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Abstract: Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous in the interstellar medium (ISM). However, their formation mechanisms and abundances are not well known. Astrochemical models are largely based on high-temperature combustion chemistry models that are unable to correctly explain PAH abundances in the ISM. Hence, alternate formation mechanisms in addition to spectroscopic measurements of PAHs are required. In this thesis, a barrierless formation mechanism for the production of the phenalenyl radical is discovered for the first time in a jet-cooled molecular beam, formed from the electrical discharge of acenaphthylene and methane. This leads to a thorough investigation of the spectroscopic properties of phenalenyl using mass-selective, multi-photon ionization techniques. Results are combined with high-level electronic structure theory that assist in their interpretation. Resonant ionization and isotopic labelling techniques show that the CH cycloaddition mechanism can convert the five-membered ring of acenaphthylene to a six-membered ring. An excitation spectrum for the phenalenyl radical is recorded across an energy range of 18350?35000 cm?1 , spanning transitions to five different excited states which are ascribed to phenalenyl through three-laser hole-burning spectroscopy. A vibronic Hamiltonian containing Jahn-Teller and pseudo-Jahn-Teller coupling gradients between the excited states of the phenalenyl radical is constructed to simulate the observed excitation spectrum at the EOMEE-CCSD level of theory. This allows for additional assignments of the D1 ← D0 transition, and a complete assignment of the D3 ← D0 transition, to be made. The ionization energy for phenalenyl is measured to be 6.496(3) eV in the absence of an electric field through pulsed grid ionization techniques. High-level calculations using the CCSD method are conducted to benchmark the accuracy of several different basis sets in predicting the ionization energy of open-shell PAH radicals based on this experimental value. The excited state lifetimes for the D1, D3 and D4 states are measured as 341 ± 14 ns, 172 ± 5 ns and 108 ± 3 ns, respectively. The applicability of the CH cycloaddition mechanism to PAH formation is extended to the formation of naphthalene from indene and methane within an electrical discharge using isotopic labelling techniques and a jet-cooled molecular beam. A spectroscopic investigation of the discharge products of an azulene and naphthalene containing molecular beam is also conducted, and the identification of 1- and 2-methylnaphthalene, 1- and 2-naphthylmethyl radicals and phenylcyclopentadiene is achieved. This thesis and the results therein represent a combined experimental and theoretical effort to investigate the formation mechanisms and abundances of PAHs in the interstellar medium. The newly identified CH cycloaddition mechanism converting a fivemembered to six-membered ring of a PAH may help to improve the accuracy of astrochemical models by addressing current deficiencies. The demonstration of vibronic coupling calculations to simulate spectra will assist in the observation of PAHs in the ISM and aid in more accurate predictions of their abundance.