The Urbach-Martienssen rule for luminescence spectra is experimentally established for the first time in PbI2 and HgI2, as theoretically suggested by Sumi and Toyozawa. This fact means that the exciton having an energy corresponding to the Urbach tail (Urbach excition) is in thermal equilibrium. It is suggested from the excitation spectra of the luminescence quantum yield that the free and Urbach excitons are not in thermal equilibrium with each other. The luminescence decay times of the free and Urbach excitons are measured in HgI2 by using a combination system of a picosecond mode-locked YAG laser and a synchroscan streak camera. As they depend strongly on the sample, the lifetimes of not only the free exciton but also the Urbach exciton are considered to be governed by the density of impurities or defects in the crystal.
ASJC Scopus subject areas
- Atomic and Molecular Physics, and Optics
- Condensed Matter Physics