Quasi-free electron scattering from highly charged ions

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Quasi-free electron scattering from highly
charged ions

• Theo J.M. Zouros
• University of Crete Heraklion
• GREECE

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The Mediterranean
Crete
CIA map
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The island of Crete
Area 8200 km2 (3200 mi2) Population
600 000 winter gt 1 000 000
summer
Two mountains 2450 m
Heraklion population 220 000
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Crete
Heraklion
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University of Crete - Heraklion
University Hospital
Physics Dept
UoC campus
Physics Biology
Medical school
From my living room
UoC 5000 students Physics Dept 600 under
grads 50 grads
30 faculty
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Recent collaborators
J.R. Macdonald Lab Kansas State
Univ.
Dr. Manolis Benis PhD 2001 Univ. of Crete JRM
Prof. Tom Gorczyca Western Michigan
Univ. R-matrix calculations
Prof. Pat Richard
Prof. (Emeritus) Chander Bhalla
Pierre Auger 1899-1993 Auger effect 1923-1925
Teck Lee Grad Student
Mikhail Zamkov Grad Student
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Ion atom/electron collisions
t -8
t -8
?t 10-17 s
Preparation
Relaxation
Interaction
Ionization Excitation Transfer combinations
e-
Non radiative Auger electrons Radiative Photon
decay
Atom/Molecule Target q 0 (neutral) Gas/Solid gs
or excited state n 1012-1020 /cm3
e-
e- beam Target q -1 I 1-50 µA n 106-108
/cm3
?
e-
Ion beam Z 1-12 I 1-100 nA q 1-12 E 0.5-2
MeV/u V 3-6 a.u. gs or metastable
Detectors projectile, recoil, photon,
electron coincidences ?O solid angle e -
efficiency
?
Black box?!
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Ion-atom/electron interactions Investigation of
the Coulomb force

• Force is Coulomb
• Potential usually known - can write down a
  Hamiltonian
• Calculate emission or interaction cross sections
• Difficulties many particles, long range force,
  correlation effects
• Model calculations
• Develop theoretical and experimental techniques
• Test approximations

Mature field more than 40 years old negative
population growth!
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Interest in ion-electron/atom collisions
Applications

• Tokomak and Astrophysical Plasmas
• Accelerator technology - Storage rings
• Radiation damage cancer therapy
• Basic atomic collisions

• Use HCI and simple targets few-electron
  systems
• Study ion excitation rather than target
  excitation
• Control charge state q of ion N number of
  electrons
• Isoelectronic sequence study same N different Z

Recent reviewElectron-Ion scattering, I.
Williams, Rep. Prog. Phys. 62 (1999) 1431
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Presentation Summary

• Resonant electron scattering (RES)
• Long range short range potential treatment
• Electron ion collision techniques
• Impulse Approximation Electron scattering model
• Advantages of quasi-free electron scattering
• RES applications He-like and H-like ions, triply
  excited states isoelectronic sequence study
• Future plans

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Electron - ion collisions Production and decay
of doubly excited states
radiative stabilization recombination DR or RTEX
Aq
capture of a (quasi-) free electron excitation
inverse autoionization
autoionization Resonant electron scattering RES
or RTEA
Ee Bn E1-E0
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Electron-ion scattering two-amplitude formula
e-
VS short range potential
q
Ion

r
R
Rutherford
Short-range
Interference
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Sum of Rutherford and Short Range amplitudes
Griffin Pinzola PRA42 (1990) 248
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Elastic scattering Rutherford term
Non resonant scattering Binary Encounter Peak
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Elastic scattering Rutherford Short-range
Resonant contributions
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Elastic scattering All 3 terms
Resonant contributions
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Elastic scattering Sum of 3 terms
Sum of resonant, non-resonant and interference
contributions
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R-matrix results e- B3 (1s2) ? B2
(1s2l2l') ? B3 (1s2) e-
Differential scattering at large angles provides
the most stringent tests of theory
(all terms!)
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Merged beam experiments at storage ring
16O7 H-like Eion 8.9 MeV/u Ee8-9keV
Eecm500 eV ?Eecm0.6 eV Iion 4 E7 ion stack 7
min
Kilgus et al PRL 1990
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Differential electron scattering
e- beam
Ion beam
J. Phys. B29 (1996) 4443
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UoC HEMISPHERICAL ANALYZER WITH 2-D PSD 0o
ELECTRON SPECTROMETER
Faraday Cup
Ion Beam
4-element lens
Inner hemisphere
Gas Cell
Pressure Gauge
Gas in
electrons
PSD X-Position Y- Position Timing
Outer hemisphere
e-
?
Resolution 0.1 ?O 1.8 x 10-4 sr
00 dgrs
Ion
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Experimental setup at J R Macdonald Laboratory
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Electron Scattering Model/Impulse Approximation
Free e--Ion ?? Bound e--Ion
Vp gtgt v
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Sliding the Compton profile across the resonances
RES
?e ??
Compton profile
Changing the ion velocity Vp slides the Compton
profile across the doubly-excited states bringing
them into resonance!
Auger decay
RES
??
Ee 184 eV (4.0 MeV)
Ee
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Comparison of signal ratesR (/s) NI ne L s
Merged beams (Heidelberg TSR) Crossed Beams (CEA Grenoble) Quasi-free e- beams (UoC - JRM)
Electrons ne (/cm3) 1 E 7 (1 A) 1.2 ? 8 (4 µ?) 3 ? 14 (10 mTorr)
Overlap L (cm) 150 0.20 5
Ions NI (/s) 3.7 E 12 (50 µA) 8.9 E 12 (10 µA) 7.8 E 10 (100 nA)
R (/s) 5.5 E 21 s 2.1 E 20 ds/dO ?O 1.3 E 26 ds/dO ?O
Electron beam ??e (eV) 0.004 0.500 120
Chamber Vacuum (Torr) 1 E -11 1 E -10 1 E -7
e- Analyzer ?Oe (sr) Resolution () Only ions measured 4 E -4 2 1.8 E -4 0.1
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Whats the use of a quasi-free electron?

• gt106 higher luminosity compared to
• crossed electron-ion beam experiments!
• Measure scattering at 1800 (very sensitive)!
• Include also resonances
• (e- energy dependence d2?/dEd?)!
• No UHV
• Spectrum in 30 minutes!

Sounds great but is it really electron-ion
scattering???
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Elastic scattering of quasi-free electrons on B4
ions
Elastic scattering of quasi-free electrons on B4
ions
Doubly Excited states
Zouros et al PRA 2003 RC
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Elastic scattering of quasi-free electrons on B3
ions
Doubly Excited states
Zouros et al PRA 2003 RC
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First Z-dependence study of a triply-excited
state
Benis et al JPBL submitted 2003
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Summary and Conclusion

• Large-angle differential electron ion
  scattering provides some of the most stringent
  tests of both atomic structure and collision
  dynamics

• State-of-the-art DDCS calculations (R-matrix)
  for free electron
• scattering from He-like and H-like ions are in
  excellent agreement with
• quasi-free electron experiments involving
  ion-H2 collisions over a wide
• energy region and many resonances

quasi-free electron scattering is
real electron scattering! (remember Compton
scattering electrons there also really
quasi-free!)

• quasi-free electron scattering provides the only
  way to presently observe differential RES!
  (particularly at the large scattering angles)

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Future

• Improve apparatus add first stage and double
  differential target
• Use Li vapor target that has narrower Compton
  profile (Laser-excited) Rydberg Li target?
• Expand studies to include
• many electron targets
• higher Z ions or L shells
• Incorporate zero-degree electron spectrometer
  system in a storage ring??!!

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Comparison of Compton Profiles
H2 target
Li vapor target
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H2 target
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He target
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Ar target
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The EndMany thanks to all my colleagues and
friends for making my sabbatical such a fun and
exciting experience!
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Summary and conclusions

• 1800 Elastic e--Ion Scattering measurements
• First differential observation of RES for He-like
  ions
• First isoelectronic sequence study of
  triply-excited state
• R-matrix calculations
• Overall good agreement with the measurements
• ESM seems to be a very good approximation
• Quasi-free e- scattering provides unique data
• Not impaired by broad Compton profile!!!
• No problem with convolution of Compton profile!!!

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Determination of the metastable 1s2s 3S
fraction
Method 1s2 1S ? 1s2p2 2D RTE 1s2s 3S ?1s2s2p
4P Capture
Two successive measurements at the same
production energy
E.P. Benis et al, PRA 65, 064701 (2002)
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1s2s 3S Metastable fraction
Foil stripping
Gas stripping
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e- scattering on ion
bjgtajLjMLjSjMSjgt
kili
De
e-
kflf
q
DEi
DEf
LSP
DEi
DEf
bf
Ione-
De
bi
Ion
Ion
Energy conservation De0 LS-coupling total
DLDSDMLDMSDP0
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Absolute doubly differential cross section
determination
Determine the overall efficiency Use BEe/elastic
non-resonant scattering peak
Determine the metastable fraction Use capture to
RTE lines ratio
B3(1s2 1S, 1s2s 3S) H2
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Elastic scattering of quasi-free electrons on B3
ions
Triply Excited States !
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Elastic scattering of quasi-free electrons on B3
ions
Triply Excited States
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B3 (1s2s 3S) ? B3 (2s2p 3P) ? B4 (1s)
eA
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Elastic scattering of quasi-free electrons on
ions(more open channels)
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Scattering of free electrons on HCI
Scattering of quasi-free electrons on HCI
Atom
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Compton Profiles
x
F(p)2
z
y
probability to find e- with component pz
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Highest Sensitivity at large-angle scattering
d?/d? 70 eV e- Li 1 1S? 2
3P DW Distorted Wave UDWUnitarized DW CC5
5-state close coupling CC11 11-state close
coupling
Largest differences between theories at 1800!!
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Resolution dependence study
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Electron scattering processes
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Summary
The impulse approximation provides

• simplification of ion-atom collisions by
  separating target-electron interactions from
  target-nucleus interactions.
• unifying view-point for treating electron
  scattering in ion-atom, e-ion and Compton.

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Differential Electron-ion scattering measurements
Recent reviewElectron-Ion scattering, I.
Williams, Rep. Prog. Phys. 62 (1999) 1431
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Kinematic Broadening
F. Fremont et al 1997
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The zero-degree Auger Projectile Spectroscopy
setup
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Sum of Rutherford and Short Range amplitudes
Griffin Pinzola PRA42 (1990) 248
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Differential e--ion scattering cross section
Griffin Pinzola PRA42 (1990) 248
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1800 differential scattering S ? S cross
sections
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Efficiency Determination ? Normalization to the
bare ion BEe peak
Normalize to the Non-bare BEe peak!
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Results

• R-matrix excellent agreement with measurements

• Impulse Approximation Valid down to VP/v t 3.5

• Resonant Excitation Strength for 2p2 1D

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Resonant Transfer Excitation
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Recombination of He-like ions
Kilgus et al PRA 1993