Max-Planck-Institut für Kernphysik Heidelberg

Collisions of electrons and positrons with atoms and molecules

Priv.-Doz. Dr. Alexander Dorn and Dr. Claus Dieter Schröter

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Experiments using laser trapped lithium at the Free Electron Laser in Hamburg (FLASH)

Free Electron Lasers (FELs) such as the Free Electron Laser in Hamburg (FLASH) provide short light pulses with ultrahigh power in the order of 1 Gigawatt in the vacuum ultraviolet (VUV) wavelength regime. These light sources now enable us for the first time to explore reactions where atoms simultaneously absorb two or more energetic photons and emit one or several electrons. The underlying basic non-linear few-photon-few-electron quantum dynamics can be explored in its most fundamental form for simple atomic targets for which theoretical approaches have just become feasible.

Lithium atoms are an ideal target for such studies since they are fairly simple containing just three electrons. Nevertheless two different electronic shells are occupied enabling various reactions and even state preparation using optical lasers.

For such experiments we have used a new apparatus the MOT-reaction microscope, MOTREMI which is shown in this figure 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.

img/ecollMOT 1.jpg
img/ecollBild 6.jpg

In this figure some results of our experiments are illustrated. For a VUV-photon energy of 50 eV (λ = 25 nm) the following reactions were observed: K-shell single ionization by absorption of two photons (a). The emitted electron has 35 eV kinetic energy corresponding to 1.6 atomic units of momentum. Due to momentum conservation the residual lithium ion which is detected in the spectrometer carries the same amount of momentum but directed to the opposite direction. Thus, its momentum distribution projected onto the Pz-Px plane which is shown in (c) is identical to the respective electron emission pattern. The angular distribution with respect to the light polarization E with four maxima corresponds to a d-angular momentum wave.

Via electron correlation or photon sharing the 2s valence electron can also be ejected giving rise to doubly charged Li2+(1s) ions. Finally, double ionization can be induced by two-photon absorption in the K-shell followed by single-photon absorption of the 2s-electron (b). Altogether three photons are absorbed and two electrons are emitted (spectra not shown here).

In a second experiment a photon energy of 59 eV (λ = 21 nm) was chosen which allows a resonant 1s -> 2p excitation of a K-shell electron. Then the absorption of further photons by both outer shell electrons gives rise to single and double ionization. The angular emission pattern for single ionization after 2p-ejection (scheme (d)) is displayed in (g). It exhibits a strongly modified angular momentum behaviour compared to non-resonant two-photon ionization (c). Again double ionization, e.g. emission of both valence electrons by two-photon and three-photon absorption is observed (not shown here).

Altogether, a number of resonant and non-resonant, basic two- and three-photon, single and double ionization reactions have been observed for the first time in a three-electron atom, lithium, at two different photon energies. Theoretical calculations have been initiated in order to obtain a quantitative understanding of the underlying few-photon few-electron dynamics. Future higher FLASH intensities will facilitate studies on non-linear reactions involving all three electrons in lithium.

Previous beamtime at 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).

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

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.

img/FLASH results.jpg

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.

Related publications

  • A. Rudenko et al., Phys. Rev. Lett. 101, 073003 (2008).
  • M. Kurka et al., New Journal of Physics 12, 073035 (2010).
  • A. Rudenko et al., J. Phys B 43, 194004 (2010).
  • G. Zhu et al., Phys. Rev. Lett. 103, 103008 (2009).
  • M. Schuricke et al., Phys. Rev. A 83, 023413 (2011).

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

img/Hall 1.jpg
img/Hall 2.jpg

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

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