X-ray signatures of charge transfer reactions involving cold, very highly charged ions

Charge transfer reactions involving highly charged ions up to U91+ are being studied using x-ray spectroscopic techniques. The measurements are performed using ions produced in situ in a cylindrical trap with very low kinetic energy (⩽3 eV/amu). The observed K-shell and L-shell x-ray emission show prominent features from high-n levels decaying to the ground level. Such emission can serve as a diagnostic marker for charge transfer processes in plasmas, for example in cometary comae or in ion traps. The emission is enabled by electron capture into states with low orbital angular momentum, mainly l=1. The observed spectra, thus, provide a handle on determining the fraction of electron capture into low-l states that can be used for testing theoretical models in the limit of low collision energy. The present paper summarizes the progress made to date.