Probing transient multi-center ionic structures in hot dense plasmas

From the very beginning of quantum molecular dynamics simulations of warm dense matter, the visualization of the electronic density (3D iso-surfaces or 2d charge density contours) has evidenced the occurrence of transient multi-center structures (TMCS), also named quasi-molecular states (see e.g. [1] for detailed description of such states for hydrogen). TMCS are commonly encountered in QMD simulations, and manifest themselves systematically at high density but not too high temperature, depending of the element (see [2] for helium). These events are thought to be the consequence of a softening of the ion-ion interactions and of the breakdown of pairwise interactions approximation. A measure of the occurrence of TMCS is provided by the pair distribution function between nuclei which, being a spatial and temporal average, records such events. By using the one component plasma (OCP) as a fully repulsive reference system, and orbital-free molecular dynamics (OFMD) simulations to account for the screening, we can compare the respective pair distribution functions (OCP, OFMD and QMD), and track the occurrence of TMCS. These structures, are attributed to the partial ionization of the system leaving 1s states candidates for transient quasi-molecular binding. We propose a X-ray diffraction experiment to probe these TMCS.

[1] L. Collins et al. ”Quantum molecular dynamics simulations of hot, dense hydrogen.” Phys. Rev. E 52 :6202–6219 (1995).

[2] A. Kietzmann, et al. “Quantum molecular dynamics simulations for the nonmetal-to-metal transition in fluid helium,” Phys. Rev. Lett., vol. 98, p. 190602, 2007.