Electronic excitations and the compressibility of deuterium

We propose an explanation of the large compressibility of deuterium observed in recent laser shock experiments, as inferred from large-scale first-principles simulations. Contrary to usual assumptions about dynamical compression of matter, we found that electronic excitations can occur during shock propagation in fluid deuterium. Their origin is traceable to nonadiabatic processes caused by transitions through avoided crossings near the Fermi level as the liquid is impacted by the shock front. Our results indicate that the observed large compressibility is determined by shock-induced electronic excitations and does not arise from new equilibrium properties of hydrogen in the liquid state.