Emission from the Excitonic Molecules in Thallous Halides

Abstract

Two emission lines which show nonlinear growth to the excitation intensity are found near the absorption edges in TlCl and in TlBr. They are investigated in a wide range of the excitation intensity at 4.2 K. From their intensity dependence on the excitation intensity, their line shapes and their energies, it is proposed that the higher energy emission line among is due to the radiative decay of the molecule of two X₆⁺×R₆⁻ excitons by leaving one X₆⁺×R₆⁻ exciton and one M-point LA-phonon through the forbidden indirect process. The lower energy emission is found to be due to the radiative decay of the excitonic molecule but its origin is not cleared. Theoretical calculation on the line shape of the emission from the molecule of forbidden indirect excitons is made.