Total and partial recombination cross sections for F
6
D. M. Mitnik and M. S. Pindzola
Department of Physics, Auburn University, Auburn, Alabama 36849
N. R. Badnell
Department of Physics and Applied Physics, University of Strathclyde, Glasgow G4 0NG, United Kingdom
Received 13 October 1998; revised manuscript received 6 January 1999
Total and partial recombination cross sections for F
6+
are calculated using close-coupling and distorted-
wave theory. For total cross sections, close-coupling and distorted-wave results, which include interference
between the radiative and dielectronic pathways, are found to be in good agreement with distorted-wave results
based on a sum of independent processes. Total cross sections near zero energy are dominated by contributions
from low-energy dielectronic recombination resonances. For partial cross sections, the close-coupling and
distorted-wave theories predict strong interference for recombination into the final recombined ground state
1 s
2
2 s
21
S
0
of F
5+
, but only weak interference for recombination into the levels of the 1 s
2
2 s 2 p configuration.
S1050-29479909205-7
PACS numbers: 34.80.Lx
I. INTRODUCTION
The free-bound spontaneous radiative emission process
plays an important role in the determination of the level
populations and ionization balance of high temperature non-
LTE laboratory and astrophysical plasmas. This process is
conventionally described as involving either direct nonreso-
nant radiative recombination RR, which is the inverse of
the ordinary nonresonant photoionization process, or two-
step resonant dielectronic recombination DR, which con-
sists of a radiationless electron capture accompanied by ex-
citation of the initial ion, to form a doubly excited
autoionizing state followed by a spontaneous radiatively sta-
bilizing transition to a bound state.
It has been pointed out in several scattering-theory inves-
tigations that the treatment of radiative and dielectronic re-
combination as two distinct, noninterfering processes is not
strictly permissible within the framework of a rigorous
quantum-mechanical theory; see, e.g., 1. Over the years a
number of theoretical approaches which unify the two re-
combination processes have been developed. A general per-
turbative projection operator approach 2–5 and nonpertur-
bative R-matrix methods 6 and a radiative optical potential
7,8 have provided computational approaches that can be
easily applied to any atomic ion.
Many experimental efforts using ion storage rings have
been made in searches for observational evidence of interfer-
ence effects in total recombination cross sections. The high
resolution achieved by these devices allows one to map out
resonance structures at the order of one-hundredth of one
electron volt 9. However, even under these conditions, the
interference effects in the total recombination remain elusive.
As has been pointed out by Badnell and Pindzola 5, the
largest DR-RR interference effects are in the weakest lines,
which are generally buried under the stronger resonances.
For most weak lines, interference effects are further sup-
pressed due to resonance decay to many final recombined
states.
There are cases in total recombination cross sections,
however, in which a weak resonance decays to only one
level. An example of this kind of resonance has been given
by Gorczyca et al. 10 for the Ar-like Sc
3 +
ion at low en-
ergies. Interference effects in the total recombination cross
section are produced by the unusual resonance structures as-
sociated with the 3 p
5
3 d
22
F terms in the recombined Sc
2 +
ion, which radiate almost exclusively to the ground levels
3 p
6
3 d
2
D . Schippers et al. 11 have investigated this re-
combination process, but the experiment does not allow for a
conclusive test of the theoretical predictions. As has been
pointed out by the authors, limitations on the accuracy of
both the calculations and the experimental measurements are
present for this ion.
Experimental efforts have also been made using ion traps
to search for observational evidence of interferences in re-
combination. Although the ion trap experiments measure a
mixture of ion stages, they have a decided advantage over
ion storage rings in that they can monitor the photon emis-
sion. This allows them to measure partial recombination
cross sections, that is, recombination to a particular final
state. However, the low-energy resolution of these devices
limits their application to very highly charged ions. The only
observation of RR-DR interference to date was reported by
Knapp et al. 12 in an ion trap experiment. For a mixture of
different ions (U
87+
to U
90+
), the KL
12
L
3
resonance mani-
fold with emission of a 100 keV photon exhibits a marked
asymmetry. The strength of the asymmetry was confirmed by
calculations of the KL
12
L
3
partial recombination correspond-
ing to the emission of a 100 keV photon in U
88+
ions 13.
In this paper we explore the possibility of finding inter-
ference effects in the total and partial recombination for low-
charged atomic ions in the well-studied Li isoelectronic se-
quence. We calculate the total and partial recombination of
F
6 +
, associated with the 1 s
2
2 pnl ( n =6,7) configurations in
the recombined F
5 +
ion. We confirm the general conclusion
that substantial interference effects are not seen in the total
recombination cross section. The most promising candidate
for an observation of interference effects in the partial re-
combination cross section are the processes in which the
PHYSICAL REVIEW A MAY 1999 VOLUME 59, NUMBER 5
PRA 59 1050-2947/99/595/35929/$15.00 3592 ©1999 The American Physical Society