Fourier transform ion cyclotron resonance mass spectrometry investigation of the photolysis of polycyclic aromatic hydrocarbons

Of the many problems currently being tackled by astrophysicists, two are of particular interest to spectroscopists and molecular physicists. These are emission observed from numerous objects in the infrared region in the interstellar medium and the absorption seen in the near infrared and visible regions. It is generally accepted that the species giving rise to these spectral features are gaseous molecules (or radical or ions), and not dust particles or grains. Polycyclic aromatic hydrocarbons (PAHs) are one class of molecules which have been proposed as carriers of these features.;In this dissertation, the effect of visible/UV irradiation on several PAHs is studied in detail. FT ICR mass spectrometry was used to determine the photolysis products of the ionized PAHs fluorene, naphthalene and 1,2-benzanthracene in the vapor phase by irradiation with a xenon arc lamp. Products of ion-molecule reactions were also characterized for all three PAHs.;Broad-band visible and ultraviolet irradiation of ionized fluorene for relatively short times (∼0.5s) is shown to lead predominantly to sequential hydrogen atom loss, with one to five hydrogens lost. Rate constants and branching ratios for the photodecomposition of the fluorene parent ion and its dehydrogenated partners have been determined. Longer irradiation times lead to various products resulting from multiple CC bond cleavages.;Density functional calculations are used to show dehydrogenation first occurs from the sp3 carbon on the five membered ring and then from only one of the six-membered rings. Possible geometries have been calculated for all of the photoproducts.;Aggregates are shown to form from the fluorene fragments reacting with neutral molecules. These aggregates are fairly stable except for dehydrogenation and slight acetylene fragmentation.;Irradiation of the naphthalene ion for short times resulted in a maximum loss of two hydrogens. CH loss was evident as well as the sequential loss of acetylene. Aggregation of the fragmented ions was also common, including the possible formation of fluoranthene. The aggregate products of naphthalene were not as stable as those of fluorene, with many of them losing acetylene, as well as hydrogen, when irradiated.;The 1,2-benzanthracene ion was more resistant to photofragmentation than fluorene or naphthalene. Although it was capable of losing eight of its twelve hydrogens, the relative abundance of the photoproducts was only 20% after two seconds of irradiation. Sequential fragmentation of carbon atoms as well as acetylene loss was observed. The primary photoproducts did not form aggregates.