Events & Seminars > Event Details


4:00 pm
Room 304, Chemistry Building

Exploring uncharted regions of atmospheric reaction pathways


Professor Marsha Lester
University of Pennsylvania

Hosted by: Professor Eric Herbst

Alkene ozonolysis is a primary oxidation pathway for alkenes emitted into the troposphere and also an important source of atmospheric hydroxyl radicals. Alkene ozonolysis takes place on a reaction path with multiple minima and barriers along the way to OH products. In particular, a key reaction intermediate, known as the Criegee intermediate, R1R2COO, had eluded detection until very recently. In this laboratory, the simplest Criegee intermediate, CH2OO, and methyl-substituted Criegee intermediates, CH3CHOO and (CH3)2COO, have now been generated by an alternative synthetic route, detected by VUV photoionization, and characterized on a strong p*¬p transition. Most recently, our studies have focused on vibrational activation of methyl-substituted Criegee intermediates in the vicinity of the barrier for 1,4 hydrogen transfer that leads to OH products. The experiments reveal infrared transitions in the CH stretch overtone region that initiate unimolecular decay as well as the rate of the appearance of OH products through direct time-domain measurements. Comparison with high level theory shows that tunneling through the barrier makes a significant contribution to the decay rate. The dissociation dynamics are also examined through the translational and internal energy distributions of the OH products, which reflect critical configurations along the reaction pathway from the barrier for hydrogen transfer to OH products. Finally, the results will be extended to thermally averaged unimolecular decay of stabilized Criegee intermediates under atmospheric conditions.


Marsha I. Lester has risen through the academic ranks at the University of Pennsylvania, where she is currently the Edmund J. Kahn Distinguished Professor in the Department of Chemistry of the School of Arts & Sciences. She completed a four-year term as Chair of the Department of Chemistry in 2009. Lester has received many honors and awards, including her election to Fellowship in the American Academy of Arts & Sciences, the Garvan-Olin Medal of the American Chemical Society, the Bourke Lectureship of the Faraday Division of the Royal Society of Chemistry, a John Simon Guggenheim Memorial Foundation Fellowship, Fellow of the American Association for the Advancement of Science, the American Chemical Society, and the American Physical Society, an Alfred P. Sloan Research Fellowship, and the Dreyfus Teacher-Scholar Award.

In 2008, Lester was appointed Editor of The Journal of Chemical Physics, the preeminent journal in her field. In the past seven years, she has reinvigorated the Journal with numerous initiatives to attract more of the best papers in the broadly defined field of chemical physics. Lester has consistently devoted a substantial amount of her time to scientific service activities. She has served on the NRC Commission on Physical Sciences, Mathematics, and Applications as well as its Board on Chemical Sciences and Technology. She has been engaged in activities of the American Chemical Society, the American Physical Society, the National Science Foundation, and the Department of Energy. She is also a founding member of the Penn Forum for Women Faculty.

Lester’s research group has developed innovative methods for stabilizing ‘entrance channel complexes’ and reaction intermediates of environmental significance. Her group has employed novel spectroscopic methods to rigorously characterize these important, yet previously uncharted, regions of chemical reaction pathways.