Luminescence of biexcitons in silver halide crystals

As it is known from Urbach rule, free excitons defining the fundamental absorption edge in AgBr1-xClx (0 less than or equal to x less than or equal to 1). The process of primary Frenkel defect creation which involves free exciton states even in AgCl and biexciton creation in AgCl are discussed. The fast luminescence band at approximate to 3.0 eV observed in AgCl under high-density pulsed electron beam and laser excitation has been studied. It is shown that its intensity is a non-linear function of the excitation density. We suggest that the fast luminescence comes from biexcitons; binding energies of such biexcitons in AgBr1-xClx (0 less than or equal to x less than or equal to 1) are estimated. (C) 1998 Elsevier Science B.V. All rights reserved.