Experimental study of the excitation threshold of fast intrinsic luminescence of CsI

A type of intrinsic luminescence in crystals has been revealed. The excitation threshold of well-known fast intrinsic luminescence of CsI (300 nm band) has been obtained from the excitation spectrum measured in the photon-energy range 5–125 eV at 300 K using synchrotron radiation. The excitation starts from an energy of about 20 eV and exponentially increases up to the 4d ${\mathrm{I}}^{\mathrm{-}}$ ionization energy (∼50 eV). Such high-energy broad threshold is not typical for any conventional mechanisms of luminescence excitation. We attribute this type of luminescence to the emission center, which is created by the interaction of several elementary excitations produced in the process of relaxation of a primary excitation. The possible role of 4d ${\mathrm{I}}^{\mathrm{-}}$ core levels is discussed. The nanosecond decay kinetics of this luminescence is nonexponential for all energies of excitation.