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Ultra fast ion collisions

Very fast highly charged ions (e.g. 1GeV/n U92+) generate a subattosecond, superintense electromagnetic pulse, which is interpreted as a broadband field of equivalent photons [1]. Ionization processes caused by these projectiles and photoionization have a common feature: During the ionization energy but no momentum is delivered by the projectile.

Previous investigations [2] performed by R. Moshammer (Robert.Moshammer) and J. Ullrich (Joachim.Ullrich) et al. at the GSI facility demonstrate that the main momentum exchange is between electron and its parent ion (fig. 1) for very fast ion collisions. The momentum signature of a given reaction and the information they carry therefore remain widely unaffected by the impact of the projectile. These projectiles provide the possibility to "softly" ionize without destroying the target in a severe collision.

Fig. 1: Momentum distribution of the electron and the He1+ recoiling target ion projected
onto the collision plane for 1 GeV/u U92+ + He2 -> He 1+ + e- + U92+. P|| is the axis parallel
to the incoming projectile beam (longitudinal), px is parallel to the transverse momentum
component of the recoiling ion.

Planned Experiments:

Utilizing the "Coltrims" reaction microscope we want to study the fragmentation processes of helium dimers (and neon dimers) when ionized by relativistic projectiles. The following topics are in the focus of our investigations:

Fig. 2: Relativistic Projectile ionizing both sites of the helium dimer.

1) Imaging the (square) nuclear wave function of He2

Recent experiments at the synchrotron radiation facility “BESSY” in Berlin allowed us to probe parts of the helium dimers nuclear wave function up to distances R of about 10 Å by examining the so called “interatomic knock-off” process. But wide areas of its wave function, which have been calculated to reach distances over 100 Å, still remain unexplored. Ionization processes in ultra fast ion collisions allow a different approach to measure even larger distances of the dimers nuclei: After subsequently singly ionizing both atoms, the dimer is left in a state, where the He+ ions repel each other and coulomb explode.

Fig. 3: Subsequent ionization process followed by coulomb explosion.

According to the classical reflection approximation as well as in a quantum mechanical treatment the kinetic energy released in this coulomb explosion is related to the distance of the two nuclei at the instant of ionization. Since the momentum vectors of all particles created in the reaction can be measured with the “Coltrims” apparatus also the kinetic energies can be determined. By that the distribution of internuclear distances can be measured which is the square of the nuclear wave function.

2) Testing the impact parameter dependence of the ionization

The above described ionization of both atoms of the helium dimer by the projectile is expected to have a strong angular dependence: The ionization process will more likely occur at small angles (and up to a maximum angle max, see fig. 5) between the molecular axis and the projectile beam axis, where the distance of both nuclei to the projectile is small. This is due to a decreasing ionization probability (fig. 6) for increasing impact parameters. Since the dimers internuclear distances are large the ionization processes can be considered as independent interactions of the projectile with each of its atoms. The angular distribution (which also can be determined from the momentum vectors) therefore contains information about the ionization probability in dependence of the impact parameter.

Fig. 4 The impact parameter dependent ionization probabilty P(b) leads to a maximum tilt angle αmax.

Fig. 5: CTMC simulation for the impact parameter dependent (single) ionization probability 120 MeV/u U90+ on He.

3) Interatomic Coulombic Decay

Interatomic (Intermolecular) Coulombic Decay has been extensively studied in synchrotron radiation experiments. Its existence in ultra fast ion collisions will be explored.

4) Neon Dimer

Also the Ne dimer, which is much more confined in space than the He dimer, will be studied in ultra fast collisions.

[1] C.F. Weizsäcker, Z. Phys. 88, 612 (1934); E.J. Williams, Phys. Rev. 45, 729 (1934)
[2] R. Moshammer et al., Phys. Rev. Lett. 79:3621, 1997