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Radiative and non-radiative processes of Ag− centres doped in NaCl single crystals

Luminescence properties including decay kinetics have been investigated for Ag− centres doped in NaCl single crystals in the temperature range from 7 to 250 K. When the C absorption band peaking at 4.5 eV is photo-excited at low temperature, the A′ and C′ luminescence bands are observed at 2.85 and 4.12 eV, respectively. As the temperature increases, the A′ luminescence grows, with compensation for the decrease of the luminescence intensity of the C′ band, reflecting a thermally activated non-radiative process from the 1T1u to 3T1u states. With increasing temperature above 130 K, the A′ luminescence intensity also decreases. Since the decay time of the A′ luminescence is almost constant at 10 µs above 80 K, the decrease of the A′ luminescence intensity is not attributed to a non-radiative process from the 3T1u state but arises from the quenching of the non-radiative relaxation from the 1T1u to 3T1u states. The temperature dependence of the luminescence intensities of the A′ and C′ bands is discussed on the basis of a simple configuration coordinate model, which consists of the 1T1u, 3T1u and 1A1g adiabatic potential energy surfaces intersecting at each other.