Self-trapping of light particles in dense fluids

Abstract

A light particle (electron, positron, or positronium atom) thermalized in a dense fluid can in a certain range of density and temperature become localized in a region of altered fluid density. Because it is the presence of the light particle, lp, that initiates the localization process it is referred to as self-trapping. Self-Trapping results in non-linear dependence upon the density in experimental measurements of lp properties such as electron mobility and positron and positronium decay rate in fluids. Because of its small mass the lp has a thermal wavelength which is much greater than the thermal wavelength of the fluid molecules. Thus it is possible to treat the translational degrees of freedom by classical mechanics and the lp's degrees of freedom by quantum mechanics. This hybrid model is referred to as the adiabatic model. Density Functional Theory, DFT, and the Path Integral Monte Carlo, PIMC, technique are two approximations used to make calculations with the adiabatic model. DFT uses the Jensen inequality to approximate the trace over the lp wavefunction and calculate important equilibrium properties of an lp in a fluid. The PIMC technique uses the classical isomorphism to relate the partition function of a quantum particle to the partition function of a classical polymer. This relation allows the computation of quantum equilibrium averages by standard classical Monte Carlo methods. In this research the simplest variant of DFT and the PIMC technique are used to simulate positronium in dense fluids. DFT is shown to be able to qualitatively recreate experimental measurements of the decay rate of positronium in ethane and Argon. However, because density fluctuations are not included in DFT's the results show an unnatural discontinuity in decay rate plots. Because the PIMC technique is a microscopic model it includes density fluctuations and more closely resembles experimental measurements than DFT. The results of both approximations indicate that the trapping of positronium results in a macroscopic decrease in the local fluid density in the vicinity of the Ps atom. Both models also indicate that trapping is most likely to occur at the fluid's liquid-vapor critical point of the fluid. This result is also indicated in experimental measurements. The decrease in fluid density near the Ps atom results in the long of lifetime of positronium at high densities and low temperatures.