Pratt’s research primarily involves experimental studies of the photoionization and photodissociation dynamics of small molecules, particularly those relevant to combustion.

Pratt first joined Argonne in 1982 as a postdoctoral appointee. He has published more than 150 journal articles, and in 1995, he was made a Fellow of the American Physical Society for fundamental contributions to molecular physics through imaginative and innovative studies that probe electron-nuclear coupling, and in particular, for his elegant experiments on molecular photoionization, predissociation, autoionization, and excited-state reactions. He has a PhD, MPhil, and MS in Chemistry from Yale University, and a BA in Chemistry from Bennington College.

Research Areas

Pratt investigates photoionization and photodissociation dynamics to learn how energy flows among the internal degrees of freedom in highly energized molecules. He is 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. His 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, Pratt and colleagues 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. They 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 their laboratory at Argonne. They have recently used the latter approach to provide a definitive assignment of the ​“new Ogawa bands” with v = 0 and 1 and are now broadening this work to address other regions of the near-threshold spectrum. The understanding they 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

Pratt is interested in both the systematic behavior 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.

Selected Publications

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 3^d 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 3^d, 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(²E_3/2)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).