Clocking Femtosecond Collisional Dynamics via Resonant x-ray Spectroscopy*

Electron-ion collisional dynamics is of fundamental importance in determining plasma transport properties, non-equilibrium plasma evolution and electron damage in diffraction imaging applications using bright x-ray free-electron lasers (FELs). Here we describe the first experimental measurements of ultra-fast electron impact collisional ionization rates using resonant core-hole spectroscopy in a solid-density magnesium plasma, created and diagnosed with the Linac Coherent Light Source x-ray FEL. By resonantly pumping the 1s-2p transition in highly-charged ions within an optically-thin plasma we have measured how off-resonance charge states are populated via collisional processes on femtosecond times scales. We present a collisional cross section model that matches our results and demonstrates how the cross sections are enhanced by dense-plasma effects including ionization potential lowering. Non-LTE (local thermodynamic equilibrium) collisional radiative simulations show excellent agreement with the experimental results, and provide new insight on collisional ionization and threebody-recombination processes in the dense plasma regime. This work has recently been published in Physical Review Letters 120, 055002 (2018).