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The radiative cooling rates of C$_N^+$ clusters ($N = 9, 11, 12, 17-27$) havebeen measured in the ultrahigh vacuum of an electrostatic storage ring tovalues on the order of $10^4$ s$^{-1}$. The rates were measured as a competingchannel to unimolecular decay, and the rate constants pertain to the excitationenergies where these two channels compete. Such high values can only beexplained as photon emission from thermally excited electronic states, amechanism that has also been seen in polycyclic aromatic hydrocarbon cations.The high rates have a very strong stabilizing effect on the clusters and theunderlying mechanism gives a high energy conversion efficiency, with thepotential to reach high quantum efficiencies in the emission process. Thecompeting decay of unimolecular fragmentation defines upper limits for photonenergies that can be down-converted to lower energy photons. Includingpreviously 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 withsize. Clusters absorbing photons of energies below these limits cool downefficiently by emission of photons via electronic transitions and theirfragmentation is strongly reduced, increasing their survival in HI regions.

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