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We present a comprehensive investigation of Raman-active modes of 3, 4, 9, 10-perylene-tetracarboxylic-dianhydride (PTCDA) thin films deposited on H-passivated Si (111) substrates. The experimental data are interpreted with a density-functional tight-binding scheme used for calculating the geometry and vibrational modes of the isolated molecule. These results for the breathing modes are applied to the optical absorption of PTCDA in solution, and the vibrational absorption bands can be related quantitatively to the elongation of the most prominent Raman modes during the highest occupied molecular orbital to lowest unoccupied molecular orbital transition. From a comparison of the calculated deformation of negatively charged PTCDA^- with Raman spectra recorded in the region of the charge-transfer exciton, a significant net charge separation during the optical absorption is highly unlikely.
Scholz et al. (Mon,) studied this question.