Exciton luminescence from NaCl

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Abstract
Electron irradiation of NaCl crystals has revealed structure in the luminescence spectra. Variations in electron energy, lifetime studies and changes in irradiation temperature enable one to resolve several of the component features. The spectra comprise overlapping emission bands centred near 2.35, 2.65 and 2.85 eV which are interpreted as the result of exciton decay at impurity sites. The intrinsic emission at 3.4 eV is also noted. Detailed studies also reveal the presence of excited excitons which have different lifetimes in the bulk and in the surface layers. The higher states of the surface exciton can be preferentially stimulated with an electron beam at the energy of excitation of the sodium Kalpha energy level. The luminescence is correlated with defect formation and ejection of atoms from the surface.