Electron excitation cross sections for the 2 s
2
2 p
2
P
o
\2 s 2 p
24
P and 2 s 2 p
22
D transitions in O
3 ¿
S. J. Smith,
1
J. A. Lozano,
1
S. S. Tayal,
2
and A. Chutjian
1
1
Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California91109, USA
2
Department of Physics, Clark Atlanta University, Atlanta, Georgia 30314, USA
Received 2 September 2003; published 24 December 2003
Experimental and theoretical electron-excitation cross sections are reported for the 2 s
2
2 p
2
P
o
→2 s 2 p
24
P and →2 s 2 p
22
D transitions in O
3+
. The transition energies are centered at 8.85 and 15.7 eV,
respectively. Measurements are made using the energy-loss merged-beams method. The center-of-mass energy
range covered is 6.1 below threshold to 19.3 eV. Comparison is made with results of new calculations in a
25-state R -matrix theory and with results in a previous eight-state R -matrix calculation. The presence of rich
resonance structure is confirmed in both experiment and theory. In most cases there is good agreement in the
magnitude of cross section, and energy location of the resonances.
DOI: 10.1103/PhysRevA.68.062708 PACS numbers: 34.80.Kw
I. INTRODUCTION
Optical emissions from highly charged ions provide im-
portant diagnostics of both electron temperature and density
in high-electron temperature plasmas. Such plasmas are en-
countered in solar and stellar atmospheres, shock-heated por-
tions of circumstellar clouds and the interstellar medium, and
in planetary magnetospheres such as the Io torus. Highly
charged ions HCI’s including carbon, nitrogen, oxygen,
nickel and iron ions are also present in fusion plasmas as
impurities sputtered into the plasma from plasma-wall colli-
sions. In all cases, knowledge of excitation cross sections are
required to understand the plasma density and temperature,
and to model the cooling rate of the plasma from radiation.
Line intensity ratios for the O
3 +
intercombination multip-
lets near 1400 Å have been used as electron density-sensitive
diagnostics for analysis of solar and stellar transition regions
1–3, novae 4, and quasars 5. The O
3 +
ground-state fine
structure splitting at 25.91 m has also been used as part of
an infrared line intensity ratio combined with an emission at
a nearby wavelength from another species, such as Ne
+
) to
search for active-galactic nuclei 6. All upper levels,
whether excited in a star or in cooler interstellar regions, are
populated predominantly by electron impact. A summary of
the O
3 +
energy levels relevant to this work is given in Fig. 1.
Theoretical studies in the O
3 +
system have included cal-
culation of energy levels and collision strengths using the
close coupling R -matrix theory 7–9, the calculation of en-
ergy levels and optical oscillator strengths using a multicon-
figuration Hartree-Fock theory 10, and a close-coupling
R -matrix approach 11.
II. EXPERIMENTAL DETAILS
Measurements of absolute cross sections were carried out
at the JPL Highly Charged Ion Facility using a 14.0 GHz
electron-cyclotron resonance ion source ECRIS and
merged-beams system. The experimental details have been
previously given in Greenwood et al. 12 and Chutjian et al.
13. The arrangement of the facility’s beam lines may be
found in Smith et al. 14. Briefly, a beam of O
3 +
ions is
derived from the ECRIS at an extraction voltage of 7.0 kV,
or at a beam energy of 7.0q keV=21.0 keV. Molecular oxy-
gen was used as the feed gas, with no other supporting gas
used. The O
3 +
ions are mass-charge analyzed in a double-
focusing 90° bending magnet. The beam is focused into the
center of the interaction region through a series of baffled
differential-pumping regions and lens systems. The O
3 +
beam is merged with a magnetically confined electron beam,
whereby the electrons are trochoidally deflected onto the ion
beam. The electrons and ions interact along the 20.0
0.3 cm merged path length. After the interaction, the con-
fined inelastically scattered electrons are then demerged from
the ions using a second trochoidal analyzer. Detection is by
means of a position sensitive detector at the exit of the sec-
ond trochoidal system. Beam-overlap profiles are measured
with both beams running, and at four locations along the
merged path using vanes with circular holes that intersect the
FIG. 1. Energy-level diagram of O
3+
indicating the fine-
structure wavelengths, including the ground-state transition at 25.91
m useful in galactic infrared line searches. The excitations
2 s
2
2 p
2
P
o
→2 s 2 p
24
P and 2 s
2
2 p
2
P
o
→2 s 2 p
22
D are studied
herein.
PHYSICAL REVIEW A 68, 062708 2003
1050-2947/2003/686/0627085/$20.00 ©2003 The American Physical Society 68 062708-1