Production and Photodissociation of Neutral Free Radicals

The primary photochemistry of several combustion-relevant free radicals have been in- vestigated via photofragment translational spectroscopy. The relevance of radical photo- chemistry will be discussed, along with methodologies and details of each experiment. The experimental apparatus will also be described, especially with regard to the recent installa- tion of a tunable energy electron ionizer. The upgraded ionizer has been a significant advance, allowing for more detailed characterization of the radical source employed in this thesis.The photochemistry of the phenyl radical (c-C6H5), a combustion intermediate and pre- cursor to polycyclic aromatic hydrocarbons, was investigated at 248 and 193 nm. At 248 nm, an H-atom loss pathway was found, while at 193 nm both H-atom loss and C2H2 loss pathways were observed. For both wavelengths, P(ET) distributions suggested internal con- version to the ground electronic state followed by energy randomization and dissociation. The branching ratio between the two 193 nm dissociation pathways was found to be 0.2 ± 0.1 in favor of H-atom loss, in good agreement with statistical Rice–Rampsperger–Kassel–Marcus (RRKM) theory.An initial investigation of the methyl perthiyl radical (CH3SS) at 248 nm suggested the surprising results of both CH3 + SS and CH2S + SH dissociation channels with no evidence for S-atom loss. In both cases, the translational energy distributions were inconsistent with the expected energetics. Upon reinvestigation, the assumption of radical production—and there- fore radical photodissociation—was shown to be incorrect. The new results demonstrated S-loss and CH3 loss pathways, with the former appearing to involve a repulsive electronic excited state

FDDYNAM16 Towards elucidating the photochemistry of the sunscreen filter ethyl ferulate using time-resolved gas-phase spectroscopy

Ultrafast time-resolved ion yield (TR-IY) and velocity map imaging spectroscopies are employed to reveal the relaxation dynamics after photoexcitation in ethyl 4-hydroxy-3-methoxycinnamate (ethyl ferulate, EF), an active ingredient in commercially available sunscreens. In keeping with a bottom-up strategy, the building blocks of EF, 2-methoxy-4-vinylphenol (MVP) and 4-hydroxy-3-methoxycinnamyl alcohol (coniferyl alcohol, ConA), were also studied to assist in our understanding of the dynamics of EF as we build up in molecular complexity. In contrast to the excited state dynamics of MVP and ConA, which are described by a single time constant (>900 ps), the dynamics of EF are described by three time constants (15 ± 4 ps, 148 ± 47 ps, and >900 ps). A mechanism is proposed involving internal conversion (IC) between the initially excited S1(11ππ*) and S2(11nπ*) states followed by intramolecular vibrational redistribution (IVR) on both states, in competition with intersystem crossing onto neighbouring triplet states (15 ± 4 ps). IVR and IC within the triplet manifold then ensues (148 ± 47 ps) to populate a low-lying triplet state (>900 ps). Importantly, the fluorescence spectrum of EF at the S1 origin, along with the associated lifetime (6.9 ± 0.1 ns), suggests that population is trapped, during initial IVR, on the S1(11ππ*) state. This serves to demonstrate the complex, competing dynamics in this sunscreen filter molecule