1. Photo Double Ionization of Helium
Mechanisms of
Photo Double Ionization of Helium
by 529 eV Photons
Alexandra Knapp
Group of R. Dörner and H. Schmidt-Böcking
University of Frankfurt, Germany
Ladies and Gentlemen,
in my lecture I will talk about the photo double ionization of Helium by photons which have an energy of 529 eV. More precise, I will talk about the mechanisms of PDI of Helium
the experiment was performed last spring at the Advanced Light Source in Berkeley

2. 79 eV and higher: single + double ionization
Single ionization
Energy
Double ionization
79 eV and higher: single + double ionization
Energy
If the photon has an energy between 25 and 79 eV, single ionization may occur
the photon is absorbed and the energy of the photon is used to take one electron into the continuum
If the photon energy is 79 and higher, single and double ionization may occur
it is believed that 2 mechanisms are responsible for the double ionization
25 eV … 79 eV: only single ionization

3. Shake off Shakeoff: First: single ionization
Followed by a rapid change of the effective nuclear charge
State of the remaining electron is not in its eigenstate and is shaken off from the nucleus
Important: fast change of the effective nuclear charge
Tendence: Mechanism is dominant at high photon energies

4. Shake off First electron absorbs photon and removes from the nucleus
First: single ionization
Followed by a rapid change of the effective nuclear charge
State of the remaining electron is not in its eigenstate and is shaken off from the nucleus
Important: fast change of the effective nuclear charge
Tendence: Mechanism is dominant at high photon energies

5. Shake off Shakeoff: First: single ionization
Followed by a rapid change of the effective nuclear charge
State of the remaining electron is not in its eigenstate and is shaken off from the nucleus
Important: fast change of the effective nuclear charge
Tendence: Mechanism is dominant at high photon energies

6. effektiv nuclear charge
Shake off
Change of the
effektiv nuclear charge
Shakeoff:
First: single ionization
Followed by a rapid change of the effective nuclear charge
State of the remaining electron is not in its eigenstate and is shaken off from the nucleus
Important: fast change of the effective nuclear charge
Tendence: Mechanism is dominant at high photon energies

7. Shake off Second electron is in its eigenstate
First: single ionization
Followed by a rapid change of the effective nuclear charge
State of the remaining electron is not in its eigenstate and is shaken off from the nucleus
Important: fast change of the effective nuclear charge
Tendence: Mechanism is dominant at high photon energies
Second electron is in its eigenstate
 Only single ionization occurs

8. Shake off Second electron is NOT in its eigenstate  Shake off
First: single ionization
Followed by a rapid change of the effective nuclear charge
State of the remaining electron is not in its eigenstate and is shaken off from the nucleus
Important: fast change of the effective nuclear charge
Tendence: Mechanism is dominant at high photon energies
Second electron is NOT
in its eigenstate  Shake off

9. Electron-electron scattering (TS1)
Shakeoff:
First: single ionization
Followed by a rapid change of the effective nuclear charge
State of the remaining electron is not in its eigenstate and is shaken off from the nucleus
Important: fast change of the effective nuclear charge
Tendence: Mechanism is dominant at high photon energies

10. Electron-electron scattering (TS1)
Shakeoff:
First: single ionization
Followed by a rapid change of the effective nuclear charge
State of the remaining electron is not in its eigenstate and is shaken off from the nucleus
Important: fast change of the effective nuclear charge
Tendence: Mechanism is dominant at high photon energies

11. Electron-electron scattering (TS1)
Shakeoff:
First: single ionization
Followed by a rapid change of the effective nuclear charge
State of the remaining electron is not in its eigenstate and is shaken off from the nucleus
Important: fast change of the effective nuclear charge
Tendence: Mechanism is dominant at high photon energies

12. Electron-electron scattering (TS1)
Shakeoff:
First: single ionization
Followed by a rapid change of the effective nuclear charge
State of the remaining electron is not in its eigenstate and is shaken off from the nucleus
Important: fast change of the effective nuclear charge
Tendence: Mechanism is dominant at high photon energies

13. Electron-electron scattering (TS1)
Shakeoff:
First: single ionization
Followed by a rapid change of the effective nuclear charge
State of the remaining electron is not in its eigenstate and is shaken off from the nucleus
Important: fast change of the effective nuclear charge
Tendence: Mechanism is dominant at high photon energies

14. Electron-electron scattering (TS1)
Shakeoff:
First: single ionization
Followed by a rapid change of the effective nuclear charge
State of the remaining electron is not in its eigenstate and is shaken off from the nucleus
Important: fast change of the effective nuclear charge
Tendence: Mechanism is dominant at high photon energies

15. Electron-electron scattering (TS1)
Shakeoff:
First: single ionization
Followed by a rapid change of the effective nuclear charge
State of the remaining electron is not in its eigenstate and is shaken off from the nucleus
Important: fast change of the effective nuclear charge
Tendence: Mechanism is dominant at high photon energies

16. Electron-electron scattering (TS1)
Shakeoff:
First: single ionization
Followed by a rapid change of the effective nuclear charge
State of the remaining electron is not in its eigenstate and is shaken off from the nucleus
Important: fast change of the effective nuclear charge
Tendence: Mechanism is dominant at high photon energies

17. Electron-electron scattering (TS1)
Shakeoff:
First: single ionization
Followed by a rapid change of the effective nuclear charge
State of the remaining electron is not in its eigenstate and is shaken off from the nucleus
Important: fast change of the effective nuclear charge
Tendence: Mechanism is dominant at high photon energies
90°
(e,2e) like collision
between the two electrons

18. Ratio of double to single ionization (theory)
Shake off + TS1
Shake off
TS1
Line presents ccc theory
On the horizontal axis
On the vertical axis
Logarithmic
60 eV over the threshold so far
We: 450 eV over the threshold
Photon energy above the threshold [eV]

19. COLTRIMS with electric field
To get the fully resolved differential cross section we need to measure the momenta of at least 2 charged particals, in our case we have measured the slow electron up to 60 eV and the recoiling ion
we did this by putting a homogeneous electric field over the targetzone, so that the recoiling ions are collected with 4 pi onto a position sensitiv channel plate detector
with only one quite small electric field we get always the recoiling ion but only the electrons if they have a small energy or if they fly directly to the other channel plate detector
a solution for this is to combine the electric with a magnetic field which force the electrons to describe cyclotron motion/ trajectory
From time of flight and position of impact the momentum of the charged partical can be deduced
with electric field

20. COLTRIMS with electric + magnetic field
Collecting electrons up to 60 eV
with 4  solid angle with a combination
of an electric and a magnetic field
To get the fully resolved differential cross section we need to measure the momenta of at least 2 charged particals, in our case we have measured the slow electron up to 60 eV and the recoiling ion
we did this by putting a homogeneous electric field over the targetzone, so that the recoiling ions are collected with 4 pi onto a position sensitiv channel plate detector
with only one quite small electric field we get always the recoiling ion but only the electrons if they have a small energy or if they fly directly to the other channel plate detector
a solution for this is to combine the electric with a magnetic field which force the electrons to describe cyclotron motion/ trajectory
From time of flight and position of impact the momentum of the charged partical can be deduced
with electric + magnetic field

21. Questions Is the assumption of a two-step model valid?
Energy sharing
Asymmetry parameter
Mechanisms of photo double ionization
Angular distributions
What are the questions?
First: Do we have a two step modell?
Answer to this gives us the energy sharing and the asymmetry parameter
And what are the mechanisms of the photot double ionization?
Answer The shape of the angular distributions of the two electrons

22. Energy sharing Extreme asymmetric energy sharing CCC-Calculation
A. Kheifets, I. Bray
Extreme asymmetric energy sharing

23. Fast electron absorbs photon energy and its angular momentum
Asymmetry parameter
Polynomial fit
Fast electron absorbs photon energy
and its angular momentum

24. Two-step model is valid
Asymmetry parameter
Polynomial fit
Two-step model is valid

25. Angular distributions Very slow electrons: 2 eV
Faster electrons: 30 eV
Shake off
TS1
If we want to learn something about the mechanisms, we must how the
This means we must fix the fast electron into the direction of the polarization and look how the slow electron is emitted in the plane which is defined by the polarisation vector and the fast electron
we can destinguish between two cases
in the first one we want to know how a 2 eV electron is emitted if the 448 eV is fixed
we can do the same with a 30 eV electron
in the first case we see a quite isotropic distribution which has its maximum at 180 degrees
this is expected from the shake off process
in the other case we see emission into a narrow cone of 90 degrees
this is what we expect from a binary encounter
in brief: very slow electrons get into the continuum via the shake off mechanism
while faster electrons get their energy from a (e,2e) like collision
CCC-Calculation
CCC-Calculation
The fast electron (indicated by the red arrow) is fixed into the direction of the polarization

26. TDCS for Eslow = 2 eV and 30 eV
q fast electron (420 eV)
q fast electron (448 eV)
q slower electron (2 eV)
q slower electron (30 eV)
These two figures are special cases where the fast electron is fixed into the same direction of the polarization
We can do the same for fixed fast electrons with different angles to the polarisation
and we can of course look at the angular distributions of the fast electron
The shake off mechanism is dominant for slow electrons which have an energy of 2 eV for all angles
and the TS1 is dominant for 20 eV electrons and higher also for all angles which is indicated by the lines

27. Shake off and TS1 can be seen for all angles
q fast electron (420 eV)
q fast electron (448 eV)
q slower electron (2 eV)
q slower electron (30 eV)
fast- slow =180°
These two figures are special cases where the fast electron is fixed into the same direction of the polarization
We can do the same for fixed fast electrons with different angles to the polarisation
and we can of course look at the angular distributions of the fast electron
The shake off mechanism is dominant for slow electrons which have an energy of 2 eV for all angles
and the TS1 is dominant for 20 eV electrons and higher also for all angles which is indicated by the lines
fast- slow =90°

28. Comparison between experiment and theory
q fast electron (420 eV)
q fast electron (448 eV)
q slower electron (2 eV)
q slower electron (30 eV)
These two figures are special cases where the fast electron is fixed into the same direction of the polarization
We can do the same for fixed fast electrons with different angles to the polarisation
and we can of course look at the angular distributions of the fast electron
The shake off mechanism is dominant for slow electrons which have an energy of 2 eV for all angles
and the TS1 is dominant for 20 eV electrons and higher also for all angles which is indicated by the lines

29. Conclusion =2 for very fast and =0 for very slow electrons
TDCS of the DPI of Helium were measured at 450 eV above the threshold
electrons are distinguishable because of the asymmetric energy sharing
=2 for very fast and =0 for very slow electrons
very slow ( 2 eV) electrons: Shake off
slightly faster ( 30 eV) electrons: TS1

30. Supported by BMBF, DFG and DOE
University Frankfurt:
Kansas State University:
LBNL:
Western Michigan University:
Australian National University Canberra:
Murdock University Perth:
Reinhard Dörner
Horst Schmidt-Böcking
Alexandra Knapp
Thorsten Weber
Sven Schößler
Till Jahnke
Jürgen Nickles
Susanne Kammer
Ottmar Jagutzki
Lothar Schmidt
C. Lewis Cocke
Timur Osipov
Michael H. Prior
Jürgen Rösch
Allen Landers
Anatoli Kheifets
Igor Bray
Supported by BMBF, DFG and DOE