Charge Exchange Spectra of Hydrogenic and He-like Iron Page: 7 of 25
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narrow CX lines (Doppler widths of order 10 eV-see 1) arising from highly ionized thermal
plasma as it interacts with neutral gas on the boundaries of dense molecular clouds.
The hardness ratio of emission from He-like ions is much less sensitive to collision energy
because the n = 2 - 1 line (Ka) always dominates. From simple spin statistics, following
electron transfer a He-like ion will have total spin S = 1 about 3/4 of the time, and S = 0
only 1/4 of the time. Since only AS = 0 transitions are allowed, none of the high-n S = 1
(triplet) states can decay to the S = 0 (singlet) 'So ground state, and instead the excited
electron cascades to one of the n = 2 triplet states from which it ultimately decays via a
forbidden or semi-permitted transition.
Within the n = 2 level, the triplet 3P2,1 and 3S1 states that give rise to the "inter-
combination" and "forbidden" lines, respectively, receive much more of the cascade-derived
population than the singlet 1P1 state that yields the "resonance" line. The triplet lines are
therefore much stronger relative to the resonance line in CX spectra than they are in thermal
plasmas. (See recent measurements by Beiersdorfer et al. (2003) and theoretical predictions
by Kharchenko et al. (2003).) Given adequate energy resolution, this is an excellent indi-
cator of CX emission, regardless of ion-neutral collision energy. As we illustrate in 5.1,
however, even if the Ka lines are instrumentally blended one may still be able to use the
energy centroid of the blend to distinguish between CX and thermal emission.
3. EXPERIMENTAL METHOD
Our experiment used the Lawrence Livermore National Laboratory (LLNL) EBIT-II
electron beam ion trap to collect Fe XXVI and Fe XXV CX spectra using N2 as the neutral
gas. The operation of EBITs has been described extensively elsewhere (Levine et al. 1988)
as has the magnetic trapping mode (Beiersdorfer et al. 1996a) used for these measurements.
To briefly summarize, Fe ions are injected into the EBIT-II trap region from a metal vapor
vacuum arc where they are further ionized and trapped, longitudinally by an electrostatic
potential and radially by a 3-T magnetic field, as well as by electrostatic attraction of the
narrow electron beam.
The neutral gas is injected directly into the trap where some N2 molecules CX with the
trapped Fe ions before being ionized and dissociated themselves. Although CX cross sections
are much larger than those for electron impact excitation (of order 10-14 cm2 versus 10-21
cm2 in this case), the relevant densities and collision velocities are much smaller for CX than
for electron-ion collisions, and the net rate of detected CX emission is less than 1% of that
from electron excitation. CX spectra are therefore collected in the magnetic trapping mode
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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/m1/7/: accessed May 24, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.