Diffusion-limited exciton fusion reaction in one-dimensional tetramethylammonium manganese trichloride (Tmmc) Kroon, R.; Fleurent, H.; Sprik, R.

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Theoretical lattice-dynamics analysis and new optical experiments on KI:Ag + reveal a nearly unstable low-temperature defect-host configuration, whose population decreases dramatically with temperature, but whose dynamics are consistent with a temperature-independent harmonic model involving collective motion. This behavior is in marked contrast to the temperature-dependent evolution of the dynamics of "soft-mode" systems. PACS numbers: 78.30.-j, 63.20.Pw, 63.20.RyThe silver ion in KI is the most thermally unstable lattice-defect combination known. At 1.2 K the Ag + ion takes the same equilibrium position as the K + ion it replaces but by 20 K it is in an off-center position. 1,2 A fundamental problem is to understand within the framework of lattice dynamics how the system moves between these two lattice-defect arrangements with temperature. 3 Missing from the characterization of the dynamics have been complementary temperature-dependent optical and Raman data and a realistic Lifshitz-type calculation of the KI:Ag + lattice dynamics.To experimentally track the temperature dependence of different aspects of the coupled defect-host system, we first present uv measurements which follow the Ag + ion itself and then Raman scattering results which monitor the disappearance of the E g mode and the appearance of a new A\ -symmetry resonant mode as the temperature is raised. Next, the spectral properties of ir-and Ramanactive features for the on-center configuration are calculated within the harmonic approximation, and the interplay between theoretical and experimental results is described for the first time. The harmonic approximation is shown to give a good account of the low-temperature ir and Raman data, for a model in which the defect and surrounding host ions remain strongly coupled yet are quite close to being unstable against T\ u displacements. Our results imply that the observed temperature dependence of the on-center spectral features simply reflects the population in this state. The system is in either the ground-state configuration or not; a continuous evolution of the on-center defect dynamics with temperature does not occur.To monitor the position of the Ag + ion with temperature we make use of the known optical behavior of this ion in alkali-halide crystals, which can be understood in terms of the 4d 10 ->4d 9 5s parity-forbidden transitions of the defect. 4 " 6 These transitions are made allowed by vibronic couplings to the odd-parity vibrational modes, or if the defect is off center, by the static odd-parity lattice displacement; the larger the odd-parity contribution the larger the line strength. Figure 1(a) shows the measured uv absorption spectrum of KI:Ag + for two different temperatures. Note that over this temperature range the center frequencies remain essentially fixed. A linear concentration depen-10 2 10 1 10°

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Joosen, W.; Fleurent, H.; Bostoen, C.; Schoemaker, D.; Haussuhl, S.