  A sputter ion source with a solid graphite target has been used to produce
dianions with a focus on carbon cluster dianions, $\text{C}_{n}^{2-}$, with
$n=7-24$. Singly and doubly charged anions from the source were accelerated
together to kinetic energies of 10 keV per atomic unit of charge and injected
into one of the cryogenic (13 K) ion-beam storage rings of the Double
ElectroStatic Ion Ring Experiment facility at Stockholm University. Spontaneous
decay of internally hot $\text{C}_{n}^{2-}$ dianions injected into the ring
yielded $\text{C}^{-}$ anions with kinetic energies of 20 keV, which were
counted with a microchannel plate detector. Mass spectra produced by scanning
the magnetic field of a $90^{\circ}$ analyzing magnet on the ion injection line
reflect the production of internally hot $\text{C}_{7}^{2-}-\text{C}_{24}^{2-}$
dianions with lifetimes in the range of tens of microseconds to milliseconds.
In spite of the high sensitivity of this method, no conclusive evidence of
$\text{C}_{6}^{2-}$ was found while there was a clear $\text{C}_{7}^{2-}$
signal with the expect isotopic distribution. An upper limit is deduced for a
$\text{C}_{6}^{2-}$ signal that is two orders-of-magnitue smaller than that for
$\text{C}_{7}^{2-}$. In addition, $\text{C}_{n}\text{O}^{2-}$ and
$\text{CsCu}^{2-}$ dianions were detected.
