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.

We report the first theoretical model for the alkali fluids which yields a liquid-vapor phase coexistence with the experimentally observed features and electrical conductivity estimates which are also in accord with observations.We have carried out a Monte Carlo simulation for a lattice gas model which allows an integrated study of the structural, thermodynamic, and electronic properties of metal-atom fluids. Although such a technique is applicable to both metallic and nonmetallic fluids, non-additive interactions due to valence electron delocalization are a crucial feature of the present model. PACS numbers: 61.25. Mv, 64.70.Fx, 71.30.+h Typeset using REVT E X 1

A theoretical interpretation of recent experiments on ruby-laser-induced breakdown in liquid helium is presented. The dynamics of unbound electrons initially at nonthermal energies in helium proceed until the electrons are finally and stably trapped in a fully thermalized electron bubble. Arguments are presented indicating that intermediate steps in the dynamical process are collisions between the unbound electron and helium atoms, followed by a lowmobility state which may be thermally destroyed, and ending with a thermally stable electron bubble which grows to its equilibrium configuration and thermalizes.Several experimental results are correlated to deduce numerical estimates for the lifetime of the low-mobility state against thermal destruction and against development into a thermally stable electron bubble. An experiment is suggested to unambiguously measure the photoionization threshold energy of an electron bubble.

The helical conformation of electric dipoles in some chiral molecules, such as DNA and bacteriorhodopsin, induces a spin-orbit coupling that results in a sizable spin selectivity of electrons. The local deformation of the molecule about the moving electron may affect the spin dynamics due to the appearance of bright solitons with well-defined spin projection onto the molecule axis. In this work, we introduce an effective model for electron transport in a deformable helical molecular lattice that resembles the nonlinear Kronig-Penney model in the adiabatic approximation. In addition, the continuum limit of our model is achieved when the dipole-dipole distance is smaller than the spatial extent of the bright soliton, as discussed by E. Díaz et al. [N. J. Phys. 20, 043055 (2018)]. In this limit, our model reduces to an extended Davydov model. Finally, we also focus on perturbations to the bright soliton that arise naturally in the context of real helical molecules. We conclude that the continuum approximation provides excellent results in more complex scenarios.

The optical absorption of linear molecular crystals is investigated theoretically. A generalization of the theory of excitons in linear molecular crystals was carried out by Merrifield who constructed the excited states of the crystal by mixing monomolecular states (Frenkel excitons) and bimolecular ionized states in which an electron is removed from one molecule in the crystal and placed on a second (charge-transfer states). If the monomolecular optical transition is allowed, we find that the optical absorption consists of a series of bound states and a continuum absorption, which gives rise to photoconductivity; the strength of all the absorption is predominantly borrowed from the Frenkel exciton. The experimental possibilities may be classified into three cases: (1) the Frenkel exciton, the charge-transfer state with electron and hole at nearest neighbors, and the state with the electron and hole at large separation are well separated in energy. This case gives rise to a strong absorption line, small charge-transfer sidebands, and very small absorption into the continuum. (2) Near resonance between Frenkel and nearest-neighbor charge-transfer states. The strong absorption is now shared by two absorption lines with weak sidebands and very small continuum absorption. (3) Resonance between the Frenkel state and the states with the electron and hole at large separation. This case corresponds to autoionization in a solid, the bound states have very weak absorption, and a strong sharp line will be observed within the continuum absorption. The width of the resonance line is directly proportional to the background absorption.