EGAS41 - Swansea University

41 st EGAS CP 117 Gdańsk 2009
Anticrossing spectroscopy of He atoms excited by
10-30 keV He + and He impact
E. Baszanowska 1,2 , R. Drozdowski 1,∗ P. Kamiński 1 , G. von Oppen 3
1 Institute of Experimental Physics, University of Gdansk, ul. Wita Stwosza 57, 80-952
Gdansk, Poland
2 Department of Physics, Gdynia Maritime University, ul. Morska 81-87, 81-225 Gdynia,
Poland
3 Technische Universitat Berlin, Hardenbergstr. 36, D - 110623 Berlin, Germany
∗ Corresponding author: fizrd@univ.gda.pl
The anticrossing spectroscopy has been used to determine the post-collisional states of He
atoms excited by He + - ion impact for a wide energy range of projectiles [1]. It was shown,
that for the intermediate energy, where the velocity of the projectiles is comparable with
the Bohr velocity v B , saddle dynamics [2] and electron promotion based on the atomic Paul
trap mechanism [3] are suitable to describe this excitation process. The post-collisional He
I states are coherent superpositions of spherical states with large electric dipole moments.
These superpositions states vary from purely parabolic states for the projectile velocity
vp ≈ 0.6 v B (30 keV) to superpositions of l ≤ 2 states for vp ≈ 1.7 v B (300 keV).
In the present investigations we analyzed the excitation of He atoms by He + - ions
and He-atoms impact for lower projectiles velocities 0.3 v B – 0.6 v B (10 – 30 keV). In this
energy range single-electron excitation cross-sections of the 1s4l and 1s5l states change
rapidly from near to minimum, to maximum at 35 keV. Therefore the anticrossing spectra
of the spectral lines λ(1s4l-1s2p 3 P)=447 nm and λ(1s5l-1s2p 3 P)=402.6 nm have been
measured for both He + -ion and He - atom projectiles for several energies. In both cases
the target and projectile atoms can be excited to the singlet states by direct excitation and
to the triplet states by electron exchange. However, the evolution of He - He system with
four equivalent electrons is more complex than the evolution of He + - He system, where
one electron is promoted on two-centre potential of the He + - ions. In the measurements
the interference filters with 2 nm half widths were used to fully separate the recorded
spectral lines. It allows us to use the distributions of the amplitudes of the anticrossing
peaks as well the shapes of the recorded anticrossing signal to determine post-collisional
states of the target atoms.
Our measurements show, that these states possess electric dipole moments, that is, the
charge distributions of the electronic clouds are asymmetric. This asymmetry is mainly
due to coherent population of the l ≤ 2 states.
Acknowledgment
This work was supported by the BW grant: 5200-5-0160-9. E.B. gratefully acknowledges additional
support from Gdynia Maritime University grant 980/BW/GU/2009.
References
[1] M. Busch, R. Drozdowski, Th. Ludwig, G. von Oppen, J. Phys. B 37, 2903 (2004)
[2] J. M. Rost, J.S. Briggs, J. Phys. B 24, 4293 (1991)
[3] G. von Oppen, Europhys. Lett. 27, 279 (1994)
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