Observation of density-driven line shifts at 100 TPa*

The deviation of an atomic transition from its isolated-atom energy is a direct consequence of a change in the local environment around the atom. At the extreme pressures characteristic of stellar interiors and inertial fusion plasmas, transition energies shift due to electrostatic interactions between the atomic states and the dense-plasma environment. Understanding the mechanism that drives these line shifts, and particularly the dependence on the thermodynamic state, will enable new spectroscopic diagnostics of dense plasmas. We present time-resolved measurements of a blue-shifting absorption spectrum of a mid-Z tracer layer compressed to 100 TPa in a stagnated, laser-driven implosion. We map the increase in energy of the 1s--2p-type inner-shell transitions and relate this to the evolving thermodynamic conditions. Plausible mechanisms for the line shifts are discussed.