  Cryogenic 22-pole ion traps have found many applications in ion-molecule
reaction kinetics and in high resolution molecular spectroscopy. For most of
these applications it is important to know the translational and internal
temperatures of the trapped ions. Here, we present detailed rotational state
thermometry measurements over an extended temperature range for the two
ion/buffer gas systems OH-/He and OD-/HD with ion-to-neutral mass ratios of
4.25 and 6 respectively. The measured rotational temperatures show a
termination of the thermalisation with the buffer gas around 25K, independent
of mass ratio and confinement potential of the trap. Different possible
explanations for this incomplete thermalisation have been investigated, among
them the thermalisation of the buffer gas and the heating due to room
temperature blackbody radiation and room temperature gas entering the trap.
