On the emergence of molecular structure

Abstract

The structure of ${{a}^{\ifmmode\pm\else\textpm\fi{}},{a}^{\ifmmode\pm\else\textpm\fi{}},{b}^{\ensuremath{\mp}}}$-type Coulombic systems is characterized by the effective ground-state density of the $a$-type particles, computed via nonrelativistic quantum mechanics without introduction of the Born-Oppenheimer approximation. A structural transition is observed when varying the relative mass of the $a$- and $b$-type particles, e.g., between atomic ${\mathrm{H}}^{\ensuremath{-}}$ and molecular ${\mathrm{H}}_{2}$${}^{+}$. The particle-density profile indicates a molecular-type behavior for the positronium ion, Ps${}^{\ensuremath{-}}$.