Nuclear proton distributions explored by relativistic resonance fluorescence

Resonance fluorescence of laser-driven atoms is studied in the relativistic regime by solving the time-dependent Dirac equation in a multi-level model. Electron spin and retardation of the electron-photon interaction give rise to phenomena such as splitting of sideband peaks and modification of the Rabi frequencies not explainable in a Schroedinger theory. The approach based on the Dirac equation allows for investigating highly relativistic ions and, consequently, provides a sensitive means to determine parameters of nuclear charge distributions by applying coherent light with x-ray frequencies. Using two driving fields, one may obtain sub-natural linewidths in the emission spectra, increasing the sensitivity of nuclear proton distribution determinations.