Regensburg 2007 – wissenschaftliches Programm
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HL: Fachverband Halbleiterphysik
HL 10: Organic semiconductors
HL 10.5: Vortrag
Montag, 26. März 2007, 15:45–16:00, H14
Resonant Raman spectra of pentacene in the UV range — •L. Gisslen1, C. Himcinschi2, J. Pflaum3, and R. Scholz1 — 1Walter Schottky Institut, Technische Universität München, Germany — 2Max-Planck-Institut für Mikrostrukturphysik, Halle, Germany — 33. Physikalisches Institut, Universität Stuttgart, Germany
Pentacene has two prominent absorption bands, one in the visible and one in the near UV, the first being based on the HOMO-LUMO transition, the second on an interference between the transitions HOMO→LUMO+2 and HOMO-2→LUMO. In order to investigate the deformation in the relaxed excited state related to the transition in the UV, we have performed resonant Raman spectroscopy in that region. The experimental observation of the Raman spectra from a pentacene crystal was realised using a LabRam HR800 UV spectrometer in a micro-configuration. Light from a He-Cd laser at a wavelength of 325 nm was focused with the microscope, resulting in a beam diameter of ≈ 1.5 µm on the sample, with a laser power of ≈ 0.5 mW measured under the objective of the microscope. Resonant Raman spectra were recorded in the Stokes region between 200 cm−1 and 2000 cm−1. The intensity distribution differs strongly from Raman data obtained in resonance with the HOMO-LUMO transition, revealing different deformation patterns. The latter are determined by geometry optimisation of the excited states with time-dependent density function theory (DFT). For both transitions, the measured intensity distributions are well reproduced by the calculations.