I am a scientist at Lawrence Berkeley National Laboratory (LBNL), Berkeley, CA, and my research is geared towards coupling novel instruments and approaches to synchrotron radiation to perform cutting edge work in chemical physics and physical chemistry. I did my doctoral work at University of Cambridge, England and was a postdoctoral research fellow at Universities of Leicester & Manchester in England and at the Max Planck Institute in Gottingen, Germany before joining LBNL in 1995. My focus is to understand and map the physical and chemical principles that govern complicated phenomena in nature. My goal is to gain a molecular level understanding of alternative carbon neutral energy sources and global climate change, which has led me to work in the fields of imaging and biological mass spectrometry, atmospheric and environmental chemistry, and dynamics of combustion processes. My current interests include: ultrafast photon science incorporating lasers with synchrotron radiation to probe charge and proton transfer dynamics in solvated systems, correlated imaging mass spectrometry/fluorescence microscopy platforms for microbiology and developing new spectroscopic and imaging tools to probe dynamics.

I am the program lead at the Chemical Dynamics Beamline located at the Advanced Light Source in Berkeley CA, within Lawrence Berkeley National Laboratory. The beamline provides one of the world's brightest sources of vacuum ultraviolet (VUV) radiation. This coupled to experimental end-stations supports a variety of projects relevant to combustion science, chemical dynamics, electronic structure, aerosol and heterogeneous chemistry and analytical mass spectrometry.

Electronic Structure, Proton Transfer and Solvation in the Building Blocks of Life.

The molecular structures of many biological molecules are well-known, in light of half century of research that has elapsed since Watson and Crick’s original postulation for the structure of DNA. However, fundamental aspects, such as the electronic structure of these building blocks of life (adenine, thymine, cytosine, guanine)  are not yet well-understood, and are being probed by vacuum ultraviolet photoionization and high level state-of-the-art ab initio calculations. 

This collaboration with Anna Krylov's group at USC have been intellectually rewarding and have taken our experimental work to a very sophisticated level. The photoionization dynamics of gas-phase DNA bases were investigated yielding the first experimental measurements of the ionization energies of their dimers. In conjunction with theory, an unprecedented insight into the effect of non-covalent interactions, i.e., hydrogen bonding, stacking, and electrostatic interactions, on the ionization energies of the individual bases was obtained. The nature of proton transfer in pi-stacked DNA base dimers is being investigated. The conventional wisdom is that proton transfer occurs through the network of hydrogen bonding - however, we have shown that it is possible to transfer protons efficiently through stacks of methylated nucleic acid bases (when hydrogen bonding is absent) upon ionization. This occurs due to changes in the geometry of the ionized stacked dimer, making proton transfer processes facile,  and has profound implications for ionization processes in biological systems. 

Using similar electronic structure theory and photoionization methods, the micro-solvation of DNA bases and small water clusters are being investigated. The ionization of water clusters is of paramount interest in a number of fields. Understanding the changes in electronic structure that occur in these ionic clusters is critical to unveil their structure and function, and is important in fields as diverse as cloud nucleation in earth’s atmosphere, radiation biology, and interstellar chemistry.  The detection of non-protonated water clusters enclosed in a nano-shell of argon has allowed for exciton and charge transfer processes to be examined in exquisite detail. These photoionization studies performed for the first time at any synchrotron provide a firm foundation on which to build the next generation of pump probe experiments using high harmonic generation (HHG) and free electron lasers.  Recent experiments have focused on following the proton transfer processes in water wires and solvated biomolecules.

A High Temperature Chemical Reactor to Probe Biofuel Combustion and Soot Formation.

In collaboration with the Kaiser group from Hawaii, we are investigating the formation mechanisms of po­ly­cyclic aromatic hydrocarbons (PAHs) with indene and naphthalene cores in hydrocarbon-based combustion processes. This will be achieved by simulating the combustion relevant conditions (pressure, tempe­ra­tu­re, re­ac­tant molecules) in a high temperature ‘chemical reac­tor’.  Recent work has involved studying the branching ratios of the reaction of phenyl radicals formed in a supersonic expansion with a number of hydrocarbons (allene, methyl­ace­ty­lene, propylene, diace­ty­lene , vinyl­ace­tylene, and 1,3-buta­di­ene. Future work will seek to decipher the oxidation pathways of these reactions.

With Barney Ellison (Colorado), John Stanton (U Texas, Austin), and John Daily (Colorado) and their crew of merry men and women we seek to understand the unimolecular decomposition of molecules relevant to biofuel combustion. Renewable biofuels are promising candidates for the transportation needs of the 21st century. One of the bottlenecks in processing biofuels from plants is the natural recalcitrance of these materials toward chemical transformation. Fundamental processes that limit this technology, and must be addressed in the future, are the relatively slow kinetics of breaking down pure cellulose into sugars, the low yields of sugar from plant polysaccharides and the removal of lignin. A molecular level understanding of the kinetics and mechanism of biomass transformation is required to facilitate the implementation of thermal, chemical or biochemical catalysts for efficient recovery of fuels from biomass. The proposed studies will represent a major advance over the largely reductionist approaches used to study these processes in the past.  There is also considerable overlap with the imaging mass spectrometry and electronic structure of biomolecules programs. For instance, the photoionization studies of lignin monomers (coniferyl alcohol) and a sugar (deoxyribose) provide the framework for studying  complicated biological polymers relevant to biofuel production. 

Spectroscopy, Astrophysics and Cosmochemistry

A laser ablation apparatus coupled to the beamline has allowed for the generation of a number of chemical species relevant to interstellar and planetary chemistry. These include molecules such as Silicon Dioxide, Silicon Clusters, Silicon Carbides, and Silicon Carbon Hydride Clusters, Carbon Nitrogen Clusters, and Polyacetylenes. The origins of the Solar System was studied via photodissociation of carbon monoxide, and hydrogen sulfide, while the ionization of nitrogen has implications for the chemical composition of distant Titan.