The role of knockout driven fragmentation in collision with isolated complex molecular systems

Abstract: This licentiate thesis covers results from collision induced dissociation (CID) experiments of Polycyclic Aromatic Hydrocarbons (PAHs), hydrogentated PAHs (HPAHs) and porphyrins. PAHs and HPAHs are believed to play key roles in the evolution of complex molecules in space, while porphyrins are essential for various functions in living organisms. The experiments were carried out in the Electrospray Ion Source Laboratory (EIS-LAB) at the Double ElectroStatic Ion-Ring ExpEriment (DESIREE) infrastructure, Stockholm University.  In these studies, molecular ions collide with neutral targets at center-of-mass energies from a few tens to a few hundreds of eV. In this energy regime, fragmentation is mainly driven by nuclear scattering processes (nuclear stopping) rather than by interactions with the molecular electronic clouds (electronic stopping). Thus, knockout of single heavy atoms from the molecule in prompt billiard-ball type collision processes may occur in competition with statistical fragmentation processes in which the excitation energy is redistributed across all degrees of freedom before the decay. In this thesis, we have investigated the importance of knockout driven fragmentation processes in a series of experiments with markedly different molecular structures ranging from pure sp2-hybridized to pure sp3-hybridized systems. The experiments are interpreted in view of molecular structure and molecular dynamics simulations. These studies show that knockout processes are important for any molecular system, but they are in general difficult to isolate by means of mass spectrometric techniques. We have seen clear fingerprints of single carbon-atom loss in the collision experiments for PAHs and porphyrins. In contrast, in the case of the hydrogenated PAHs the fragmentation is dominated by the statistical loss of single carbon atoms due to their lower dissociation energy. We further report the threshold displacement energy for PAH, hydrogenated PAHs and porphyrin molecules--the key intrinsic property determining the cross section for prompt atom knockout.