Michael Gatchell (Stockholm University, Department of Physics)
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.