This thesis presents experimental studies of mutual-neutralization reactions between H^- and the cations ⁷Li⁺, ¹⁶O⁺, ¹²C⁺ and ¹⁴N⁺ at the double electrostatic ion-beam storage ring DESIREE. By overlapping two keV ion beams, with corresponding speeds of ca. 1000 km/s, and matching their velocities with applied electric fields, the center-of-mass collision energies are reduced to a few tens of meV. Time- and position-sensitive detectors allow the measurement of the  separation of the two formed neutral products, which depends on the kinetic-energy release in the charge-transfer reaction. The branching fractions into the different quantum states are extracted by binning or modelling of the measured product-separation distributions.

Due to the difficulty of merged-beams experiments with high mass ratio between the ions, ¹H^- is often replaced with ²H^-. We were able to merge beams with a mass ratio of up to 16 between the ions, which allowed us to use ¹H^- in all studies presented here. For H^-+⁷Li⁺ and H^-+¹⁶O⁺, we included both hydrogen isotopes, and found a significant isotope effect. The lighter hydrogen isotope leads to an increased population in lower excited states in both cases.

Our data constitute important benchmarks for theoretical calculations that are needed in the modelling of stellar photospheres.We compare our experimental findings to different theoretical models and discuss their strengths and shortcomings.

In order to achieve these scientific results, the thesis work involved the development of data preparation and analysis for a frame- and an event-based detector, as well as the development of experimental methods.