Max-Planck-Institut für Kernphysik Heidelberg


Atoms and molecules in ultra-short laser pulses

Priv.-Doz. Dr. Robert Moshammer

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A New Lithium Magneto-Optical Trap Target for the Free Electron Laser at DESY/Hamburg (FLASH)

In April 2008 we had a first beam time at FLASH where photo-ionization of magneto-optically trapped and cooled lithium atoms by FEL radiation was investigated. The idea was to laser-excite the outer 2s-valence electron and thereby align the orbital spatially in the first step and to ionize one inner-shell 1s-electron by absorption of a FEL-photon in the second step. The emitted photo-electron has a certain chance to collide and thereby to excite or knock-out the outer electron while leaving the atom. This interaction should depend on the alignment, i.e. the charge cloud distribution of the laser excited state with respect to the photo-electron emission direction. This was observed using a new apparatus in which for the first time a magneto-optical trap was combined with a recoil-ion and electron momentum spectrometer (Reaction Microscope).

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Double ionization of a lithium atom after absorbtion of one energetic FEL photon (Eg > 81 eV).

In this figure the experimental apparatus is shown schematically. Lithium atoms are trapped and cooled (T 1 mK) in the crossing point of six pairwise counter-propagating laser beams (red) and ionized by an intense pulsed photon beam (blue). Charged fragments are momentum analyzed by the Reaction Microscope.

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In the diagram below first, preliminary results are shown for K-shell ionization of ground state lithium atoms by 90 eV FLASH photons.

The ion time-of-flight (TOF) allows to separate different charge states, i.e. single and double ionization and furthermore contains the momentum information along the FEL polarization. The detection position (x, y) contains the transversal momentum information.

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Here different ionization mechanisms giving rise to the observed singly and doubly charged ion momentum distributions are depicted.

The main double ionization pathway in our experiment is a collision of the electron which absorbed the photon and leaves the atom with the outer electron.

Different single ionization reactions. Left: ionization of the outer 2s electron with rather low cross section and resulting in a high recoil ion momentum. Middle: K-shell ionization with rather large cross section and smaller ion momentum. Right: K-shell ionization with subsequent excitation of the 2s electron.

Some pictures from our beam-time at FLASH / DESY in Hamburg during April 2008.

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Alexander Dorn

Priv.-Doz. Dr. Alexander Dorn

Electron collisions and Photoionization of Lithium

Room: Bo. 261b
Tel.: +49 (0) 6221 516 - 513
Email: alexander.dorn@please delete this@mpi-hd.mpg.de


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