Contribution Contributed Talk
Rotational Cooling of CH + and OH - at the Cryogenic Storage Ring
The Cryogenic Storage Ring (CSR)  located at the Max-Planck-Institut für Kernphysik in Heidelberg is an electrostatic storage ring with a circumference of 35 m in a fully cryogenic environment. Due to the low temperature of the inner experimental chambers (~6K) and an extreme vacuum, with residual gas densities below 140 cm$^-3$ , the CSR is able to imitate the conditions of interstellar space and at the same time reach very long storage times . The first experiments at the CSR aimed at verifying whether the rotational states of stored molecular ions cool down to temperatures of interstellar clouds. Hence the population of states was monitored for CH$^+$ by photodissociation  and for OH$^-$ by near-photodetachment spectroscopy as a function of the storage time. Equilibrium populations of nearly 60% in the ground state for CH$^+$  and over 90% for OH$^-$ lead to temperatures well below typical kinetic temperatures in interstellar clouds. These measurements pave the way for collision and reaction studies under true interstellar conditions.
 R. von Hahn et al., Review of Scientific Instruments 87, 063115 (2016)  A. P. O’Connor et al., PRL 116, 113002 (2016).