Mutual neutralization is the process where an electron is transferred from an anion to a cation forming two neutral atoms or molecules. This project's primary purpose is to measure mutual neutralization with control of  internal degrees of freedom (i.e. the quantum states occupied forboth ions) and their relative motion. In the cryogenic electrostatic double ion-storage ring, DESIREE, beams can now be stored for long times in
cryogenic environments allowing the ions to cool down internally.  
We use these unique conditions to investigate this process for a wide range of atomic and small molecular ions of fundamental interest. We investigate mutual neutralization processes of relevance in the interstellar medium and between H- and positive metal ions, which is of significance for the understanding of stellar atmospheres and stellar metal abundances.