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Atomic and molecular signatures for charged-particle ionization


The way in which atoms and molecules are ionized by the impact of charged particles has important consequences for the behaviour of many physical systems, from gas lasers to astrophysical plasmas. Much of our understanding of this process has come from ionization measurements of the energy and angular distribution of electrons ejected in the same plane as the trajectory of the incident ionizing beam. Such studies suggest that the mechanisms governing the ionization of atoms and molecules are essentially the same. But by measuring the electrons ejected from a gas in a plane perpendicular to the incident beam, we show this is not always the case. Experiments and quantum mechanical calculations enable us to construct a remarkably accurate classical picture of the physics of charged-particle ionization. This model predicts that the differences in ionization behaviour arise in molecules that do not have nuclei at their centres of mass.

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Figure 1: The experimental geometry.
Figure 2: Experimental and theoretical DCS data in the perpendicular plane for He and H2 targets, normalized to unity at the experimental maximum.
Figure 3: Different mechanisms that may lead to ionization in the perpendicular plane.
Figure 4: Averaging of the electronic and nuclear structure of the targets as experimental constraints mean that the orientation of the molecule cannot be determined.
Figure 5: Change in the calculated ionization DCS for H2 in the perpendicular plane as a function of the size of the spherically averaged nuclear shell, normalized to unity at the experimental maximum.
Figure 6: DCS for ionization of CO2 in the perpendicular plane normalized to unity at the experimental maximum.

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We thank the EPSRC (UK) for supporting these experiments, and the NSF for support of the theoretical work under grant No. PHY-0757749.

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O.A.-H. and D.M. carried out the theoretical calculations for this work, whereas C.K. and A.M. carried out the experimental investigations that are presented.

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Correspondence to Don Madison or Andrew James Murray.

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Al-Hagan, O., Kaiser, C., Madison, D. et al. Atomic and molecular signatures for charged-particle ionization. Nature Phys 5, 59–63 (2009).

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