Optical Absorption and Luminescence Spectra of Pb ²⁺-Aggregated Centers in CdI ₂

Change in absorption and luminescence spectra with the concentration of Pb ions doped in a CdI2 crystal is investigated up to a few mol%. Absorption bands due to dimmer, trimmer and further aggregated centers are found to appear in the low energy side of Pb2+ monomer absorption band. Excitation in these absorption bands induces the luminescence from relaxed excited states in the respective centers whose peak energy decreases with increase in the size of aggregated centers. Further increase in the concentration beyond 5 mol%, gives rise to an edge luminescence arising from an excitonic state formed in PbI2 clusters.