Mechanisms of

Photo Double Ionization of Helium

by 529 eV Photons

Alexandra Knapp

Group of R. Dörner and H. Schmidt-BöckingUniversity of Frankfurt, Germany

0

Ene

rgy

Single ionization

25 eV … 79 eV: only single ionization

0

Ene

rgy

Double ionization

79 eV and higher: single + double ionization

Shake off

Shake off

First electron absorbs photon and removes from the nucleus

Shake off

Shake off

Change of the effektiv nuclear charge

Shake off

Second electron is in its eigenstate Only single ionization occurs

Shake off

Second electron is NOT in its eigenstate Shake off

Electron-electron scattering (TS1)

Electron-electron scattering (TS1)

Electron-electron scattering (TS1)

Electron-electron scattering (TS1)

Electron-electron scattering (TS1)

Electron-electron scattering (TS1)

Electron-electron scattering (TS1)

Electron-electron scattering (TS1)

Electron-electron scattering (TS1)

90°

(e,2e) like collision between the two electrons

Photon energy above the threshold [eV]

TS1

Shake off

Shake off + TS1

Ratio of double to single ionization (theory)

%

with electric field

COLTRIMS

with electric + magnetic field

COLTRIMS

Collecting electrons up to 60 eV with 4 solid angle with a combination

of an electric and a magnetic field

Questions

• Is the assumption of a two-step model

valid?

• Energy sharing

• Asymmetry parameter

• Mechanisms of photo double ionization

• Angular distributions

Energy sharing

Extreme asymmetric energy sharing

CCC-Calculation A. Kheifets, I. Bray

Asymmetry parameter

Fast electron absorbs photon energy and its angular momentum

Polynomial fit

Asymmetry parameter

Two-step model is valid

Polynomial fit

Shake off TS1

CCC-Calculation CCC-Calculation

Very slow electrons: 2 eV

Angular distributions

Faster electrons: 30 eV

The fast electron (indicated by the red arrow) is fixed into the direction of the polarization

fast electron (420 eV)

fast electron (448 eV)

s

low

er e

lect

ron

(2 e

V)

s

low

er e

lect

ron

(30

eV)

TDCS for Eslow = 2 eV and 30 eV

fast- slow =180

fast- slow =90

fast electron (420 eV)

fast electron (448 eV)

s

low

er e

lect

ron

(2 e

V)

s

low

er e

lect

ron

(30

eV)

Shake off and TS1 can be seen for all angles

fast electron (420 eV)

fast electron (448 eV)

s

low

er e

lect

ron

(2 e

V)

s

low

er e

lect

ron

(30

eV)

Comparison between experiment and theory

Conclusion

• 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

Reinhard Dörner Horst Schmidt-BöckingAlexandra KnappThorsten WeberSven SchößlerTill JahnkeJürgen Nickles Susanne KammerOttmar JagutzkiLothar Schmidt

C. Lewis CockeTimur Osipov

Michael H. PriorJürgen Rösch

Allen Landers

Anatoli Kheifets

Igor Bray

University Frankfurt:

Kansas State University:

LBNL:

Western Michigan University:

Australian National University Canberra:

Murdock University Perth:

Supported by BMBF, DFG and DOE