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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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]

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

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

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

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

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

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

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

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

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°

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

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

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