EGAS41 - Swansea University

41 st EGAS CP 152 Gdańsk 2009
Large linear magnetic dichroism in laser–excited K atoms
S. Heinäsmäki 1,∗ , J. Schulz 2 , R. Sankari 3 , H. Aksela 1
1 Department of Physical Sciences, University of Oulu, P.O. BOX 3000, 90014 University of
Oulu, Finland
2 Centre for Free-Electron Laser Science, DESY, 22607 Hamburg, Germany
3 MAX–lab, Lund University, Box 118, SE-22100 Lund, Sweden
∗ Corresponding author: sami.heinasmaki@oulu.fi
Photoionization of polarized atoms opens up the possibility to study various phenomena
related to electron emission in the presence of selectively populated magnetic substates,
which under normal circumstances are degenerate [1]. These orientational features are
all the more important when various correlation (i.e. many–electron) effects are studied,
since these are often very sensitive to the symmetry properties of the electronic states.
Alkali atoms are especially suitable targets since they possess an s type electron outside
a nobel gas –like closed shell structure. Laser pumping can be used to excite this valence
electron into the corresponding p orbital, leading to orientation or alignment of the excited
atoms. Orientation can be achieved by using circularly polarized laser radiation, which
gives rise to uneven population of magnetic substates corresponding to ±|m|. Linearly
polarized laser radiation gives rise to alignment, where those magnetic substates are evenly
populated.
In this work we discuss the observation of large linear magnetic dichroism (LMD) of
the conjugated shakedown satellites in the 3p photoelectron spectrum of the 2 S 1/2 → 2 P 1/2
laser–excited potassium. The photoelectron spectra were recorded at the undulator beamline
I411 of the storage ring MAX II in Lund. In a previous work [2] these satellite lines
have been identified as originating from two–electron transitions 4p 1/2 → 4s, 3p 1/2,3/2 →
ǫp or 3p 1/2,3/2 → 4s, 4p 1/2 → ǫp. This leads to same final ionic states as in direct photoionization
but with a shift in energy corresponding to the initial laser excitation energy.
The LMD amounts to using left– and right handed circularly polarized light in the
laser excitation, and measuring the respective photoionization intensities, using linearly
polarized synchrotron light. The difference of the two spectra yields the LMD.
Within the traditional picture the LMD for these satellite lines should be very close
to zero, since it depends only on the difference of the phases of the continuum waves,
which are only of the p 1/2 and p 3/2 type here, thus having a very small phase difference.
A possible way to have nonzero LMD would involve large electron-electron interactions
in the laser-excited states, thus enabling other continuum channel through configuration
interaction effects. We therefore propose that the LMD gives an opportunity to study the
degree of electron correlation in the ionic states.
References
[1] S. Baier, A.N. Grum–Grzhimailo, N.M. Kabachnik, J. Phys. B: At. Mol. Opt. Phys.
27, 3363 (1994)
[2] J. Schulz, S. Heinäsmäki, R. Sankari, T. Rander, A. Lindblad, H. Bergersen, G.
Öhrwall, S. Svensson, E. Kukk, S. Aksela, H. Aksela, Phys. Rev. A 74, 012705-1 (2006)
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