Max-Planck-Institut für Kernphysik Heidelberg


Collisions of electrons and positrons with atoms and molecules

Priv.-Doz. Dr. Alexander Dorn and Dr. Claus-Dieter Schröter

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Single Ionization of Atoms: (e,2e)

Electron impact single ionization of atoms is extensively studied with conventional experiments where two electron spectometers are used to detect both outgoing electrons in coincidence. Most experiments looked at so-called coplanar collisions where the ionized electron is ejected into the plane defined by the incoming and scattered projectile (see figure 1). Some experiments measured particular other geometries but a complete picture of the cross section could not be obtained.

We are investigating the single ionization process applying the new reaction microscope covering the full solid angle for electron emission.

An example is shown in figure 2. The projectile (E0 =102 eV) emerges from the bottom, hits the target in the center and is deflected to the left. The probability for emission of the second electron (E2 = 10 eV) into a particular direction is given by the distance from the origin to the 3D surface shown.

The electron is emitted with highest probability to the right corresponding to a binary knock-out by the scattered projectile. If the target electron while leaving the atom additionally scatters off the ionic potential it can be emitted into the opposite direction giving rise to the lobe on the bottom right.

Surprisingly the 3D image of the cross section reveals some structures corresponding to electron emission perpendicular to the scattering plane which is indicated by the dashed frame in the plot. Standard theories as the 3C calculation displayed in the lower panel nicely reproduce the two main lobes but do not show this out-of-plane contribution.

A more quantitative comparison is possible applying cuts through the 3D surface inside the scattering plane (dashed frame in Figure 2) and the perpendicular plane (dotted frame) which are shown below.

The experimental data are compared to different calculations. The out-of-plane contributions are visible in particular for larger scattering angles (see Figure 3) as side peaks around 70 and 290. While the 3C calculation failes to show these feature, more sophisticated calculations nicely reproduce them.

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Figure 1: Conventional (e, 2e) set-up

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Figure 2: Measured cross-section fr single ionisation of Helium

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Figure 3: calculated cross-section for single ionisation of Helium


Single ionization of aligned Molecules

It is expected that the details of an ionizing collision should critically depend on the alignment angle between the molecular axis and the projectile beam, e.g. if molecular axis is aligned along the projectile beam or perpendicular. Nevertheless, practically all existing (e,2e) experiments for molecular targets average over the orientation of the molecular axis.

Molecular collision

In a first step we started performing (e,2e) experiments on molecular hydrogen where a few percent of the residual ion fragments into atomic H and a proton. The detection of the proton allows the determination of the molecular axis alignment during the collision. An example is shown in the figure below. For 200 eV electron impact ionization the ionized electron angular distribution is plotted for three different molecular alignment angles. For parallel alignment the overall cross section is significantly higher than for perpendicular alignment. The calculated results using a distorted wave approximation (Don Madison, Missouri S&T University, USA) reproduce this trend.

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In future we will investigate how multiple-scattering reactions can be enabled or disabled by the molecular alignment angle. Furthermore, multiple-centre interference effects should become pronounced for fixed-in-space molecular axis.

Positron impact ionization of atoms

Collisions involving positrons are expected to be significantly different from analogous collisions involving electrons in particular in the low energy range up to  100 eV. Reasons are the absence of the Pauli exclusion principle and therefore of the exchange interaction characteristic for electrons, the added richness of the positronium channel (the positron captures an atomic electron) and the repulsive short-range positron-atom interaction, in contrast to the attractive electron-atom interaction. Experimental data in particular for differential cross sections are scarce. For ionization, so far, mainly integrated cross sections were measured. Reasons are the lack of available intense positron beam sources and the low efficiency of conventional experimental coincidence spectrometers. The project described here uses a multi-particle imaging spectrometer (reaction microscope) with large phase space acceptance and high efficiency. We had a first pilot beam time at the NEPOMUC positron beam facility at the neutron research reactor FRMII in Garching which can provide a beam of up to one billion positrons per second.

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In the top diagram of the next Figure the ionization cross section is plotted as function of the longitudinal momentum of the scattered positron as well as function of the ejected electron. For comparison a respective cross section for electron impact ionization at slightly higher impact energy of 100 eV is also shown (bottom diagram). Since in this case ejected and scattered electrons cannot be distinguished only one curve is plotted. It shows a sharp peak at high forward momentum which can be assigned to forward scattered projectiles and a broad maximum around zero momentum corresponding to ionized electrons. For positron impact, the projectile and the ejected electron can be distinguished. As expected a clear forward emission of the positron is found with a rather sharp cut-off close to the maximum possible momentum at 2.1 a.u. corresponding to a forward scattered positron which has lost just the He ionization energy. Interestingly also the ionized electrons are strongly forward emitted with the cross section maximum close to 1 a.u. This is in strong contrast to the results obtained for electron impact ionization where the maximum of the emitted electrons is found at zero momentum. A likely explanation for this behaviour is the different post collision interaction (PCI) in both cases. For positron impact, the emitted electron is attracted by the scattered positron and dragged forward. For electron impact the ionized electron is repelled by the scattered projectile. As a result, the projectile peak is also broader for positron impact, since there is additional energy loss due to PCI while the contrary is true for electron impact.

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Alexander Dorn

Priv.-Doz. Dr. Alexander Dorn

Electron collisions and Photoionization of Lithium

Room: Bo. 261b
Tel.: +49 (0) 6221 516 - 513
Email: alexander.dorn@please delete this@mpi-hd.mpg.de



Xueguang Ren

Dr. Xueguang Ren

Electron impact ionization of molecules and atoms

Room: Bo. 325
Tel.: +49 (0) 6221 516 - 609
Email: xueguang.ren@please delete this@mpi-hd.mpg.de


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