Electron self-trapping in liquids and dense gases

The basic physics regarding self-trapping of light particles in simple fluid hosts is reviewed pedagogically. Electron and positronium self-trapping in fluid helium is taken as a historical starting point. The theoretical context consists of simplified continuum models with averaged interactions, but required improvements are discussed. Experimental examples are chosen to illustrate bulk, surface, and impurity effects. Equilibrium and dynamical aspects of the field are illustrated. In noting applications to more complex systems, reference is made to recent developments using path-integral and computer simulation methods. The article spans certain aspects of studies in this fascinating area over the last 30 years.