doi:10.1016/S0168-583X(03)00949-2 
Copyright © 2003 Elsevier Science B.V. All rights reserved.
Cascade transition X-rays from electron capture into highly charged ions in collisions with neutral gas targets
a NIST, Gaithersburg, MD 20899-8421, USA
b Debrecen University, Bem tér 18/a, H-4026, Debrecen, Hungary
c Institute of Nuclear Research of the Hungarian Academy of Sciences (ATOMKI), Bem tér 18/c, H-4026, Debrecen, Hungary
Available online 21 February 2003.
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Abstract
X-rays originating from a series of the cascades after electron capture into highly excited Rydberg states have been observed from low energy, highly charged Krq+ ions (q=27–36) colliding with neutral Ar atoms. The intensity ratio between L (n=3→2) X-rays and the sum of M X-rays (n=4→3, n=5→3, n=6→3, etc.) is drastically changed from Kr27+ to Kr28+ and constant for higher ion charge states (q=29–36). This feature can be understood to be due to the metastable states formed during cascades after electron capture into Kr27+ ions. This is also supported by time-dependent population calculations.
Author Keywords: X-rays; Charge exchange; Electron capture; Highly charged ion
PACS classification codes: 52.70.2.m; 32.30.Rj; 34.50.Fa; 34.80.Dp
Fig. 1. L and M X-rays from collisions of Krq+ ions (q=27–36) with neutral argon atoms. The strong line around 2 keV corresponds to n=3 to 2 transitions, L (n=3→2). Higher n transitions to the n=2 level (L (n=4→2), L (n=5→2), etc.) can be seen on the high-energy side of the L (n=3→2) line. The strong line below 1 keV is made up the M (n=4→3) transitions. Here also the higher energy structure corresponds to higher n transitions to the same n=3 level as of the M (n=4→3).
Fig. 2. CTMC calculation of the (n,l) distribution of the levels where the initial electron capture takes place in collisions of Krq+ ions with neutral argon atoms. Going from Kr27+ to Kr35+ the initial n state smoothly shifts from about 12 to 15. At the same time the most probable l quantum number shifts 10 to about 11.
Fig. 3. Ratio of the different L line intensities relative to the sum of all the M transitions. The ratio of the L (n=3→2) line to the sum of the M lines is close to 1 except for Kr27+ where the intensity ratio is about 0.5. In almost all cases the L (n=4→2) line is about the same intensity as the sum of all the higher n L transitions (L (n=5→2), L (n=6→2), etc).
Fig. 4. Calculation of the time dependence of the relative level populations in Kr28+ and Kr27+. The metastability of the 2s22p53s 3P2 state formed in collisions of Kr27+ explains the drop in the L (n=3→2) to M line intensity.
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