Oliver S. Wenger, G. Mackay Salley and H. U. Güdel
Effects of High Pressure on the Luminescence and Upconversion Properties of Ti²⁺-Doped NaCl
J. Phys. Chem. B 106, 10082-10088 (2002)                       

Abstract: The luminescence and upconversion properties of 0.8% Ti²⁺-doped NaCl at 15 K are studied as a function of external hydrostatic pressure. Luminescence band maxima shifts are analyzed with a ligand field point charge model, allowing an estimate of the compressibility of the spectroscopically active TiCl₆^4- unit in the NaCl host matrix. The pressure dependencies of the vibrational fine structures in the two Ti²⁺ luminescence bands are analyzed and interpreted in terms of pressure-induced changes in equilibrium distortions and force constants of the emitting electronic states ³T_2g(t_2ge_g) and ³T_1g(t_2ge_g). Time-resolved luminescence measurements are used to study the effects of pressure on the excited-state dynamics. 15 K near-infrared excitation at 9399 cm^–1 leads to upconversion luminescence in the red spectral region both at ambient pressure and 34 kbar. On the basis of time-dependent upconversion luminescence experiments, two fundamentally different upconversion mechanisms are found to be dominant under these two experimental conditions. In particular, pressure is found to switch on an efficient upconversion mechanism, which is inactive at ambient pressure, leading to an estimated order-of-magnitude enhancement of the overall upconversion efficiency at 34 kbar. This additional mechanism involves energy transfer (ET) between two excited Ti²⁺ ions. Its occurrence only at high pressure is interpreted in terms of a strongly pressure-dependent spectral overlap integral governing the efficiency of the ET step.