If swift heavy ions (5 MeV/u) hit solid targets energy is transferred to the electrons of the solid. Caused by the high electronic energy deposition along the flight path of the ion a cylindrical shaped chemical or structural defect cluster is formed. This kind of defect cluster together with its electronic and atomic precursors is called ion track. Ion‐track formation is mainly described by two different mechanisms: At the beginning of the ion‐track formation, within a timescale of 10−17 s, the energy is transferred into the electronic system and the electrons move away from the center of the track where a positively charged inner cylinder remains. If the charge‐neutralization time of these positive ions along the track is sufficiently long so that they can repel each other by Coulomb forces, the Coulomb‐explosion model can describe the track formation. Otherwise (short charge‐neutralization time) the energy is transferred into the atomic system by electron‐phonon coupling (within a picosecond timescale), what is included in the thermal‐spike model. During the energy transfer from the electronic to the atomic system atoms are set in motion and charged as well as neutral particles are sputtered. By detecting these particles energy resolved information on the track forming mechanism can be achieved.
Within the talk different experimental approaches to the ion‐track formation realized in the ion‐beam laboratory (ISL) of the Hahn‐Meitner‐Institut (HMI) are presented.
The electron dynamics takes place within a femtosecond timescale. Auger‐electrons leaving the target surface are suitable for investigating the short‐time dynamics because typical Auger‐decay times are in the order of a few femtoseconds. The width of the Auger‐profiles mainly reflects a convolution of the energy distribution of the electrons involved in the Auger process. An energy transfer from the projectile to the electronic system of the target causes a change of the energy distribution of the valence electrons and the shape of Auger‐profiles respectively.
For analyzing the atom dynamics (picosecond timescale) sputtered ions as well as neutral particles have to be detected. The desorbed ions are detected by a conventional energy resolved mass‐spectrometer. For the detection of the sputtered neutrals a new experimental setup is installed. The neutrals are ionized by non‐resonant multiphoton ionization with an amplified pulsed Ti:Sapphire laser and analyzed by a time‐of‐flight mass spectrometer.
- Energy transfer
- Femtosecond dynamics
- Charge transfer
- Electron transfer
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