| Popis: |
The component of the triplet-exciton diffusion along the crystal $a$ axis was measured in anthracene crystals at 118, 160, 298, and 371 \ifmmode^\circ\else\textdegree\fi{}K, and found to be ${D}_{\mathrm{aa}}=(4.0\ifmmode\pm\else\textpm\fi{}0.5),(2.5\ifmmode\pm\else\textpm\fi{}0.3),(1.5\ifmmode\pm\else\textpm\fi{}0.2)$, and (1.6 \ifmmode\pm\else\textpm\fi{} 0.3) \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}4}$ ${\mathrm{cm}}^{2}$ ${\mathrm{sec}}^{\ensuremath{-}1}$, respectively. The polarized-excitation spectra (0, 0 line) for delayed fluorescence have also been measured. The inferred values for the Davydov splitting $\ensuremath{\Delta}$ and the effective scattering rate $\ensuremath{\Gamma}$ at the corresponding temperatures are $\ensuremath{\Delta}=18\ifmmode\pm\else\textpm\fi{}2,18\ifmmode\pm\else\textpm\fi{}6,17\ifmmode\pm\else\textpm\fi{}3$, and 19 \ifmmode\pm\else\textpm\fi{} 2 ${\mathrm{cm}}^{\ensuremath{-}1}$ and $\ensuremath{\Gamma}=14\ifmmode\pm\else\textpm\fi{}1,30\ifmmode\pm\else\textpm\fi{}2,51\ifmmode\pm\else\textpm\fi{}1$, and 65 \ifmmode\pm\else\textpm\fi{} 2 ${\mathrm{cm}}^{\ensuremath{-}1}$. The simultaneous measurement of these parameters, in conjunction with an expression for diffusion derived from a phenomenological model of triplet-exciton scattering, allows the assessment of the relative importance of local vs nonlocal scattering mechanisms in the triplet-exciton motion. The nonlocal scattering rate, due to fluctuations in the exciton-transfer matrix elements between molecules separated by $\ifmmode\pm\else\textpm\fi{}\frac{1}{2}(a\ifmmode\pm\else\textpm\fi{}b)$, is estimated to be \ensuremath{\sim} 0.1 ${\mathrm{cm}}^{\ensuremath{-}1}$, and appears to be temperature insensitive. The local scattering mechanism is dominant, but the nonlocal fluctuation rates can make a sizable contribution to the rate of triplet transport. The spectroscopic measurements show that the hopping model for transport is applicable in the temperature range studied. |
