The activation energy for thermally generating conduction electrons in fluids such as Hg, Cs, and Rb decreases with increasing fluid density and eventually vanishes resulting in a transition to semimetallic behavior. A dominant contribution to this evolution is held to be the cohesion within thermally generated positive molecular ions.

• Received 27 February 1984

DOI:https://doi.org/10.1103/PhysRevLett.53.2320