Resonant Raman spectroscopy of 3,4,9,10-perylene-tetracarboxylic-dianhydride epitaxial films

We present a comprehensive investigation of Raman-active modes of 3,4,9,10- perylene-tetracarboxylicdianhydride (PTCDA) thin films deposited on H-passi vated Si(lll) substrates. The experimental data are interpreted with a dens ity-functional tight-binding scheme used for calculating the geometry and v ibrational modes of the isolated molecule. These results for the breathing modes are applied to the optical absorption of PTCDA in solution, and the v ibrational absorption bands can be related quantitatively to the elongation of the most prominent Raman modes during the highest occupied molecular or bital to lowest unoccupied molecular orbital transition. From a comparison of the calculated deformation of negatively charged PTCDA(-) with Raman spe ctra recorded in the region of the charge-transfer exciton, a significant n et charge separation during the optical absorption is highly unlikely.