An ensemble of molecular ions interacting with a black-body field is expected to equilibrate to a internal temperature equal to that of the environment. At thermal equilibrium the probability of the ions to occupy a certain quantum state is given by the Boltzmann distribution. For experiments on molecular ions it would be greatly beneficial if all ions were in the same quantum state.
In this work we present an experiment with OH− stored in one of the DESIREE storage rings. Due to the excellent vacuum of DESIREE storage times of several minutes are obtained. On this time scale we are able to study the rotational relaxation of the ions as they interact with black-body radiation from the cold environment.
Using a photodetachment thermometry technique we measure the populations of the ions in the ground and excited rotational states which yields the internal temperature of the ions. At thermal equilibrium the internal temperature agrees well with the macroscopic temperature of the environment. We also apply selective photodetachment to manipulate the distribution of rotational states resulting in a beam with > 99 % of the ions in the ground state.