Physical Chemistry: Molecular Spectroscopy utilizing a variety of techniques including Resonance Enhanced Multiphoton ionization, Zero Electron Kinetic Energy (ZEKE) photoelectron spectroscopy, Mass Analyzed Threshold Ioniztion (MATI) spectroscopy, and IR-UV double resonance spectroscopy. These techniques are applied to the study of isolated, gas phase large molecules and clusters.
The recent focus of our research has been on studying the detailed interactions which determine conformational structure in large molecules and molecular clusters including the evolution of structure associated with dynamical process such as conformational interconversion and dissociation. Our primary approach is experimental measurement of isolated gas phase molecules and clusters using a variety of laser spectroscopic techniques. These include excited state vibronic spectroscopy using resonance enhanced multi-photon ionization (REMPI), ground and excited state vibrational spectroscopy using ultra-violet Infrared (UV-IR) double resonance spectroscopy and cation vibronic spectroscopy using zero electron kinetic energy (ZEKE) spectroscopy. The diversity of techniques used allows us to map out the molecular conformations and dynamics in several electronic states helping to unravel how electronic excitation influences the molecular properties. In support of these experimental studies we have been utilizing increasingly sophisticated quantum chemical calculations to give a more complete picture of the molecular structure and dynamics.
The following are several recent projects:
Aniline-Arn dissociation pathways. One of molecular properties which is difficult to accurately measure is the dissociation energy. In this work we used Mass Analysed Threshold Ionizaton (MATI) spectroscopy to accurately measure the dissociation energy of one and two Ar atoms bound to Aniline. An unexpected low energy dissociation pathway for Aniline-Ar2 in the cation was revealed and led to the identification of a new local minimum for the Aniline-Ar in the cation. Link to Publication Here
Accurate Determination of a water-carboxylic acid binding energy
Using an IR-UV double resonance technique we have been able to accurately measured the binding energy of a carboxylic acid (9-hydroxy fluorine carboxylic acid – 9HFCA) to water with an accuracy of better than 0.1 kcal/mole. A subsequent paper focused on the interaction of multiple hydrogen bonds within clusters of 9-HFCA with a series of smaller acid binding partners. In 9-HFCA the 9-hydroxy is internally H-bound to the carbonyl oxygen. When dimers of 9-HFCA are formed, through two additional intermolecular H-bonds, the strength of the internal 9-hydroxy bond is changed. This change was monitored via IR spectroscopy and was found to report very accurately on the nature of the intermolecular bonds. Extensive quantum chemical calculations were performed to model these effect and a correlation was observed between the measured 9-OH frequency and the calculated Natural Bond Orbital (NBO) sigma* population. Calculations were also performed on a number of model systems including the much simpler, but related, glycolic acid. Link to Two Publications Here
Zero kinetic energy photoelectron spectroscopy of tryptamine and the dissociation pathway of the singly hydrated cation cluster, Quanli Gu and J. L. Knee, J. Chem. Phys. 137, 104312 (2012). doi:10.1063/1.4752080
Communication: Frequency shifts of an intramolecular hydrogen bond as a measure of intermolecular hydrogen bond strengths, Quanli Gu, Carl Trindle and J. L. Knee, J. Chem. Phys., 137, 091101 (2012).
Applying 6-Methylisoxanthopterin-Enhanced Fluorescence To Examine Protein–DNA Interactions in the Picomolar Range, Andrew Moreno, Joseph Knee, Ishita Mukerji, Biochemistry, 51, 6847 (2012), doi: 10.1021/bi300466d
Communication: Spectroscopic measurement of the binding energy of a carboxylic acid-water dimer Quanli Gu and J. L. Knee, J. Chem. Phys. 136, 171101 (2012); doi: 10.1063/1.4711862
Characterization of two adenosine analogs as fluorescence probes in RNA, Ying Zhao, Joseph L. Knee, Anne M. Baranger, Bioorganic Chemistry, 36, 271 (2008).
Cation Spectroscopy and Binding Energy Determination for 1,4-benzodioxan -Ar1 and -Ar2 Complexes, Quanli Gu and J.L. Knee, J. Phys. Chem. A, 112 (30), 6823–6828, (2008).
Photoionization spectroscopy of even-parity autoionizing Rydberg states of argon: Experimental and theoretical investigation of Fano profiles and resonance widths, D. Wright, T. Morgan, Leping Li, Quanli Gu, J. L. Knee, I. D. Petrov, V. L. Sukhorukov and H. Hotop, Phys. Rev. A 77, 062512 (2008).
Characterization of the Dynamics of an Essential Helix in the U1A Protein By Time-Resolved Fluorescence Measurements, Divina Anunciado, Michael Agumeh, Bethany L. Kormos, David L. Beveridge, J. L. Knee, and Anne M. Baranger, J. Phys. Chem. B, 112 (19), 6122 -6130, (2008).
Two-color photoexcitation of Rydberg states via an electric quadrupole transition.Leping Li; Quanli Gu; J.L. Knee; J.D. Wright; J.M. DiSciacca; T.J. Morgan Journal of the Optical Society of America B: Optical Physics, 25, 334 (2008).
Binding energies and dissociation pathways in the Aniline-Ar2 cation complex , Quanli Gu, and J. L. Knee, J. Chem. Phys., 128, 064311/1-064311/8 (2008).
Tryptophol Cation Conformations studied with ZEKE Spectroscopy, Quanli Gu, Swarna Basu and J. L. Knee, J. Phys. Chem. A., 111, 808 (2007).
B.A. 1979 State University of New York, Binghamton
Ph.D. 1983 State University of New York, Stony Brook