Energy Transfer in Solid Rare Gases

Abstract

Three types of energy transport are essential in condensed rare gases: migration of free excitons, transfer by localized centers and mass diffusion of electronically excited centers. The first two processes are observed in solid rare gases, a combination of the last two processes in liquid rare gases. Free and localized exciton states are described and the balance between localization and migration of free excitons is analyzed because it determines the transfer range of free excitons. Photoelectron and luminescence experiments are discussed which monitor the migration of free excitons. Forster‐‐Dexter type of energy transfer for localized centers is illustrated by host to guest and guest to guest electronic and vibrational transfer. The competition between transfer and non- radiative relaxation is crucial and examples for transfer prior to electronic or vibrational relaxation are presented. The long lifetime of triplet states especially in liquid helium causes transfer ranges of the order of centimeters. Finally the application of emission from selftrapped excitons in rare gas crystal for vacuum ultraviolet solid state lasers is shown. The effeciency is determined by losses due to energy transport. At these high excitation densities an extremely hot electron plasma is created in the crystal which induces additional transport and scattering processes.