  We apply near-threshold laser photodetachment to characterize the rotational
quantum level distribution of OH$^-$ ions stored in the cryogenic ion-beam
storage ring, DESIREE, at Stockholm University. We find that the stored ions
relax to a rotational temperature of 13.4$\pm$0.2 K with 94.9$\pm$0.3 % of the
ions in the rotational ground state. This is consistent with the storage ring
temperature of 13.5$\pm$0.5 K as measured with eight silicon diodes, but in
contrast to all earlier studies in cryogenic traps and rings where the
rotational temperatures were always much higher than those of the storage
devices at their lowest temperatures. Furthermore, we actively modify the
rotational distribution through selective photodetachment to produce an OH$^-$
beam where 99.1$\pm$0.1 % of approximately one million stored ions are in the
$J$=0 rotational ground state.
