Low-Energy Electron Impact Fragmentations of the Polycyclic Aromatic Hydrocarbon Anthracene

The aim of the study undertaken for this Masters of Science thesis was to investigate the fragmentation processes induced by low-energy electron impact with anthracene. This research is relevant in the study of low-energy electron interactions with molecules in gaseous environments, including potentially in the interstellar medium where secondary electrons may be produced by UV-irradiation of interstellar dust grains. An effusive, molecular beam of anthracene, generated in a resistively heated oven, is crossed with a beam of low-energy electrons to form positively charged fragments. A reflectron time-of-flight mass spectrometer with a microchannel plate detector is used to mass-resolve and detect the fragment ions. A multichannel scaler card is used for acquiring single mass spectra. Data acquisition is controlled by LabVIEW code, which ramps the electron impact energy from 0 to 100 eV in 0.5 eV steps, acquires mass spectra as a function of electron impact energy, and adds each mass spectrum to the data that has already been accumulated. The full data set consists of a two-dimensional array of ion yield as a function of time-of-flight and of electron impact energy. Ion yield curves of most of the fragment ions have been determined by fitting groups of adjacent peaks in the mass spectra with sequences of normalized Gaussians. This is done using in-house LabVIEW software. The appearance energies for these fragments have been determined. The groups of fragments containing 8 to 13 carbon atoms provide evidence for hydrogen rearrangements during fragmentation, involving the retention or loss of one or two additional hydrogen atoms. Groups of fragments with 6 and 7 carbon atoms clearly show the presence of doubly-charged fragments. We see broadened peaks in the lower-mass groups with 1 to 4 carbon atoms, which may be due to energetic charge-separation fragmentations of doubly-charged anthracene