A Spectroscopic Investigation of the Phenalenyl Radical and its Formation

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 is 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 18350-35000cm-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 \textit{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 +- 14ns, 172 +- 5ns and 108+-3ns, 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