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MO: Fachverband Molekülphysik
MO 18: Femtosekundenspektroskopie
MO 18.3: Poster
Dienstag, 11. März 2008, 14:00–19:00, Poster C1
Influence of environment on rotational wave packets in gases — •Jochen Maurer, Nina Owschimikow, Falk Königsmann, and Nikolaus Schwentner — Institut für Experimentalphysik, Freie Universität Berlin, Arnimallee 14, 14195 Berlin
A linearly polarized short laser pulse interacts with a molecule with anisotropic polarizability via non-resonant Raman transitions and induces recurrent transient alignment of the molecular axis along its electric field direction. This rotational wave packet leads to a time-dependent change in birefringence of the sample which we detect using optical Kerr effect spectroscopy (OKE). We use a Ti:Sa laser (wavelength 780 nm, pulse length 150 fs) that is focused to an intensity of up to 1014 W/cm2 to create alignment of N2 molecules in the gas phase. Our experiments cover pressures ranging from few mbar to 5 bar and temperatures from 80 K to 300 K. We find a quadratic increase in signal intensity with density characteristic for coherent superpositions. In addition to this effect the signal amplitude grows in accordance with theoretical predictions on the susceptibility χ(2), as with decreasing temperature the thermal occupation of rotational levels is narrowed. The homodyne detected signal is proportional to the square of the degree of alignment <cos(θ)2>2 and decays with time as the wave packet dephases due to bimolecular collisions. We study the temporal evolution of the degree of alignment depending on pressure in pure N2 and mixtures of N2 with the noble gases He, Ar and Kr. From the rate of decay of revivals we extract interaction radii for dephasing and find that dephasing is mainly due to inelastic collisions.