Nucleus-electron model for states changing from a liquid metal to a plasmaand the Saha equation

We extend the quantal hypernetted-chain (QHNC) method, which has been prove d to yield accurate results for liquid metals, to treat a partially ionized plasma. In a plasma, the electrons change from a quantum to a classical fl uid gradually with increasing temperature; the QHNC method applied to the e lectron gas is in fact able to provide the electron-electron correlation at an arbitrary temperature. As an illustrating example of this approach, we investigate how Liquid rubidium becomes a plasma by increasing the temperat ure from 0 to 30 eV at a fixed normal ion density 1.03 x 10(22)/cm(3). Th, electron-ion radial distribution function (RDF) in liquid Rb has distinct i nner-core and outer-core parts. Even at a temperature of 1 eV, this clear d istinction remains as a characteristic of a liquid metal. At a temperature of 3 eV, this distinction disappears, and rubidium becomes a plasma with th e ionization 1.21. The temperature variations of bound levels in each ion a nd the average ionization are calculated in Rb plasmas at the same time. Us ing the density-functional theory, we also derive the Saha equation applica ble even to a high-density plasma at low temperatures. The QHNC method prov ides a procedure to solve this Saha equation with ease by using a recursive formula; the charge population of differently ionized species are obtained in Rb plasmas at several temperatures. In this way, it is shown that, with the atomic number as the only input, the QHNC method produces the average ionization, the electron-ion and ion-ion RDF's, and the charge population t hat are consistent with the atomic structure of each ion for a partially io nized plasma. [S1063-651X(99)04809-6].