# Rotationally cold OH$^-$ ions in the cryogenic electrostatic ion-beam storage ring DESIREE

## TL;DR

This study demonstrates that OH$^-$ ions stored in a cryogenic ring reach near the ring's temperature with most ions in the ground rotational state, and shows how to actively prepare ions in the lowest rotational state.

## Contribution

It provides the first detailed measurement of rotational cooling in a cryogenic storage ring and introduces a method to selectively produce ions in the ground rotational state.

## Key findings

- Ions relax to a rotational temperature of 13.4 K, matching the ring temperature.
- 94.9% of ions are in the rotational ground state at equilibrium.
- Active photodetachment can produce nearly pure ground state ion beams.

## Abstract

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.