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Thermal radiative cooling of carbon cluster cations C$_N^+$, $N = 9, 11,12, 17-27$

The radiative cooling rates of C$_N^+$ clusters ($N = 9, 11, 12, 17-27$) have been measured in the ultrahigh vacuum of an electrostatic storage ring to values on the order of $10^4$ s$^{-1}$. The rates were measured as a competing channel to unimolecular decay, and the rate constants pertain to the excitation energies where these two channels compete. Such high values can only be explained as photon emission from thermally excited electronic states, a mechanism that has also been seen in polycyclic aromatic hydrocarbon cations. The high rates have a very strong stabilizing effect on the clusters and the underlying mechanism gives a high energy conversion efficiency, with the potential to reach high quantum efficiencies in the emission process. The competing decay of unimolecular fragmentation defines upper limits for photon energies that can be down-converted to lower energy photons. Including previously measured cluster sizes provides the limits for all clusters C$_N^+$, $N=8-27$, of values that vary from 10 to 14.5 eV, with a general increase with size. Clusters absorbing photons of energies below these limits cool down efficiently by emission of photons via electronic transitions and their fragmentation is strongly reduced, increasing their survival in HI regions.