Osnabrück 2002 – wissenschaftliches Programm

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MO: Molekülphysik

MO 19: Poster II (Energietransfer, Reaktionen, Photoionisation, Femtosekundenspektroskopie, ultrakurze Pulse)

MO 19.12: Poster

Donnerstag, 7. März 2002, 17:30–19:30, Schloss

Non-Resonant anti-Stokes Raman Scattering detected after Ultrafast Internal Conversion — •Qingrui An, Wolfgang Zinth, and Peter Gilch — Sektion Physik, Ludwig-Maximilians-Universität München, Oettingenstr. 67, D-80538 München

Excited vibrational modes of polyatomic molecules hold informations on the reaction pathway of preceding ultrafast processes like internal conversion or photochemistry. In principle, an ideal tool to monitor excited vibrations is anti-Stokes Raman scattering. Due to the weakness of Raman cross sections these experiments are usually performed under resonance conditions. Here we demonstrate the feasibility of non-resonant anti-Stokes scattering after ultrafast internal conversion.

In this experiment para-nitroaniline (pNA) which undergoes internal conversion in a few 100 fs was electronically excited by a blue ∼ 150 fs laser pulse. The excitation of the symmetric stretch mode of the NO₂ group of pNA at 1315 cm⁻¹ was monitored via anti-Stokes scattering induced by a suitable delayed probe pulse. This anti-Stokes signal rises with the decay of the excited state of pNA. At its maximum (∼ 0.8 ps) a pronounced shift to lower wavenumbers of ∼ 40 cm⁻¹ with respect to the Stokes position of the mode is observed. This shift relaxes in ∼ 1 ps. The overall decay of the vibrational excitation is somewhat slower, of the order of 3 ps. Possible mechanisms for this huge shift are discussed.