Reciprocity in the electronic stopping of slow ions in matter |
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Authors: | P Sigmund |
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Institution: | (1) Department of Physics and Chemistry, University of Southern Denmark, 5230 Odense M, Denmark |
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Abstract: | The principle of reciprocity, i.e., the invariance of the
inelastic excitation in ion-atom collisions against interchange of
projectile and target, has been applied to the electronic stopping cross
section of low-velocity ions and tested empirically on
ion-target combinations supported by a more or less adequate amount of
experimental data. Reciprocity is well obeyed (within ~10%) for
many systems studied, and deviations exceeding ~20% are
exceptional. Systematic deviations such as gas-solid or metal-insulator
differences have been looked for but not identified on the present basis.
A direct consequence of reciprocity is the equivalence of Z1 with
Z2 structure for random slowing down. This feature is reasonably well
supported empirically
for ion-target combinations involving carbon, nitrogen, aluminium and
argon.
Reciprocity may be utilized as a criterion to reject questionable
experimental data. In cases where a certain
stopping cross section has not been or cannot be measured, the
stopping cross section for the inverted system may be available and
serve as a first estimate.
It is suggested to build in reciprocity as a fundamental requirement into
empirical interpolation schemes directed at the stopping of low-velocity
ions. Examination of the SRIM and MSTAR codes reveals cases where
reciprocity is obeyed accurately, but deviations of up to a factor of two
are common. In case of heavy ions such as gold, electronic stopping
cross sections predicted by SRIM are asserted to be almost an order of
magnitude too high. |
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Keywords: | 34 50 Bw Energy loss and stopping power 34 50 Fa Electronic excitation and ionization of atoms |
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