  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.
