UC San Diego

Small organic acids are ubiquitous in the atmosphere in both gaseous and aerosol phases. As a result, they are used in atmospheric models for volatile organic compounds, but their photochemistry is relatively unknown. This dissertation delves into the photochemical transformations induced by UV-A and UV-C irradiation of three organic acid anions. The application of photoelectron-photofragment coincidence (PPC) spectroscopy allows for the elucidation of photodissociation and dissociative photodetachment pathways of these anions, shedding light on the intricate photophysics that govern their behavior in the presence of solar radiation.

The energetics and dissociation dynamics of pyruvate, the simplest α-keto acid anion, were characterized via PPC spectroscopy at two photon energies and two accumulation trap temperatures. Measurements at hν = 4.66 eV at room temperature show two distinct pathways: a photodissociation + subsequent autodetachment pathway and a direct dissociative photodetachment (DPD) pathway. At hν = 3.49 eV, the primary channel is photodissociation + subsequent autodetachment, but a high-energy photoelectron is also observed. The high energy photoelectron is produced by photodetachment of an anionic product of the photodissociation by a second UV photon, and is believed to be indicative of a three-body dissociation. At liquid nitrogen temperatures, the distribution of the kinetic energy release in the photodissociation channel narrows, indicating parent ion vibrational cooling. Believing that the three-body dissociation is stepwise on a nanosecond time scale, a pump-probe experiment at hν = 3.49 eV was set up to induce photodetachment ~5 nanoseconds after initial photodissociation. The next most simple α-keto acid anion, α-ketobutyrate, was also studied at hν = 4.66 eV and 3.49 eV at room temperature. Similarly to pyruvate, photodissociation + autodetachment was the predominant channel at both photon energies. At hν = 4.66 eV, an analogous direct DPD pathway is accessed. At hν = 3.49 eV, the high-energy photoelectron is also observed. α-Ketobutyrate was studied at two beam energies to better understand the timescale of the high-energy photoelectron process. PPC spectroscopy was used to disentangle the contributions from direct DPD and ionic photodissociation for the first time.

Finally, gas-phase dissociation dynamics of the phenylacetic acid anion were studied forthe first time. PPC experiments on phenylacetate at hν = 4.66 eV were compared to previous PPC experiments of other, structurally similar, aromatic acid anions. C6H5CH2CO2− → C6H5CH2 + CO2 + e− was observed to be the only accessible dissociative photodetachment channel. There was no evidence observed of any other photodissociation or stable direct photodetachment pathway.