QUANTUM SIZE EFFECT OF EXCITONIC MOLECULES IN CUCL MICROCRYSTALS

The quantum size effect of excitonic molecules confined in microcrystals of CuCl is studied theoretically by a variational calculation for a simplified model. The excitons are approximated as quasiparticles confined in a microsphere with the effective radius R* and interacting with each other through the Morse potential. It is shown that the peak energy of the excitonic molecule luminescence shifts to the lower energy side from the bulk value for relatively large values of R* but shifts to the higher energy side as R* is decreased further, in agreement with the experimental observation. By fitting the theoretical value of the peak energy shift with the experimental values, it is shown that the minimum point of the Morse potential is at about 1 nm and the separation between the center of mass of the two excitons is about 1.5 nm in the bulk crystal.