Ionization Potential Depression in Partially Ionized Hydrogen

The problem of determining the ionization potential depression in the medium or the so-called continuum lowering has a long history and dates back to the 1960s of the past century, when two competing approaches were proposed. The first method of Stewart and Pyatt [1] was based on the approximation of an electron-nucleus electrostatic interaction by the Thomas-Fermi potential at a finite temperature. The other approach was formulated by Ecker and Kröll [2] and stemmed from the generalized Saha equation incorporating the chemical potential of the surrounding plasma. The experimental capabilities of that time refrained from making a univocal choice in favor of one of the two concepts, but the situation has recently changed dramatically. Rather fresh experiments with matter at high-energy densities [3]–[5] have revived the subject by demonstrating that the problem of the ionization potential depression still requires further elaboration