MODEL CALCULATION OF THE OPTICAL PROPERTIES OF 3,4,9,10 - PERYLENE -
- TETRACARBOXYLIC - DIANHYDRIDE (PTCDA) THIN FILMS
The marked difference between the optical absorption of dissolved
PTCDA monomers and crystalline films is analysed in terms of the
deformation of an effective internal vibrational mode of the
molecule and transfer of Frenkel excitons between different
molecular sites in the crystal. It is shown in detail that the
coupling between equivalent molecules in different crystal unit
cells, especially among stack neighbours, dominates over exciton
transfer between the two different molecules in the crystal basis.
The first type of interaction leads to a pronounced deformation of
the expected Poisson progression over subsequent vibrational levels
while the latter is responsible for a small Davydov splitting and
consequently different absorption lineshapes along different crystal
axes.
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