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The breakup of helium dimers investigated in fast ion collisions

With a binding energy of only 95neV the helium dimer is the most weakly bound atomic system. We have investigated the (He+,He+) fragmentation of an helium dimer after colliding with an alpha particle (He2+) with an energy of 150 keV/u..If the dimer ion breaks up in two singly charged ions (back to back) the momentum vectors are equal in magnitude but oppositely oriented. Thus, momentum conservation enables us to distinguish these events from random events. By using the ColTRIMS technique these momentum vectors can be obtained. In our measurement we focused our attention on double capture (DC) where two electrons end up in a projectiles bound state

DC:    He2+ + He2  →  He0 + He+ + He+

and transfer ionization (TI) where one electrons is captured and the second is released by the interaction with the projectile:

TI:    He2+ + He2  →  He+ + He+ + He+ + e-


Surprisingly, the results show more than one process leading to different kinetic energy releases (KERs). Figure 2 illustrate the KER distribution for DC and TI. It illuminates that the distribution for TI includes three peaks while the distribution for DC shows only two peaks. As it is explained below the broad maximum between 1eV and 5 eV indicated by 1 can be related to the two-step-process while the maximum at 9 eV marked by 2 originates from a one-step-process. The maximum pictured by number 3 belongs to a process termed Interatomic Coulombic Decay which is observed in an ion collision for the first time here.

Peak 1: two-step-process

The angular distributions of the dimer axis in respect to the projectile direction for TI and DC are seen in (C) and (D), respectively. As these spectra show there is a strong alignment of the dimer axis in respect to the beam direction. Here the projectile (purple arrow) interacts independently with both atoms. In this case the dimer atoms have to be located within the geometrical cross section (illustrated by the circular area in figure 1A) which is given by the impact parameter distribution. Consequently, only those orientations of the dimer axis are selected which fulfill this condition. This leads to the characteristic angular distribution around the projectile direction. The internuclear distance is obtained by the classical approximation:
KER (a.u.) = 1/R.

Peak 2: one-step-process

The angular distribution (B) is isotropic. Furthermore the very small internuclear distances (C) which are calculated from the KERs by the classical approximation implies a contraction of the dimer ion before breaking up. Here an -step interaction occurs: as it is demonstrated in figure (A) two electrons can be removed from one atom of the dimer. In this case the projectile leaves a stable dimer ion consisting of a doubly charged helium ion on the one side and its neighbouring neutral partner on the other side. Attracted by Coulomb force the dimer ion begins to shrink. At internuclear distances of 3 A an electron exchange between the two centers takes place.

Peak 3: Interatomic Coulombic Decay

The KER maximum at 8 eV seen only for TI belongs to a diagonal structure, which has so far been observed only in photo ionization experiments. It is a t ypical fingerprint of a process termed Interatomic Coulombic Decay (ICD) which is characteristic for van der Waals or hydrogen bound systems. As ICD takes place excitation energy is transferred via virtual photon exchange from one atom to a neighbouring atom which thus is ionized, even if both are separated by several angstroms. Figure 4 (A) show how the mechanism proceeds in an ion collision. Capturing an electron the projectile simultaneously deposits energy in one of the dimer atom by excitation of the remaining electron. After a delay the neighbouring atom is ionized by ICD leading to the fragmentation.


Jasmin Titze,
Die Untersuchung der Ionisationsdynamik von Heliumdimeren in Stößen mit Alpha-Teilchen
You will find this publication here

J. Titze, M. S. Schöffler, H.-K. Kim, F. Trinter, M. Waitz, J. Voigtsberger, N. Neumann, B. Ulrich, K. Kreidi, R. Wallauer, M. Odenweller, T. Havermeier, S. Schössler, M. Meckel, L. Foucar, T. Jahnke, A. Czasch, L. Ph. H. Schmidt, O. Jagutzki, R. E. Grisenti, H. Schmidt-Böcking, H. J. Lüdde, and R. Dörner
Ionization Dynamics of Helium Dimers in Fast Collisions with He++
Physical Review Letters, 106, 033201 (2011)
DOI: 10.1103/PhysRevLett.106.033201
Download this file here: titze2011prl.pdf (496kb)

H.-K. Kim, H. Gassert, J. N. Titze, M. Waitz, J. Voigtsberger, F. Trinter, J. Becht, A. Kalinin, N. Neumann, C. Zhou, L. Ph. H. Schmidt, O. Jagutzki, A. Czasch, M. Schöffler, H. Merabet, H. Schmidt-Böcking, T. Jahnke, H. J. Lüdde, A. Cassimi, and R. Dörner
Orientation dependence in multiple ionization of He2 and Ne2 induced by fast, highly charged ions: Probing the impact-parameter-dependent ionization probability in 11.37-MeV/u S14+ collisions with He and Ne
Physical Review A, 89, 022704 (2014)
DOI: 10.1103/PhysRevA.89.022704
Download this file here: Kim2014pra.pdf (1,119kb)