Interaction dynamics in collisions of ions with molecules and clusters

PhD ceremony: Mr. J.J.U. Postma, 16.15 uur, Doopsgezinde kerk, Oude Boteringestraat 33, Groningen

Dissertation: Interaction dynamics in collisions of ions with molecules and clusters

Promotor(s): prof. R.A. Hoekstra

Faculty: Mathematics and Natural Sciences

In the first part of this thesis a pioneering experimental study on (keV) ion interactions with polycyclic aromatic hydrocarbon molecules (PAHs) is presented. The obtained experimental data are necessary to obtain detailed insight into ion-PAH interaction processes occurring in many regions of interstellar space. Time-of-flight mass spectra for keV proton and alpha particle collisions with the polycyclic aromatic hydrocarbon anthracene (C14H10) are presented. An electronic stopping model, using realistic anthracene electron densities, is employed to estimate the molecular excitation due to the inelastic part of the interaction. To explore the sub-keV range of ion kinetic energies where elastic interactions dominate, a molecular dynamics simulation code was developed and employed. A systematic investigation of energy deposition as a function of impact site reveals that the conventionally applied binary collision model clearly overestimates direct fragmentation. In subsequent Monte Carlo type simulations, anthracene excitation energy distributions due to collisions with He projectiles were determined for a wide range of He kinetic energies. This allowed to estimate a probability for dissociation of the molecule in competition with non-destructive relaxation through Infra-Red (IR) photon emission. In the second part of the thesis a new apparatus was constructed and commissioned that allows for detailed kinematic studies of ions interacting with molecules and clusters. This setup entailed the development of a supersonic jet molecular beam source serving as a cold and localized target, to be crossed with a projectile beam, and a recoil ion momentum spectrometer (RIMS). Such a crossedbeam detector combination allows to determine a wealth of kinematic parameters involved in ion- PAH reactions.