In the present experiment, beams of Li+ and D− were produced in separate ion sources and stored in the two DESIREE storage rings. In the common straight section, the ions may interact and undergo a process known as mutual neutralization where the excess electron in D− is transferred to Li+, resulting in neutral Li and D atoms. In this reaction, energy is released in the form of kinetic energy. The amount of kinetic energy released depends on the initial and final quantum states involved in the reaction, and by measuring the final-state center-of-mass kinetic energy, we can deduce in which states the products were formed.

To determine the kinetic energy release, the two ion beams are overlapped in the section where the two storage rings merge, and their relative velocity is brought close to zero using a system of drift tubes. Once the reaction has occurred, the neutral products are no longer affected by the steering elements of the storage rings, and they travel in free flight to a position- and time-sensitive detector. Depending on the kinetic energy release, the trajectories of the products are altered differently, and by measuring the distance between the products as they hit the detector the different quantum states involved in the reaction can be separated. The figure shows a histogram of the distances between the products, labelled r, and two peaks are clearly observed. The right peak corresponds to the 3s excited state in neutral Li. The left peak results from a combination of the 3p and 3d states, as these states are too close in energy to be resolved in the experiment. By measuring the number of observations in each channel it is possible to determine the branching fraction of the 3s channel which was found to be 57±0.7%. Additional measurements were performed at higher relative velocities, all in agreement with recent theoretical and experimental work.

The present results are of astrophysical interest. For example, the determination of the Li abundance in stars. Abundance measurements provide information of stellar evolution and nucleosynthesis. Li abundances are deduced from observations and modeling of stellar spectra, and collisional processes such as mutual neutralization are an important part of the models.

The present study was proposed by Paul Barklem and Jon Grumer from Uppsala university who work on the theoretical description of the mutual neutralization process and the modelling of stellar atmospheres. The experiment was performed in collaboration with the local DESIREE group, and the results represent a major milestone for DESIREE, as they demonstrate the first successful merged-beams experiment performed at the facility. More results from merged-beams studies from DESIREE are expected in the near future, including studies of mutual neutralization for Na+ and Mg+ with D− also in collaboration with the researchers from Uppsala and also with the astrophysical application as the primary motivation.

Link to the article:

For the DESIREE scientific and technical staff,
Gustav Eklund, PhD student in the atomic physics division.