EVOLUTION OF EXCITONIC ENERGY-LEVELS IN AR-N CLUSTERS - CONFINEMENT OF BULK, SURFACE, AND DEEP VALENCE SHELL EXCITONS

The evolution of excitonic energy levels (Wannier and Frenkel type) is investigated for ATN clusters in the range N=200-10(6) using fluoresc ence excitation spectroscopy. In the case of Wannier excitons, a prono unced blue shift of the absorption bands relative to the position in t he infinite solid is observed. As a consequence of the lower dimension ality, the shift of the transition energy of surface excitons is consi derably smaller than that of the bulk states of clusters. The evolutio n with size is discussed within several theoretical models for exciton confinement. In addition, model calculations are performed for bulk e xcitons which give good quantitative agreement with the experimental r esults. In the case of n=1 Frenkel or intermediate type excitons, ther e are blue and red shifts observed. The spectral shift of(3p-->4s) and deep valence (3s-->4p) excitations differs considerably. From the shi ft of the transition energies the exciton mass of the (3p-->4s) excito n is derived. (C) 1996 American Institute of Physics.