We report the observation of trielectronic recombination with simultaneous excitation of a $K$-shell and an $L$-shell electron, hence involving three active electrons. This process was identified in the x-ray emission spectrum of recombining highly charged Kr ions. An energy resolution three times higher than any reported for this collision energy range around 10 keV resulted in the separation of the associated lines from the stronger dielectronic resonances. For ${\text{Kr}}^{30+}$, intershell trielectronic recombination contributions of nearly 6% to the total resonant photorecombination rate were found.

Resonance fluorescence of laser-driven highly charged ions is investigated with regard to precisely measuring atomic properties. For this purpose an ab~initio approach based on the Dirac equation is employed that allows for studying relativistic ions. These systems provide a sensitive means to test correlated relativistic dynamics, quantum electrodynamic phenomena and nuclear effects by applying x-ray lasers. We show how the narrowing of sidebands in the x-ray fluorescence spectrum by interference due to an additional optical driving can be exploited to determine atomic dipole or multipole moments to unprecedented accuracy.

Double-polarization observables in the reaction $\vec{e}p\to e^{\prime }\vec{p}{}^{\prime }\gamma$ have been measured at $Q^2=0.33{(\mathrm{GeV}/c)}^2$. The experiment was performed at the spectrometer setup of the A1 Collaboration using the 855 MeV polarized electron beam provided by the Mainz Microtron (MAMI) and a recoil proton polarimeter. From the double-polarization observables the structure function $P_{LT}^{\perp }$ is extracted for the first time, with the value $(-15.4\pm 3.3_{(\mathrm{stat}.)}{}_{-2.4}^{+1.5}{}_{(\mathrm{syst}.)}){\mathrm{GeV}}^{-2}$, using the low-energy theorem for virtual Compton scattering. This structure function provides a hitherto unmeasured linear combination of the generalized polarizabilities of the proton.