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Regensburg 2007 – scientific programme

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HL: Fachverband Halbleiterphysik

HL 10: Organic semiconductors

HL 10.5: Talk

Monday, March 26, 2007, 15:45–16:00, H14

Resonant Raman spectra of pentacene in the UV range — •L. Gisslen1, C. Himcinschi2, J. Pflaum3, and R. Scholz11Walter 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.

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