Physical Chemistry, Molecular Structure and Dynamics
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. Recent highlights of our research are described below.
Recently, we demonstrated a new photoionization scheme for sensitive, state-selective detection of OH radicals. Our approach combines UV excitation on the well-characterized A2Σ+ – X2Π band system, typically used for laser-induced fluorescence detection, with fixed-frequency VUV ionization via autoionizing Rydberg states that converge on the OH+ A3Π limit. This new scheme is expected to be widely applicable for state-selective detection of OH radicals for fundamental studies of photodissociation, inelastic and reactive scattering dynamics, and in situ diagnostics.
This laboratory obtained the first infrared spectrum of the hydrogen trioxide (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. We have characterized the geometric structure, vibrational frequencies, and stability of the cis and trans conformers of HOOO and its deuterated analog. In particular, 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 25% of the OH radicals in the vicinity of the tropopause may be bound to O2, rather than free OH radicals. Studies of combination bands in the fundamental OH stretch region reveal nearly all other vibrational modes of trans- and cis-HOOO. We have subsequently derived a torsional potential from our spectroscopic data to obtain the relative stability of the cis and trans conformers and isomerization barrier, which are critical for atmospheric modeling of HOOO.
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 A2Σ+ 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 A2Σ+ electronic state by introducing nonradiative decay pathways. The quenching of OH A2Σ+ 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. Our studies examined the Doppler profiles for the H/D-atom products of reactive quenching, which show that most of the excess energy results in vibrational excitation of ‘hot’ water products. Our work also focused on characterizing the nonreactive quenching process, where OH X2Π products are generated with a remarkably high degree of rotational excitation and lambda-doublet specificity. The OH quantum state distribution directly reflects the anisotropy and A′ symmetry of the conical intersection region. We also demonstrated for H2 and D2 collision partners that reaction accounts for nearly 90% of the quenched products. These distinctive dynamical signatures of passage through a conical intersection region have sparked intense theoretical interest in this system.
Quenching of electronically excited OH radicals proceeds through a conical intersection, leading to 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 under Grant No. NSF CHE-1112016 and the Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Sciences, Office of Science, U.S. Department of Energy under Grant No. DE-FG02-87ER13792.
- B.A. Douglass College, Rutgers University (1976)
- Ph.D. Columbia University (1981)
- NSF Postdoctoral Fellow, AT&T Bell Laboratories (1981-82)
- Camille and Henry Dreyfus Young Faculty Award (1982), Teacher-Scholar Award (1986)
- Alfred P. Sloan Research Fellow (1987)
- National Science Foundation Career Advancement Award (1988)
- Broida Prize awarded by the International Symposium on Free Radicals (1995)
- Fellow of the American Physical Society (1993), the American Association for the Advancement of Science (1997) and the American Chemical Society (2010)
- Guggenheim Fellowship (2002-03)
- Visiting Miller Research Professor, Berkeley (2003)
- Distinguished Traveling Lecturer, Division of Laser Science, American Physical Society (2002-06)
- Bourke Lectureship, Faraday Division of the Royal Society of Chemistry (2005)
- Fellow, American Academy of Arts and Sciences (2008)
J. H. Lehman, M. I. Lester, and D. H. Yarkony, “Reactive quenching of OH A2Σ+ by O2 and CO: Experimental and nonadiabatic theoretical studies of H- and O-atom product channels”, J. Chem. Phys. 137, 094312 (2012).
B. A. O’Donnell, J. M. Beames, and M. I. Lester, “Experimental characterization of the CN X2Σ+ + Ar and H2 potentials via IR-UV double resonance spectroscopy”, J. Chem. Phys. 136, 234304 (2012).
J. M. Beames, F. Liu, M. I. Lester, and C. Murray, “Communication: A new spectroscopic window on hydroxyl radicals using UV+VUV resonant ionization”, J. Chem. Phys. 134, 241102 (2011).
J. H. Lehman, J. Bertrand, T. A. Stephenson, and M. I. Lester, “Reactive quenching of OD A2Σ+ by H2: Translational energy distributions for H- and D-atom product channels”, J. Chem. Phys. 135, 144303 (2011).
J. M. Beames, M. I. Lester, C. Murray, M. E. Varner, and J. F. Stanton, “Analysis of the HOOO Torsional Potential”, J. Chem. Phys. 134, 044304 (2011).
J. H. Lehman, L. P. Dempsey, M. I. Lester, B. Fu, E. Kamarchik, and J. M. Bowman, “Collisional quenching of OD A2Σ+ by H2: Experimental and theoretical studies of the state-resolved OD X2Π product distribution and branching fraction”, J. Chem. Phys. 133, 164307 (2010).
C. Murray, E. L. Derro, T. D. Sechler, and M. I. Lester, “Weakly bound molecules in the atmosphere – a case study of HOOO”, Acc. Chem. Res. 42, 419-427 (2009).
E. L. Derro, T. D. Sechler, C. Murray, and M. I. Lester, “Observation of combination bands of the HOOO and DOOO radicals using infrared action spectroscopy”, J. Chem. Phys. 128, 244313 (2008).
B. A. O’Donnell, E. X. J. Li, M. I. Lester, and J. S. Francisco, “Spectroscopic identification and stability of the intermediate in the OH + HONO2 reaction”, Proc. Natl. Acad. Sci. 105, 12678-12683 (2008).
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).