# Incomplete rotational cooling in a 22-pole ion trap

## TL;DR

This study investigates the limitations of rotational cooling in a cryogenic 22-pole ion trap, revealing incomplete thermalization around 25K despite various potential causes being examined.

## Contribution

It provides detailed rotational thermometry measurements for specific ion/buffer gas systems and identifies the temperature limit of thermalization in the trap.

## Key findings

- Rotational temperatures plateau around 25K, indicating incomplete thermalization.
- Buffer gas thermalization and blackbody radiation do not fully explain the temperature limit.
- The confinement potential does not significantly affect the thermalization termination.

## Abstract

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.