München 2004 – scientific programme

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MO: Molekülphysik

MO 13: Spektroskopie von Molekülen in He-Clustern

MO 13.1: Fachvortrag

Tuesday, March 23, 2004, 16:30–17:00, HS 355

Spectroscopy of Organic Molecules in Helium Nanodroplets — •Wolfgang E Ernst¹, Frank Ruediger Bruehl² und Shuo Jiang³ — ¹Institut fuer Experimentalphysik TU Graz — ²Physikalisch-Technische Bundesanstalt Berlin — ³Tsinghua University Beijing PRC

Helium nanodroplets of 0.4 K internal temperature are generated in a molecular beam experiment. They provide an almost non-interacting matrix for the study of molecular spectra at such low temperature. We doped large helium clusters with organic molecules such as pentacene to investigate the influence of helium on the spectral line shapes. The inhomogeneous contribution to the linewidth of about 12 GHz is due to the size distribution of helium clusters in the beam. Various porphyrin type molecules were embedded in helium and their relaxation after electronic excitation was measured. Regardless of the excitation wavelength, we observe emission emission only from the ground vibrational level of the S1 state. This demonstrates that the vibrational relaxation in the droplet is complete within the fluorescence lifetime. For zinc tetrabenzoporphine, four sets of excitation and emission spectra shifted by 30 to 50 cm-1 with respect to one another, seem to indicate the presence of four different conformers of this molecule in helium. In the spectra of free base tetraphenylporphine, the low frequency torsional modes are strongly broadened in helium.