SELF-TRAPPING IN MIXED-CRYSTAL - CLUSTERING, DIMENSIONALITY, PERCOLATION

Self-trapping of a Frenkel exciton in binary mixed crystal ACB1-C, with excitation energy difference Δ=εB—εA and exciton-phonon coupling constant g, is studied. Impurity-assisted self-trapping is possible through cooperation of Δ and g even when neither of them alone is enough for localization. In this situation, one expects amalgamated absorption spectrum representing unrelaxed extended exciton and Stokes-shifted broad emission band representing relaxed localized exciton. We study whether relaxed exciton is localized (S-state) at a particular site of, or extended (B-state) throughout, the A-cluster of various sizes, and how such cluster state changes, as c increases, into free (F) state extended throughout the crystal. The results are compared with recent experiments on TlBrcCl1-c. Sudden appearance of edge emission (due to F) at c∼0.3, at the cost of broad band (due to S and B), is ascribed to the combined effect of size-dependent discontinuity of S-B transition in Br-clusters and their percolation. © 1979, THE PHYSICAL SOCIETY OF JAPAN. All rights reserved.