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A: Fachverband Atomphysik

A 26: Posters: BECs, ultracold gases and plasmas

A 26.3: Poster

Thursday, March 13, 2008, 16:30–18:30, Poster C3

Observations and Simulation of the evaporation process in an optical dipole trap — •Christoph Käfer¹, Thomas Boll¹, Riad Bourouis^{1, 2}, Jürgen Eurisch¹, and Hanspeter Helm¹ — ¹Department of Molecular and Optical Physics, Stefan-Meier-Str. 19, 79104 Freiburg — ²Institut für Angewandte Physik, Universität Bonn

We investigate the temporal development of atom number and atom temperature in an optical dipole trap which is established by a focused CO₂ laser. At full power the trap frequencies in radial and longitudinal direction are v_r= 2.4 kHz and v_z= 160 Hz. When forcing the evaporation process by lowering the laser power, these trap frequencies and the elastic scattering rate decrease and spilling of atoms prior to rethermalization is playing an ever more important role as the evaporation process proceeds. Also the effect of gravity becomes a crucial factor at the end of the evaporation process. To our knowledge no published models account for all these parameters, and they cannot quantitatively explain the observations. We present a numerical model which can quantitatively account for all these parameters. In our model, the atoms are distributed according to a chopped Maxwell-Boltzmann distribution, where atoms with a kinetic energies above the potential depth are removed from the sample. After calculating the redistributed kinetic energy of two randomly chosen particles after scattering, the energy distribution of the entire sample can be updated and chopped again. In this fashion natural evaporation and forced evaporation can be traced out numerically. We compare predictions of this recursive model to our experimental result.