Relaxation processes among adiabatic potential energy surfaces of Tl+ and Au− centers in NaCl crystals

Abstract

We investigated temperature dependencies of the temporal behaviors of luminescence bands attributed to the radiative transitions from adiabatic potential energy surfaces (APESs) of 1T1g, 3T2u+3Eu, 3T1g, and 3A1u of Tl+ and Au− centers in NaCl crystals. In addition to NaCl:Au−, a luminescence band with the sharp zero-phonon luminescence line was observed in NaCl:Tl+, a strong electron-lattice interaction system. A time-resolved study revealed that this luminescence band with the zero-phonon line in NaCl:Tl + exhibited the decay time of 4.3 ms at low temperatures attributed to the radiative transition from 3A1u APES. The characteristic temperature dependences of the decay time constant and luminescence intensity of the luminescence from the 3T1g and 3A1u APESs were discussed based on a configuration coordinate scheme. The intensities and decay time constants of luminescence bands caused by radiative transitions from the 1T1g, 3T2u+3Eu, 3T1g, and 3A1u APESs changed with temperature through thermally activated nonradiative transitions among APESs. The temperature dependencies of luminescence intensities and decay time constants were analyzed using rate equations that comprise radiative and nonradiative transitions from APESs and among APESs, respectively. The activation energies of APESs were estimated from the best-fit results. The characteristics of thermally activated processes among APESs are revealed in weak and strong electron-lattice interaction systems.