Isomer selective interrogation of combustion and atmospheric intermediates

University Of The Pacific, Stockton CA

Investigators

Abstract

In this project funded by the Chemical Structure Dynamics and Mechanism-A (CSDM-A) program of the Chemistry Division, Professor Bálint Sztáray of the University of the Pacific is using a combination of mass spectrometry and photoelectron spectroscopy to study the chemical reactions of unstable and elusive molecules that are relevant to atmospheric chemistry. Mass spectrometry is a technique that separates molecules or atoms according to their mass and electrical charge. Photoelectron spectroscopy is a technique that uses light (typically from a laser) to eject electrons from atoms or molecules. The energy of the ejected electrons gives information about how strongly the electron was bound to the molecule. Professor Sztáray combines these two techniques to form photoelectron-photo-ion coincidence spectroscopy or "PEPICO", which provides a much more detailed molecular fingerprint, and enables the analysis of gaseous mixtures of many components. With a prototype instrument in Switzerland and with a new Sandia National Lab apparatus at the Advanced Light Source in Berkeley, reactions of unstable intermediate species, relevant in combustion, atmospheric, or interstellar environments are studied. The students involved in this project are receiving training in a cutting-edge experimental, highly collaborative environment. As the research in the Sztáray laboratory involves custom-built instruments, graduate student researchers learn unique design and fabrication skills which are highly valued in the high-tech sector. Graduate and undergraduate researchers also have opportunities to work with collaborators at US National Laboratories (Sandia and Lawrence Berkeley National Laboratories) as well as international facilities (Swiss Light Source). In PEPICO coincidence detection, each photoelectron is "labeled" with the mass-to-charge ratio (m/z) of the photoion it corresponds to, essentially combining the advantages of mass spectrometry and photoelectron spectroscopy. Using a side-sampled, pulsed-laser photolysis flow-tube reactor, the project focuses on the spectroscopy and thermochemistry of elusive intermediates, such as alkylperoxy and hydroperoxyalkyl radicals, ketohydroperoxides and Criegee intermediates. Utilizing the isomer selectivity of PEPICO detection, a systematic study is conducted on the reactions of small carbon-containing radicals with small hydrocarbons. Recombination reactions of propargyl, allyl, and benzyl radicals play a crucial role in molecular weight growth processes that ultimately lead to soot formation. Here, PEPICO detection offers a step forward in understanding these reactive pathways and providing isomer selective branching ratios and reaction rates. Ultimately, by identifying key intermediates, their reaction mechanisms, and rates, more complete combustion models can be constructed, leading to better predictions of combustion products, including gaseous and aerosol pollutants. These predictions can then aid in the development of low-emission engine technologies.