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

Fachbereich Physik, Theoretische Physik, Universita¨t Gesamthochschule Paderborn, D-33098 Paderborn, Germany~Received 6 July 1999; revised manuscript received 7 December 1999!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 areinterpreted with a density-functional tight-binding scheme used for calculating the geometry and vibrationalmodes of the isolated molecule. These results for the breathing modes are applied to the optical absorption ofPTCDA in solution, and the vibrational absorption bands can be related quantitatively to the elongation of themost prominent Raman modes during the highest occupied molecular orbital to lowest unoccupied molecularorbital transition. From a comparison of the calculated deformation of negatively charged PTCDA