Ionization and Recombination with Electrons:
Laboratory Measurements and Observational
Consequences
Daniel Wolf Savin
Columbia Astrophysics Laboratory

Collaborators
Warit Mitthumsiri, Michael Schnell – Columbia University
Mark Bannister – Oak Ridge National Lab (ORNL)
Martin Laming, Enrico Landi – Naval Research Laboratory
Andreas Wolf – Max Planck Institute for Nuclear Physics
Alfred M üller, Stefan Schippers – University of Giessen

Outline
I.
Motivation
II.
Types of Cosmic Plasmas
III.
Electron Impact Ionization (EII)
IV.
Dielectronic Recombination (DR)
V.
Future Needs

Spectra observations can be used to infer properties of the cosmos.
The aim of laboratory astrophysics is to reduce atomic physics uncertainties so that discrepancies between spectral observations and models tells us something about the properties of the observed sources and cannot be attributed to errors in the atomic data used in the models.

Ionization balance calculations are used to infer the properties of cosmic objects.
For example, to infer relative abundances, we note that
I line
 n q n
A n
A n
H
 line
.
Rewriting this gives n
A n
H


I line line n q n
A
.
Clearly, accurate ionization and recombination data are needed for reliable ionization balance calculations to get reliable relative abundances.

Outline
I.
Motivation
II.
Types of Cosmic Plasmas
III.
Electron Impact Ionization (EII)
IV.
Dielectronic Recombination (DR)
V.
Future Needs

Cosmic plasmas can be divided into two broad classes:
Collisionally-ionized
(stars, galaxies,...)
• Ionization due to electrons.
• In equilibrium an ion forms at T e
~ I p
/2.
• High T e
DR dominant recombination process.
Photoionized (PNe,
IGM, XRBs, AGN,…)
• Ionization due to photons and resulting electrons.
• In equilibrium an ion forms at T e
~ I p
/20.
• Low T e
DR dominant recombination process.

Outline
I.
Motivation
II.
Types of Cosmic Plasmas
III.
Electron Impact Ionization (EII)
IV.
Dielectronic Recombination (DR)
V.
Future Needs

Electron impact ionization (EII) e + O 7+ → e + e + O 8+
EII requires E k
> E b
.

Published recommended EII rate coefficients have yet to converge.

Errors in EII data translate directly into errors in predicted line ratios.
In collisional ionization equilibrium (CIE) we have n e n q
C q
 n e n q

1
 q

1
.
Rewriting gives n q

1 n q

C q
 q

1
.
Errors in either the ionization or recombination data will affect predicted or interpreted line ratios involving ions q and q+1.

We are carrying out a series of new EII measurements at ORNL.
Ionization data can be collected for collision energies
3-2000 eV.
(Bannister 1996, Phys. Rev. A 54, 1435)

We have carried out preliminary measurements for EII of Be-like C 2+ → C 3+
Ground-state (2s 2 1 S
0
)
IP = 47.89 eV
Metastable (2s2p 3 P)
IP = 41.39 eV
Lifetime = 9.7 ms (J=1)
≥ 200s (J=0,2)

Initial C 2+ EII measurements are discrepant with theory.
Arrows indicate threshold for metastable and ground-state
C 2+ .
Metastable fraction inferred by comparing electron impact excitation data (using same ion source) to theory.
Curve shows configurationaverage distorted-wave theory for our mixed state ion beam.

Extracted ground state cross section is a factor of 2 smaller than published theory.
Lotz formula used for energy dependence of EII cross sections σ
G and σ
M
.
Fit to lab data gives σ
G
σ
M
(solid curves).
and
Also shown are distorted wave theory (dashed curve,
Younger, 1981) and the recommended data (dash-dot curve, Bell et al., 1983)

Outline
I.
Motivation
II.
Types of Cosmic Plasmas
III.
Electron Impact Ionization (EII)
IV.
Dielectronic Recombination (DR)
V.
Future Needs

Dielectronic Recombination (DR) e + Fe 23+ ↔ (Fe 22+ )** → (Fe 22+ )* + h n
Energy conservation requires ΔE = E k
Both ΔE and E b quantized

E k
+ E quantized.
b
.
Low temperature DR occurs for E k
<< ΔE .
High temperature DR occurs for E k
~ ΔE .

DR theory for L- and M-shell ions are theoretically and computationally challenging.
Until recently modelers have had few modern calculations to use.
Comparisons show these data to have factor of 2 or more uncertainties.
(Savin et al. 2002, ApJ, 576, 1098)

In photoionized gas DR uncertainties affect predicted temperature and gas stability.
Using XSTAR and varying the low T e
DR data for Fe 17+ to Fe 23+ by a factor of 2.
Temperature
Phase diagram
Line emission seen from ions predicted to form in region of thermal instability.
(Savin et al. 1999, ApJS, 123, 687)

In electron-ionized plasmas DR errors affect predicted relative abundances.
Using older DR data inferred relative abundances in the solar corona can be a factor of 5 smaller or
1.6 times larger.
Line Ratio
Mg VI/Ne VI
Mg VII/Ne VII
Mg IX/S IX
Mg IX/S X
Si IX/S IX
Si IX/S X
Si X/S X
Variation
Minimum Maximum
0.60
1.11
0.67
0.33
0.51
0.20
0.36
1.22
1.29
1.64
1.01
1.14
0.43
1.60
(Savin & Laming 2002, ApJ, 566, 1166)

We are carrying out a series of DR measurements using the Test Storage Ring (TSR).

Schematic of the electron cooler

Measurements can be carried out for low and high temperature DR.
DR of O-like Fe XIX forming F-like Fe XVIII
(Savin et al. 1999, ApJS, 123, 687; 2001, ApJ, 576, 1098)

We can use these data to produce Maxwellian rate coefficients for plasma modeling.
Pre-experiment Post-experiment
Measurements are used to benchmark modern DR theory which is then used to calculate DR for other ions in the tested isoelectronic sequence.

Even with benchmarking modern DR theory has still not converged for all L-shell ions

Current AGN spectral models over-predict the ionization stages of M-shell iron ions.
Models that match spectral features from abundant 2nd row elements, over-predict the average Fe charge stage.
This is believed to be due to the absence of low T e
DR data for Mshell Fe (Kraemer et al. 2004; Netzer 2004).
(Netzer et al. 2003, ApJ, 599, 933)

Published laboratory work supports that poor Fe M-shell DR data is the cause.
Published DR data were for tokamaks, stars, etc. and did not attempt to treat properly the low energy DR resonances.
This is an example of how better communication between atomic physics and astrophysics could have predicted this problem
DR of Fe XVI forming Fe XV
(Müller 1999, Int. J. Mass Spectrom. 192, 9)

We are carrying out further M-shell Fe DR measurements to address this issue.
DR of Fe XV forming Fe XIV

Conclusions
• Significant errors exist in EII data base.
• Much experimental and theoretical EII work needs to be done.
• L-shell DR data has improved recently but room remains for theoretical improvement.
• More L-shell benchmark DR measurements needed.
• Lots of experimental and theoretical work is needed to improve the M-shell DR data.
• More accurate structure calculations are needed for low-lying autoionization levels.

We have added a beam attenuation cell to determine directly the metastable fraction.
If the electron capture cross section for metastable and ground state ions differ significantly, then one state will be lost first as the target gas density increases.
Plot of the log of ion current vs. target gas density is bilinear (slopes proportional to capture cross sections) and can be used to infer relative populations of ground-state and metastable ions.
(Zuo et al. 1995, ApJ, 440, 421)

These DR uncertaintes also affect predicted line emission.
XSTAR spectra for gas at log( ζ)=2.1 erg cm s -1
(Savin et al. 2000, AIP CP547, 267)

In CIE, errors in DR data translate directly into errors in predicted line ratios.
In CIE we have n e n q
C q
 n e n q

1
 q

1
.
Rewriting gives n q

1 n q

C q
 q

1
.
Errors in either the ionization or recombination data will affect predicted or interpreted line ratios involving ions q and q+1.

Theory has also has a problem with high charge states for L-shell ions.

Recent AGN Observations have indicated the importance of Fe M-shell DR.
A new AGN spectral feature at λ ≈ 16-17 Å has been identified as being due to absorption in M-shell iron ions.
(Sako et al. 2001, A&A, 365, L168)