Charge Exchange Spectra of Hydrogenic and He-like Iron Page: 6 of 25
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ionization potential) as
n ~ I 1/2 (l-1/2
nmax ~ CI g \1+ , q (1)
where q is the ion charge, In is the ionization potential of the neutral species, and IH is
the ionization potential of atomic H (13.6 eV). For Fe26+ and Fe25+ colliding with H at
low energies, nmax is therefore expected to be -12. Molecular nitrogen has an ionization
potential of 15.6 eV (14.5 eV for atomic N), so nmax for CX with N2 is nearly the same as
with H. (The second ionization potentials of N and N2 are roughly 30 eV, so the transfer level
for the second electron in double-transfer is lower, nmax - 8.) At low collision energies, the
n-distribution has a fairly sharp maximum, but gradually broadens to its widest at - 25q0-5
keV amu-1. At even higher energies, nmax slowly decreases and the distribution narrows
again (Ryufuku & Watanabe 1979).
The angular momentum (l) distribution varies more strongly with collision energy. The
details of this energy dependence are important because they affect how the excited ion can
radiatively decay, e.g., directly to ground if initial - lgrund = +1, or via cascades for large
values of initial l. The l distribution is especially important in the CX of fully stripped
ions, which yields excited hydrogenic ions. For example, if the initial excited level is an
11p state, it can decay directly to the is ground state yielding a Lyr, photon. If the ion
starts from an s, d, f, g, or other state, however, it cannot decay to ground because of the
Al = +1 selection rule. Instead, the ion is likely to end up decaying along the "yrast chain"
in sequential Al = An = -1 steps with l = n - 1 (- - - 4f - 3d - 2p - 1s), ultimately
resulting in Lya emission.
At low collision energies, low-i states are most likely to be populated (Ryufuku & Watan-
abe 1979) and the combined intensity of high-n lines (n > 3 - 1) may exceed that of Lya
(Beiersdorfer et al. 2000). As energy increases, however, the l distribution becomes more
statistical in nature (in proportion to 2l + 1) and fewer of the initial states can decay directly
to ground, resulting in a higher fraction of Lya emission. The hardness ratio of high-n versus
Lya emission can thus be used as a diagnostic of collision energy, as illustrated for O VIII
and Ne X by Beiersdorfer et al. (2001).
At the higher energies of relevance for cosmic-ray CX (- 100 keV amu-1) only a few
percent of the X-ray emission is from high-n states. The absence of significant high-n Fe lines
in observations of diffuse emission from the Galactic Ridge and Galactic Center therefore
does not necessarily indicate the absence of cosmic-ray CX emission (cf. Masai et al. (2002)).
Enhanced high-n emission is expected, however, when collision energies are low, e.g., for
highly charged thermal ions (with collision energies < 1 keV amu-1 even for kT ~ tens of
keV). Such a situation may occur in some locations in the Galactic Center, with relatively
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Wargelin, B J; Beiersdorfer, P; Neill, P A; Olson, R E & Scofield, J H. Charge Exchange Spectra of Hydrogenic and He-like Iron, article, April 27, 2005; Livermore, California. (https://digital.library.unt.edu/ark:/67531/metadc879558/m1/6/: accessed May 25, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.