Marsha I. Lester

Edmund J. Kahn Distinguished Professor and Department Chair
Physical Chemistry: Molecular Structure and Dynamics

Office: 262 T
Lab: 236- 39N
Phone: (215) 898-4640
Fax: (215) 573-2112
Email: milestersasupennedu

Jump to: Research Statement | Education and Academic History | Selected Publications

Research Statement
Our research combines new experimental and theoretical approaches to probe intermolecular potential energy surfaces between reactive partners. These potentials control the approach and recoil of molecules in both inelastic and reactive encounters. We have extensively studied intermolecular interactions and reactions involving the hydroxyl radical, which plays a critical role in combustion and atmospheric chemistry. Three recent highlights of our research are described below.

This laboratory obtained the first high-resolution spectrum of peroxynitrous acid (HOONO), a key intermediate in atmospheric chemistry. HOONO had been proposed as a secondary product of one of the most important reactions in the chemistry of the Earth's lower atmosphere, namely OH + NO2 → HONO2, which controls the conversion of reactive NO2 into chemically inactive nitric acid (HONO2). A significant production of HOONO is expected to have a profound impact on modeling of NOx chemistry in the lower atmosphere, including its role in the ozone budgets of both the troposphere and stratosphere. Our spectroscopic studies of HOONO in the OH overtone region provide the first definitive identification of the trans-perp (tp) conformer of HOONO. In addition, the OH overtone excitation imparts sufficient energy to dissociate tp-HOONO, allowing us to determine the HOONO bond energy and thereby estimate its yield (up to 20%) under atmospheric conditions. Further studies have identified transitions associated the cis-perp (cp) conformer of HOONO, providing insight on the complex torsional motion and conformational dynamics of HOONO when compared with theoretical predictions. Parallel studies on HONO2 with complementary theory have identified the three strongly coupled states involved in a Fermi resonance that initiate rapid intramolecular vibrational energy redistribution following OH overtone excitation.


The structure and stability (in kcal/mol) of peroxynitrous acid (HOONO), a secondary product of a key atmospheric reaction between OH and NO2, is characterized using infrared action spectroscopy. HOONO is identified in trans-perp (tp) and cis-perp (cp) configurations in a supersonic expansion, where OH overtone excitation provides sufficient energy to break the peroxide bond, enabling sensitive ultraviolet (UV) detection of the OH photofragments. The cis-cis (cc) conformer is not observed because overtone excitation falls short of the energy required for dissociation.

This laboratory has recently obtained the first infrared spectrum of the hydrogen trioxide (trans-HOOO) radical, an intermediate invoked in the H + O3 and O + HO2 atmospheric reactions as well as the efficient vibrational relaxation of OH radicals by O2. There had been much debate in the literature as to whether HOOO is stable or metastable with respect to the OH + O2 limit, as well as the relative stability of the cis and trans conformers. By measuring the OH product state distribution following IR excitation of HOOO, we have directly determined the stability of trans-HOOO and shown that is much greater than prior estimates. As a result, HOOO may act as temporary sink for OH radicals and be present in measurable concentrations in the Earth's atmosphere. The experimental stability indicates that up to 50% of the OH radicals in the vicinity of the tropopause may be bound to O2, rather than free OH radicals. Our work has continued with studies of HOOO in the fundamental OH stretch region, including combination bands that reveal information on low frequency torsional and bending modes, and analogous studies of DOOO.


IR action spectrum of cis- and trans-HOOO in the OH overtone region (left), and fraction of atmospheric OH predicted to exist as HOOO (right).

Collisional quenching of electronically excited OH A 2Σ+ radicals has been extensively investigated because of its impact on OH concentration measurements in atmospheric and combustion environments. Yet little is known about the outcome of these events, except that they facilitate the efficient removal of OH population from the excited A 2Σ+ electronic state by introducing nonradiative decay pathways. The quenching of OH A 2Σ+ by H2 and D2 has emerged as a benchmark system for studying the nonadiabatic processes that lead to quenching. Theoretical calculations indicate that a conical intersection funnels population from the excited to ground electronic surfaces. Previously, this laboratory observed bimodal Doppler profiles for the H/D-atom products of reactive quenching, which was attributed to direct and indirect dynamical pathways through the conical intersection region. Our recent work is focused on characterizing the nonreactive quenching process, where OH X 2Π products are generated with a remarkably high degree of rotational excitation and lambda-doublet specificity. The OH quantum state distribution directly reflects the anistropy and A′ symmetry of the conical intersection region. Surprisingly, we also find that reaction accounts for nearly 90% of the quenched products.


Quenching of electronically excited OH radicals proceeds through a conical intersection, leading to H + H2O and OH X 2Π + H2 products. Pump-probe laser methods are used to examine the outcomes of these reactive and nonreactive quenching processes, providing new dynamical signatures of the nonadiabatic process.

We gratefully acknowledge financial support from the National Science Foundation, the Department of Energy, and the Air Force Office of Scientific Research.


Summer ’07 Bowling Party: Ian Spalding, Tim Sechler, Craig Murray, Logan Dempsey, Marsha Lester, Patti Cleary, Erika Derro, Robin Lefkowitz, Bridget O’Donnell and Ergin Ahmed (not pictured).

Education and Academic History

Selected Publications
L. P. Dempsey, C. Murray, and M. I. Lester, “Product Branching between Reactive and Non-reactive Pathways in the Collisional Quenching of OH A 2Σ+ Radicals by H2”, J. Chem. Phys. [Communication] 127, 151101 (2007).

E. L. Derro, C. Murray, T. D. Sechler, and M. I. Lester, “Infrared action spectroscopy and dissociation dynamics of the HOOO radical”, J. Phys. Chem. A 111, 11592 -11601 (2007).

C. Murray, E. L. Derro, T. D. Sechler, and M. I. Lester, "Stability of the HOOO Radical via Infrared Action Spectroscopy", J. Phys. Chem. A [Letter] 111, 4727-4730 (2007).

P. A. Cleary, L. P. Dempsey, C. Murray, M. I. Lester, J. Klos and M. H. Alexander, "Electronic Quenching of OH A 2Σ+ Radicals in Single Collision Events with Molecular Hydrogen: Quantum State Distribution of the OH X 2Π Products", J. Chem. Phys., 126, 204316 (2007).

I. M. Konen, E. X. J. Li, M. I. Lester, J. Vazquez, and J. F. Stanton, "Infrared Overtone Spectroscopy and Vibrational Analysis of a Fermi Resonance in Nitric Acid: Experiment and Theory", J. Chem. Phys. 125, 074310 (2006).

E. X. J. Li, I. M. Konen, M. I. Lester, and A. B. McCoy, "Spectroscopic Characterization of Peroxynitrous Acid in cis-perp Configurations", J. Phys. Chem. A 110, 5607-5612 (2006).

I. M. Konen, I. B. Pollack, E. X. J. Li, M. I. Lester, M. E. Varner, and J. F. Stanton, "Infrared Overtone Spectroscopy and Unimolecular Decay Dynamics of Peroxynitrous Acid", J. Chem. Phys. 122, 094320 (2005).

M. D. Marshall, J. B. Davey, M. E. Greenslade, M. I. Lester, "Evidence for Partial Quenching of Orbital Angular Momentum upon Complex Formation in the Infrared Spectrum of OH-Acetylene", J. Chem. Phys. 121, 5845-5851 (2004).

I. B. Pollack, I. M. Konen, E. X. J. Li, and M. I. Lester, "Spectroscopic Characterization of HOONO and its Binding Energy via Infrared Action Spectroscopy", J. Chem. Phys. [Communication] 119, 9981-9984 (2003).

M. I. Lester, B. V. Pond, D. T. Anderson, L. B. Harding, and A. F. Wagner, "Exploring the OH + CO Reaction Coordinate via Infrared Spectroscopy of the OH-CO Reactant Complex", J. Chem. Phys. [Communication] 113, 9889-9892 (2000).

D. T. Anderson, M. W. Todd, and M. I. Lester, "Reactive Quenching of Electronically Excited OH Radicals in Collisions with Molecular Hydrogen", J. Chem. Phys. [Communication] 110, 11117-11120 (1999).