Self-trapped exciton in caesium iodide. III. Polarisation of the emissions

For pt.II see ibid., vol.13, no.6, p.993 (1980). The radiative recombination of (100) oriented self-trapped holes and conduction electrons has been measured in CsI:Na between 1.4 and 50K. At the lowest temperatures, both intrinsic emissions (290 nm and 338 nm) exhibit a partial plane polarisation perpendicular to the (100) axis of the parent V K centres. The 338 nm emission polarisation remains independent of temperature but the 290 nm emission polarisation decreases with temperature and eventually reverses its sign near 11K. Calculations show that both emissions come from a split triplet state located slightly below a singlet state which has an allowed transition to the ground state for the 290 nm emission but is forbidden for the 338 nm emission. This model explains reasonably well all the previously measured properties of self-trapped exciton emissions in CsI such as intensities, polarisations and decay times.