In this thesis a series of experiments on collisions between atomic projectile ions at keV energies and target vapors of either isolated molecules or van der Waals clusters is presented and analyzed. The atomic ions are produced in an Electron Cyclotron Resonance (ECR) ion source, accelerated and guided into the target volume. The charged target collision products are mass-to-charge analyzed in a time-of-flight spectrometer. The Polycyclic Aromatic Hyrdrocarbons (PAHs) Anthracene (C₁₄H₁₀), Coronene (C₂₄H₁₂), two C₁₆H₁₀ isomers, Pyrene and Fluoranthene, and the fullerene C₆₀ are examined.

For projectile ions in low charge states, small impact parameter collisions dominate, which leads to internal heating of the target. With isolated molecules as targets, this typically results in ionization and often also in fragmentation. For cluster targets energy and charge are rapidly distributed among the cluster building blocks. This is followed by cluster evaporation and very limited fragmentation of the individual molecules. C₁₁₉⁺ and C₁₁₈⁺ are observed as products. These are due to the formation of the reactive C_58/59⁺ ions by direct knockout processes, which react with another C₆₀ of the cluster to form dumb-bell shaped molecules.

For projectile ions of high charge (Xe²⁰⁺) larger impact parameters dominate, leading to little internal heating. For isolated molecule targets, intact molecular ions are the main collision products. Charged fragments stem mostly from multifragmentation following ionization to high charge states. For cluster targets, the collision products consist mainly of singly charged monomers. Fragmentation of the individual molecules is comparatively strong. This suggests a quick distribution of charges followed by a Coulomb explosion, which leads to internal heating.

The results show that weakly bound clusters do not sustain the impact of keV-ions and that it is possible to form new molecular structures.

This licentiate thesis covers experimental and theoretical studies of the fragmentation and sub- sequent reactions of fullerenes or Polycyclic Aromatic Hydrocarbons (PAHs). I present results from experiments where atoms or ions collide with fullerenes, PAHs, or clusters of these mole- cules at energies from 100 eV up to tens of keV.

The results of these experiments are interpreted using quantum chemical calculations, mol- ecular dynamics simulations, and a stopping model that we have developed. With these models I show that different energy transfer processes dominate at different collision energies, from nuclear stopping in low energy (~ 100 eV) collisions to electronic stopping at higher energies (~ 10 keV). I particularly focus on non-statistical fragmentation induced by nuclear stopping which can lead to the formation of exotic new species such as the odd-numbered C₁₁₉ fullerene- like molecule. 

In this work we will discuss fragmentation and molecular growth processes in collisions of Polycyclic Aromatic Hydrocarbon (PAH) molecules, fullerenes, or their clusters with atoms or atomic ions. Simple collision models as well as molecular structure calculations are used to aid the interpretations of the present and other experimental results. Fragmentation features at center-of-mass collision energies around 10 keV are dominated by interactions between the fast ion/atom and the electron cloud in the molecules/clusters (electronic stopping processes). This electronic excitation energy is rapidly distributed on the vibrational degrees of freedom of the molecule or of the molecules in a cluster and may result in fragmentation. Here, the fragmentation is statistical and favors the lowest-energy dissociation channels which are losses of intact molecules from clusters, H- and C₂H₂-losses from isolated PAHs, and C₂-loss from fullerene monomers. We will also discuss the possibility of formation of molecular H₂ direct from native PAHs which reach high enough energies when interacting with ions, electrons, or photons.

For the experiments at lower center of mass collision energies (~100 eV) a single atom may be knocked out in close atom-atom interaction. Such non-statistical fragmentation are due to nuclear stopping processes and gives highly reactive fragments which may form covalent bonds with other molecules in a cluster on very short time scales (picoseconds). This process may be important when considering the formation of new species. For collision between 12 keV Ar²⁺ and clusters of pyrene (C₁₆H₁₀) molecules, new molecules, e.g. C₁₇H_10⁺, C₃₀H_18⁺, C₃₁H_19⁺, etc are detected. We also observe molecular fusion processes for He and Ar ions colliding with clusters of C₆₀ molecules. These and related molecular fusion processes may play a key role for understanding molecular growth processes under certain astrophysical conditions.

When molecules are excited by photons or energetic particles, they will cool through the emission of photons, electrons, or by fragmenting. Such processes are often thermal as they occur after the excitation energy has been redistributed across all degrees-of-freedom in the system. Collisions with atoms or ions may also lead to ultrafast fragmentation in Rutherford-like scattering processes, where one or several atoms can literally be knocked out of the molecule by the incoming projectile before the energy can be completely redistributed. The resulting fragmentation pathways can in such knockout processes be very different from those in thermal processes.

This thesis covers extensive studies of collisions between ions/atoms and isolated Polycyclic Aromatic Hydrocarbon (PAH) molecules, isolated fullerene molecules, or clusters of these. The high stabilities and distinct fragmentation channels make these types of molecules excellent test cases for characterizing knockout-driven fragmentation and the reactions that these processes can lead to. I will present experimental measurements for a wide range of energies and compare them with my own molecular dynamics simulations and quantum chemical calculations. In this thesis, I present an in-depth study of the role of knockout in the energetic processing of molecules and clusters. The competition between knockout and thermally driven fragmentation is discussed in detail.

Knockout-driven fragmentation is shown to result in exotic fragments that are far more reactive than the intact parent molecules or fragments from thermal processes. When such reactive species are formed within molecular clusters efficient molecular growth can take place on sub-picosecond timescales. The cluster environments are crucial here because they protect the newly formed molecules by absorbing excess energy. This is a possible pathway for the growth of large PAHs, fullerenes, and similar carbonaceous complexes found in, for instance, the interstellar medium.