, the decay is found to be dominated by the RF process with decay rate coefficients on the order of 5 × 10⁴ s^-1. The result clearly demonstrates the presence of the RF process in C6 and illustrates the importance of this process in the production and cooling of isolated molecules of astrophysical interest.