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.