The general characteristics and spectrometric features of a high resolution four-crystal reflection x-ray monochromator with wavelength analysis installed at the HASYLAB beam line L at DESY are presented. The monochromator is part of a spectrometer developed to calibrate x-ray absorption edge spectra in the energy range of 6–36 keV with a relative uncertainty ΔE/E from 10−5 to 10−6. This requires an extremely effective suppression of harmonics and also a negligible instrumental influence in order to obtain almost intrinsic spectra. As the results show, the monochromator fulfills the requirements, including very high stability. An example of the calibration procedure for the copper K edge is given as well as a comparison of the remeasured absorption edge energies with the previously tabulated data.
Ultracold collisions between Cs atoms and Cs2 dimers in the electronic ground state are observed in an optically trapped gas of atoms and molecules. The Cs2 molecules are formed in the triplet ground state by cw photoassociation through the outer well of the 0-(g) (P3/2) excited electronic state. Inelastic atom-molecule collisions converting internal excitation into kinetic energy lead to a loss of Cs2 molecules from the dipole trap. Rate coefficients are determined for collisions involving Cs atoms in either the F=3 or F=4 hyperfine ground state, and Cs2 molecules in either highly vibrationally excited states (nu'=32-47) or in low vibrational states (nu'=4-6) of the a3 summation(u)+ triplet ground state. The rate coefficients beta approximately 10(-10) cm3/s are found to be largely independent of the vibrational and rotational excitation indicating unitary limited cross sections.
We simultaneously trap ultracold lithium and cesium atoms in an optical dipole trap formed by the focus of a CO2 laser and study the exchange of thermal energy between the gases. The optically cooled cesium gas efficiently decreases the temperature of the lithium gas through sympathetic cooling. Equilibrium temperatures down to 25 microK have been reached. The measured cross section for thermalizing 133Cs-7Li collisions is 8 x 10(-12) cm(2), for both species unpolarized in their lowest hyperfine ground state. Besides thermalization, we observe evaporation of lithium purely through elastic cesium-lithium collisions (sympathetic evaporation).
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