Carbon, hydrogen, oxygen, and nitrogen are the most common elements necessary for life and in combination create a multitude of molecules. These include highly reactive species with complicated electronic structures, such as oxygenated hydrocarbon radicals and nitrogen oxides. These short-lived species can be important in atmospheric and combustion processes, but difficult to characterize due to their transient nature. Photoelectron-photofragment coincidence spectroscopy is a technique designed to study short-lived species thought photodetachment of the corresponding stable anion and measure the energy partitioning within the neutral species.

The oxyallyl anion was photodetached to the nearly degenerate states, 3B2 and 1A1 as well the 3B1 excited state, which were correlated to predominately stable C3H4O neutral. The photoelectron spectrum of the oxyallyl diradical and the acetone enolate radical, C3H5O, are the correct assignment for the C2O2 and HC2O2 photoelectron spectra reported in the literature. The oxyallyl diradical was observed to have a minor dissociation channel, at both Ehν = 3.20 eV and 1.60 eV. At Ehν = 3.20 eV, photodetachment to the 1A1 state is accessible, and results in dissociation to CO + C2H4. Dissociation was also observed from the triplet states, occurring after intersystem crossing. This system is further complicated by the observation of dipole bound states. The 1A1 state has a zwitterionic character resulting in observation of photodetachment ~0.3 eV below threshold. A stable neutral was the result of delayed photoemission during cyclization, as well as dissociative photodetachment resulting from a two photon process were observed.

The complicated electronic structure of nitrogen oxides, N2O2 and NO3 exhibit photodissociation pathways due the several neutral dissociation channels accessible at Ehν = 3.20 eV. For N2O2, photodissociation resulted in vibrationally excited NO anions which undergoes autodetachment, as well as photodissociation of NOˉ (ν=0). Photodissociation was also observed to result in N2O + Oˉ which undergoes sequential photodetachment. The peroxynitrite anion, a high energy isomer of the nitrate anion, was also observed to produce similar photodissociation pathways. Vibrationally excited NOˉ followed by autodetachment was observed, as well as the observation of sequential photodetachment of O2ˉ via a two-photon process.

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.

The hydroxyl radical plays a critical role in several contexts including combustion, atmospheric chemistry, and low-temperature extraterrestrial environments. OH has been observed in the interstellar medium and is important in the gas phase chemistry of interstellar clouds. Additionally, the hydroxyl radical is the dominant oxidizer of volatile organic compounds in the atmosphere. Here, the reactions between OH and simple alkanes, alkenes, and alcohols were investigated as these bimolecular reactions can expand fundamental understanding of gas phase reaction dynamics. Photoelectron-photofragment coincidence (PPC) spectroscopy was used to characterize the energetics and dynamics of the OH + CH4 → H2O + CH3 reaction initiated by photodetachment of the OH−(CH4) anion complex. PPC measurements at a photon energy of 3.20 eV yielded stable and dissociative channels. Interpretation of the experimental results was supported by quantum chemistry and quasiclassical trajectory calculations showing that most of the trajectories yield slowly recoiling OH + CH4 reactants while some are trapped in the entrance channel van der Waals well. Extending the study to hydroxy radical reactions with the simplest alkene, ethylene, yielded similar dissociative photodetachment dynamics (DPD) to PPC studies on OH−(CH4). The dominant channel was DPD to OH + C2H4 + e− with a low kinetic energy release (KER) of the dissociating fragments, consistent with weak repulsion between the OH + C2H4 reactants near the transition state. To model the DPD process, an impulsive model was used to account for rotational energy partitioning in C2H4 photofragments and showed good agreement with the experimental results. PPC studies probing the OH + CH3OH → H2O + CH3O reaction had the best Franck-Condon overlap with the exit channel. High-level coupled cluster calculations of the stationary points on the anion surface show that CH3O−(H2O) is the stable minimum. Photodetachment leads to H2O + CH3O + e− products with both direct dissociative photodetachment as well as resonance mediated processes observed on the neutral surface. The partitioning of total kinetic energy in the system indicates that water stretch and bend excitation is induced in DPD, and evidence for long-lived complexes consistent with production of vibrational Feshbach resonances in the exit channel is reported.

Atmospheric aerosols are ubiquitous, and they have substantial impacts on climate and human health thus it is important to understand their physical and chemical transformation. However, physicochemical properties of aerosols are poorly understood due to the complexity of the system and difficulties in experiment. To accurately simulate the atmospheric aerosols in the laboratory environment, particle levitation and non-destructive probing are crucial to monitor the change in physicochemical properties in real-time without contact. An environment-controlled mobile electrodynamic balance (EDB) coupled with dual-beam laser spectroscopy setup was developed, achieving stable trapping of single droplet over a long period of time (~72 hours) and providing information on size, phase, and chemical composition of trapped droplet by Mie scattering imaging (MSI) and Raman spectroscopy.The implementation of the custom-built EDB apparatus revealed new findings about the physical and chemical evolution of aerosol droplets. First, kinetic limitation of water diffusion was observed by the measurements of water diffusion coefficients in aqueous sucrose and aqueous citric acid droplets. The rates of water diffusion in single levitated droplets over a wide range of relative humidity (RH) were directly measured by H2O/D2O isotopic exchange. Although sucrose droplets show slower rate of water diffusion, the incomplete exchange of H2O to D2O was only observed for citric acid droplets. Second, the evolution of hydrogen bond interactions in sucrose and citric acid droplets versus RH was thoroughly investigated by 2D correlation analysis of Raman spectra. It was found that the hydrophilic functional groups govern the structures of hydrogen bond interactions with water, especially at lower RH conditions where the droplets experience metastable states. Next, the accelerated keto-enol tautomerization kinetics of malonic acid were quantized by spectroscopically monitoring the rate of C-H to C-D exchange at α-carbon. Lastly, reactions of nitrate anion (NO3ˉ) by the 266 nm photoexcitation of sulfanilic acid were investigated in single acidic droplets. Absorption-emission analysis supported by computational calculations of excitation energies suggest that the triplet-triplet energy transfer from sulfanilic acid triggers the reactions of nitrate. The experimental results and analysis presented in this thesis can improve the fundamental understanding of physical and chemical transformations of atmospheric aerosols.

Small particle interactions with surfaces dominate processes from industrial manufacturing to micrometeorite impacts in space. For particles that are optically visible a wide range of modeling and studies exist to characterize the mechanical behavior and material properties that govern these interactions. The properties of materials at smaller scales, below microns in size, can differ from bulk measurements and the body of literature examining these smaller interactions lacks the same variety of experimental data. Presented herein is a new tool, the Aerosol Impact Spectrometer (AIS), for the characterization of submicron particles impacting on surfaces.

The AIS is composed of a particles source, particle mass spectrometer, linear accelerator, impact target, and post impact detectors. Highly charged particles are generated using an electrospray ionization source before being injected into vacuum through an aerodynamic lens. A subset of the particle beam is selected using a quadrupole deflector and individual particles are injected into a linear electrostatic trap. As particles oscillate in the trap charge detection mass spectrometry is performed to determine individual particle mass and charge before being ejection into a linear accelerator. A series of high voltage elements either accelerates or decelerates each particle to a select final energy before impacting a target surface. The subsequent behavior of the particle is examined with a variety of charge sensitive detectors including measurements of particle sticking and rebound velocity.

The rebound velocities over a wide range of initial energies has been demonstrated using polystyrene latex spheres, tin metal particles, and frozen water-ice. The behavior of each species is distinct and further analysis of the tin metal particles shows behavior consistent with non-bulk material properties of the metal. Evidence for water ice melting and fragmentation have also been observed with high impact energies. Finally, the techniques employed to examine particle behavior after impact have also been applied to examine durability in freestanding ultrathin films under particle bombardment and measure the efficacy of large particle detection using a microchannel plate detector.

Transient species play a central role in governing the chemical dynamics of reactions, but are difficult to study due to their short lifetime and high reactivity. By using kinematically complete techniques such as photoelectron-photofragment coincidence (PPC) spectroscopy, characteristics such as energetics and dissociative pathways for transient species can be studied. One major limitation of such a technique is limited knowledge of the internal energy of the precursor ion prior to photoexcitation. While for some species, this information is less critical and can be inferred from combining theoretical calculations with experimental observations, other species are much more ambiguous.

One example is the tert-butoxy radical (CH3)3CO which is a reaction intermediate in the combustion of tert-butyl alcohol, a candidate for alternative fuels. Due to the lack of α hydrogens with respect to the radical center leading to a low reactivity with O2 as well as a large barrier to isomerization, unimolecular decomposition as the primary gas phase reaction pathway for this radical. Due to the method of ion selection being by mass, isomers of the tert-butoxy radical were taken into consideration for determining the dissociation dynamics observed. Ab initio calculations found that the dissociation of highly vibrationally excited tert-butoxy to methyl radical + acetone products is energetically very similar to theoretical ab initio calculations for the dissociative dynamics of its carbanion isomer ((CH3)2C(CH2)OH¯) to methyl radical + propen-2-ol. Utilizing Franck-Condon simulations the observed dissociation dynamics was assigned to the photoexcitation of highly vibrationally excited tert-butoxide anion precursor.

While efforts to generate cold precursor ions to photoelectron photofragment coincidence spectroscopy have been carried out through the installation of an electrostatic ion beam trap, studies on tert-butoxy radical have shown that high frequency vibrations are not sufficiently cooled through supersonic expansion alone. To address this issue, the source for the PPC spectrometer has been redesigned to include a Wiley McLaren style ion extraction coupled with a cryogenic octopole accumulation trap (COAT) to allow for the preparation of both hot and cold of ions for PPC spectroscopy. The demonstration of the effectiveness of COAT is demonstrated through the measurement of the dissociation dynamics of ozonide (O3−). Photoexcitation of O3ˉ at 3.20 eV results in a stable channel and two dissociative pathways, Oˉ + O2 and O + O2ˉ. The O + O2ˉ channel in particular is observed as an autodetachment channel where an electron in O2ˉ autodetaches when v > 3. At hv = 3.20 eV, the product O2 (v > 3) is only energetically accessible from vibrationally excited O3ˉ making this channel particularly sensitive to vibrational excitation of the precursor ion, O3ˉ. Utilizing different trapping times and temperatures for COAT, the intensity of the autodetachment channel demonstrates the ability to generate not only vibrationally cold ions, but hot ions as well. Additionally, it was found that the autodetached electrons from O2ˉ(v″ = 4) exhibits resolved features consistent with bend (ν2), asymmetric stretch (ν3) and a stretching combination band (ν1 + ν3) in the intermediate electronic state.

As the simplest carboxyl free radical the formyloxyl radical, HCO2, serves as a prototype for more complex carboxyl free radicals such as CH3CO2, C6H5CO2 and HC2CO2. It is also an intermediate in the OH + CO ⟶ H + CO2 reaction, of increasing interest for both atmospheric and combustion chemistry. The predissociation dynamics for the DCO2¯ + h𝜈 → D + CO2 + e¯ reaction were studied by DPD of DCO2¯. Photodetachment near threshold (hν = 4.27 eV) accesses all three of the lowest-lying electronic states (2B2, 2A1, and 2A2) of the formyloxyl radical. The photoelectron spectrum indicates vibrational excitation dominated by the O–C–O bending mode in the 2A1. In the PPC spectra for DCO2¯, each vibrational state of the nascent neutral is coupled to bending excitation in the resulting CO2 product observed in the eKE-gated state-resolved translational energy distribution. Each CO2 product-state distribution exhibits a substantial level of bending excitation, more than predicted by recent quantum-dynamics calculations on an ab initio potential energy surface.

Extending our work on carboxyl free radicals, the dissociation dynamics of the propiolyl radical, HCCCO2, and its related isomer CCCO2H were studied by DPD of their respective anions using PPC spectroscopy. At hν = 4.27 eV, the HCCCO2¯ anions is found to readily dissociate to HCC + CO2. Similar to the formyloxyl radical, dissociation to HCC + CO2 results in a series of resolved peaks in the translational energy release spectrum corresponding to the O–C–O bending mode in the CO2 fragment. The isomeric CCCO2H were not observed. High level ab initio calculations and Franck-Condon simulations were carried out to reproduce the stable photoelectron spectrum.

Extending the recent work on the F + H2O benchmark F initiated hydrogen abstraction reaction, the effects of vibrational excitation of the precursor anion was explored. To carry out this experiment, an IR laser was coupled into the existing PPC spectrometer using a mirror on the existing neutral beam block. Excitation of the internal hydrogen bond F—H—OH of the anion causes a noticeable change in dissociation dynamics, increasing access to the HF + OH products. Finally, the effects of intramolecular vibrational relaxation of the excited anions was explored by looking at how the anions aged during confinement in the electrostatic ion beam trap.

Finally, the F + HOCH3 → HF + OCH3 reaction was studied; expanding our work on F initiated hydrogen abstraction reactions. The HF product is formed with considerable excitation while the OCH3 product behaves as a spectator to the reaction. Both the metastable reactant channel van der Waals complex and stable product hydrogen bonded complexes were observed, along with a series of long-lived metastable vibrational Feshbach resonances.

Aerosols are ubiquitous in the atmosphere and play a major role in balancing the incoming solar radiation directly through scattering and absorption, and indirectly by acting as seeds to nucleate clouds. The ability of an aerosol to uptake water is fundamental to the condensational growth necessary for cloud formation, and additionally alters aerosol properties such as concentration, density, refractive index, and viscosity. The growth rate under particular relative humidity is dependent on the mobility of water molecules in the aerosol, reported as the water diffusion coefficient, and becomes extremely slow at high viscosity.

Ocean-derived sea spray aerosol (SSA) is one of the most abundant natural sources of atmospheric aerosol and exhibits complex chemical composition. SSA contains both inorganic and organic compounds, including inorganic salts (NaCl and CaCl2) and saccharides. A number of recent studies revealed that model SSA organic compounds can achieve high-viscosity, semi-solid phase states at low humidity. The relationship between viscosity and diffusion based on the Stokes-Einstein (S-E) relation is used to estimate water diffusion coefficients in aerosol because of the supersaturated concentrations not accessible to bulk diffusion techniques. However, the S-E relation has been shown to under-predict water mobility by several orders of magnitude using recently developed single aerosol diffusion techniques. Only a few methods currently exist for measuring diffusion in single aerosol, limiting the database of concentration-dependent water diffusion coefficients necessary for improving the parameterization between viscosity and diffusion. In this thesis, a new method is presented for directly measuring the diffusion coefficient of water that applies the Raman spectroscopic isotope tracing method to single, charged viscous aerosols trapped in a newly built electrodynamic balance (EDB) under controlled humidity. The EDB Raman isotope tracing method is validated by comparing results of the concentration-dependent water diffusion coefficient using sucrose as a model system. We then apply the method to directly measure the water diffusion coefficient in ternary sucrose-NaCl and sucroseCaCl2 mixtures for the first time, and model the results with a Vignes-type diffusion model. Further application of the EDB isotope tracing method will continue to clarify the role of chemical composition on aerosol physical properties.