Luminescence of biexcitons in silver halide crystals

As it is known from Urbach rule, free excitons defining the fundamental absorption edge in AgBr_{1 - x}Cl_x (0 ≤ x ≤ 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 ≈ 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 AgBr_{1 - x}Cl_x(0 ≤ x ≤ 1) are estimated.