Scientists at the Chemical Dynamics Beamline of the Advanced Light Source in Berkeley are continuously reinventing synchrotron investigations of physical chemistry and chemical physics with vacuum ultraviolet light. One of the unique aspects of a synchrotron for chemical physics research is the widely tunable vacuum ultraviolet light that permits threshold ionization of large molecules with minimal fragmentation. This provides novel opportunities to assess molecular energetics and reaction mechanisms, even beyond simple gas phase molecules. In this perspective, significant new directions utilizing the capabilities at the Chemical Dynamics Beamline are presented, along with an outlook for future synchrotron and free electron laser science in chemical dynamics. Among the established and emerging fields of investigations are cluster and biological molecule spectroscopy and structure, combustion flame chemistry mechanisms, radical kinetics and product isomer dynamics, aerosol heterogeneous chemistry, planetary and interstellar chemistry, and secondary neutral ion-beam desorption imaging of biological matter and materials chemistry.

Femtosecond x-ray transient absorption spectroscopy is used in conjunction with selective bond breaking to characterize the electronic structures of substituted radicals. By utilizing the A band photodissociation of iodides, a series of carbon-centered radicals is produced and their frontier molecular orbitals are probed via core-level transitions by x-ray photons produced through high harmonic generation, whose limit has been extended to 390 eV with the apparatus detailed in the second chapter.

Two types of substituted radicals are studied in this work to demonstrate substitution effects: halogenated radicals and alkyl substituted radicals. The third chapter focuses on the halogenated radicals, in particular ·CH2Cl and ·CH2Br, which are produced under ultraviolet radiation of CH2ICl and CH2IBr, respectively. The ·CH2Cl radical is probed via the carbon K edge and the chlorine L2,3 edges to reveal the electron withdrawing effect impacting different atoms in the photodissociation of CH2ICl. Core-level transitions in ·CH2Br are studied in comparison to ·CH3 and ·CH2Cl to understand possible stabilization of the frontier molecular orbital by π bonding and the strength of such stabilization. The fourth chapter turns to the alkyl substituted radicals, where the methyl, ethyl, isopropyl and tert-butyl radical are produced from methyl, ethyl, isopropyl and tert-butyl iodide, respectively. While the characteristic carbon 1s→σ*(C-I) transition takes place in the parent iodides at similar energies, the transition from carbon 1s to the frontier singly occupied molecular orbital differs drastically in the radicals. This can be explained by either a significant change in the carbon 1s orbital of the radicals because of methyl substitution or complications due to vibrational excitation. The results point to new directions for core-level theoretical investigations.

Understanding the heterogeneous oxidation of organic particulate matter ("aerosol") is an active area of current research in atmospheric and combustion chemistry. The chemical evolution of organic aerosol is complex and dynamic since it can undergo multiple oxidation reactions with gas phase oxidants to form a mixture of different generations of oxidation products that control the average aerosol mass and volatility. In many of these systems, hydrocarbon free radicals, formed by reaction with gas phase oxidants, play key roles as initiators, propagators and terminators of surface reactions. This dissertation presents a detailed study of the reaction kinetics and mechanisms of the heterogeneous oxidation of unsaturated organic aerosol, and aims to provide new molecular and mechanistic insights into the reaction pathways in heterogeneous organic aerosol oxidation.

The heterogeneous oxidation of unsaturated fatty acid (oleic acid C18H34O2, linoleic acid C18H32O2 and linolenic acid C18H30O2) aerosol by hydroxyl (OH) radicals is first studied in Chapter 2 to explore how surface OH addition reactions initiate chain reactions that rapidly transform the chemical composition of unsaturated organic aerosol. Oleic acid, linoleic acid and linolenic acid have the same linear C18 carbon backbone structure with one, two and three C=C double bonds, respectively. By studying carboxylic acids with different numbers of C=C double bonds, the role that multiple reactive sites plays in controlling reaction rates can be observed. The kinetic parameter of interest in these studies is the effective uptake coefficient, defined as the number of particle phase unsaturated fatty acid molecules reacted per OH-particle collision. The effective uptake coefficients for the unsaturated fatty acids are larger than unity, providing clear evidence for particle-phase secondary chain chemistry. The effective uptake coefficients for the unsaturated fatty acids decrease with increasing O2 concentration, indicating that O2 promotes chain termination in the unsaturated fatty acid reactions. The kinetics and products of squalene (a C30 branched alkene with 6 C=C double bonds) oxidation are compared to that of the unsaturated fatty acids in Chapters 3 and 4 to understand how molecular structure and chemical functionality influence reaction rates and mechanisms. The squalene effective uptake coefficient, which is also larger than one, is smaller than that of linoleic acid and linolenic acid despite the larger number of C=C double bonds in squalene. In contrast to the unsaturated fatty acids, the squalene effective uptake coefficient increases with O2 concentration, indicating that O2 promotes chain propagation in the squalene reaction. Elemental and product analysis of squalene aerosol shows that O2 promotes particle volatilization in the squalene reaction, suggesting that fragmentation reactions are important when O2 is present in the OH oxidation of branched unsaturated organic aerosol. In contrast, elemental and product analysis of linoleic acid aerosol shows that O2 does not influence the rate of particle volatilization in the linoleic acid reaction, suggesting that O2 does not alter the relative importance of fragmentation reactions in the OH oxidation of linear unsaturated organic aerosol.

Lastly, depending on the aerosol phase (e.g. solid and semi-solid) and the timescale for homogeneous mixing within the aerosol particle, the chemical composition may vary spatially within an aerosol particle. This necessitates the need for new techniques to characterize the interfacial chemical composition of aerosol particles. In the last portion of the dissertation, direct analysis in real time mass spectrometry (DART-MS) is used to analyze the surface chemical composition of nanometer-sized organic aerosol particles in real time at atmospheric pressure. By introducing a stream of aerosol particles in between the DART ionization source and the atmospheric pressure inlet of the mass spectrometer, the aerosol particles are exposed to a thermal flow of helium or nitrogen gas containing some fraction of metastable helium atoms or nitrogen molecules. In this configuration, the molecular constituents of organic aerosol particles are desorbed, ionized and detected with reduced molecular ion fragmentation, allowing for compositional identification. The reaction of ozone with sub-micron oleic acid particles is also measured to demonstrate the ability of DART-MS to identify products and quantify reaction rates in a heterogeneous reaction.

Nanostructured materials have emerged as new building blocks for future innovative devices. The size-tunable properties exhibited across a wide variety of nanostructures, ranging from quantum confinement to optical confinement, can have a direct impact on how efficiently energy is transferred, stored, and utilized. Two distinct types of nanostructured materials in different size regimes are described in this dissertation: Pt-tipped CdSe/CdS nanorods and ZnO tetrapods.

Charge separation dynamics are investigated in Pt-tipped and non-tipped CdSe (quantum dot)/CdS nanorods by ultrafast white light transient absorption spectroscopy. Charge transfer is observed from the CdS and CdSe regions, with evidence for electron transfer to the Pt tip occurring over both sub-ps and sub-100 ps timescales. By exciting the CdSe quantum dot directly at 550 nm and probing the quantum dot directly at 580 nm, electron transfer to the Pt tip is monitored from a local region of the nanorod that is separated from the CdS-Pt interface.

To investigate the lasing action in nanostructured cavities in detail, single ZnO tetrapod structures are isolated with a home-built confocal microscope and excited with a sub-ps UV pump pulse (267 nm). The lasing thresholds are observed to be in the range of ~50- 200 J/cm2. To characterize the Fabry-Pérot-type cavities further, micro-Raman studies are performed and rate equations for microcavity lasers are numerically solved to investigate the near-threshold lasing dynamics.