The Gas-Phase Chemical Dynamics Group that he leads is primarily focused on developing the foundations for predictive models of combustion chemistry. The group currently includes 12 staff scientists and a complement of postdoctoral fellows, part-time employees, students, and visitors. A significant aspect of this group is the strong interactions between theoretical and experimental efforts.
Dr. Pratt started at Argonne as a postdoctoral appointee. Then he became an assistant chemist and a chemist. Currently, he is a senior chemist, group leader of the Gas-Phase Chemical Dynamics Group, and Division Theme Leader for Fundamental Interactions.
Dr. Pratt has written approximately 150 refereed journal articles, with a focus on experimental studies of the photodissociation and photoionization dynamics of small molecules.
He has a PhD, MPhil, and MS in Chemistry from Yale University and a BA in Chemistry from Bennington College.
My research involves the study of photoionization and photodissociation dynamics as a means of learning about how energy flows among the internal degrees of freedom in highly energized molecules. I am also interested in determining photoionization and photoabsorption cross sections of both stable and reactive species, and in developing a systematic understanding of these cross sections. My experimental research program involves both laboratory-based studies using lasers for multiphoton excitation and pump-probe experiments, and in facility-based studies using synchrotron sources for single-photon photoabsorption and photoionization studies of a wide range of small molecules. In some instances, these studies provide a framework for new work using vacuum-ultraviolet (vuv) and x-ray free-electron lasers to study ultrafast processes in these same systems.
Near-Threshold Photoionization Dynamics
As an example, we have recently been focused on the near-threshold photoionization and photoabsorption spectrum of N2. Although it is the dominant component of the atmosphere, much of the structure in the near-threshold photoionization spectrum of N2 is only tentatively assigned. We are currently trying to address this difficulty with a combination of three experimental approaches: very high resolution photoabsorption measurements using the vuv Fourier-transform absorption spectrometer at the SOLEIL Synchrotron, high-resolution photoelectron-photoion coincidence measurements at the SOLEIL Synchrotron, and double-resonance photoionization and photoelectron images studies in our laboratory at Argonne. We have recently used the latter approach to provide a definitive assignment of the “new Ogawa bands” with v = 0 and 1, and we are now broadening this work to address other regions of the near-threshold spectrum. The understanding we are developing for N2 will also be used in the design and interpretation of new ultrafast experiments using the FERMI vuv free-electron laser.
Photoionization Cross Sections
I am interested in both the systematic behaviour of photoabsorption and photoionization cross sections, and in the determination of absolute photoionization cross sections for reactive species like free radicals. As in the study of photoionization dynamics, both synchrotron and laboratory experiments have provided insight into the cross sections. In particular, absolute photoionization cross section measurements have been performed by combining photodissociation, ion-imaging, and vuv photoionization in the laboratory, and by using flow reactors and vuv photoionization at the synchrotron facility.
Ananya Sen and Stephen T. Pratt, “Double-Resonance Studies of Electronically Autoionizing States of Molecular Nitrogen,” Mol. Phys., in press.
R. Forbes, A. De Fanis, C. Bomme, D. Rolles, S. T. Pratt, I. Powis, N. A. Besley, S. Nandi, A. R. Milosavljević, C. Nicolas, J. D. Bozek, J. G. Underwood, and D. M. P. Holland, “Auger Electron Angular Distributions Following Excitation or Ionization of the I 3d Level in Methyl Iodide,” J. Chem. Phys. 149, 094304 (2018).
R. Forbes, A. De Fanis, C. Bomme, D. Rolles, S. T. Pratt, I. Powis, N. A. Besley, M. Simon, S. Nandi, A. R. Milosavljevic, C. Nicolas, J. D. Bozek, J. G. Underwood, and D. M. P. Holland, “Photoionization of the Iodine 3d, 4s and 4p Orbitals in Methyl Iodide,” J. Chem. Phys. 149, 1443012 (2018).
A. M. Chartrand, E. F. McCormack, U. Jacovella, D. M. P. Holland, Bérenger Gans, Xiaofeng Tang, G. A. Garcia, L. Nahon, and S. T. Pratt, “Photoelectron Angular Distributions from Rotationally Resolved Autoionizing States of N2,” J. Chem. Phys. 147, 224303 (2017).
Min Xie, Zhitao Shen, S. T. Pratt, and Yuan-Pern Lee, “Vibrational Autoionization of State-Selective Jet-Cooled Methanethiol (CH3SH) Investigated with Infrared + Vacuum-Ultraviolet Photoionization,” Phys. Chem. Chem. Phys. 19, 29153-29161 (2017).
Ananya Sen, Stephen T. Pratt, and Katharine L. Reid, “Circular Dichroism in Photoelectron Images from Aligned Nitric Oxide Molecules,” J. Chem. Phys. 147, 013927 (2017).
A. Pićon, C. S. Lehmann, C. Bostedt, A. Rudenko, A. Marinelli, T. Osipov, D. Rolles, N. Berrah, C. Bomme, M. Bucher, G. Doumy, B. Erk, K. R. Ferguson, T. Gorkhover, P. J. Ho, E. P. Kanter, B. Krässig, J. Krzywinski, A. A. Lutman, A. M. March, D. Moonshiram, D. Ray, L. Young, S. T. Pratt, and S. H. Southworth, “Hetero-Site-Specific Ultrafast Intramolecular Dynamics,” Nat. Commun. 7, 11652 (2016).
U. Jacovella, D. M. P. Holland, S. Boyé-Péronne, B. Gans, N. de Oliveira, K. Ito, D. Joyeux, L. E. Archer, R. R. Lucchese, Hong Xu, and S. T. Pratt, “A Near-Threshold Shape Resonance in the Valence Shell Photoabsorption of Linear Alkynes,” J. Phys. Chem. A. 119, 12339-12348 (2015).
Hong Xu and S. T. Pratt, “Photodissociation of Methyl Iodide via Selected Vibrational Levels of the B(2E3/2)6s Rydberg State,” J. Phys. Chem. A, 119, 7548-7558 (2015).
S. T. Pratt, “Charge Transfer Goes the Distance,” Science 345, 267-268 (2014).
H. Xu and S. T. Pratt, “The Photoionization Cross Section of Propargyl Radical and Some General Ideas for Estimating Radical Cross Sections,” J. Phys. Chem. A 117, 9331-9342 (2013).
H. Xu, U. Jacovella, B. Ruscic, S. T. Pratt, and R. R. Lucchese, “Near-Threshold Shape Resonance in the Photoionization of 2-Butyne,” J. Chem. Phys., 136, 154303 (2012).
S. T. Pratt and Ch. Jungen, “The Isotope Dependence of Dissociative Recombination via the Indirect Mechanism,” J. Chem. Phys. 137, 174306 (2012).