People | Faculty | Peter Siska
(Deceased 2/27/09)
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Peter Siska Department of Chemistry |
Within this general theme our group has been studying the chemistry of electronically excited atoms, specifically their interactions and reactions with ground-state atoms and small molecules. Excited atoms represent a new frontier in chemical reactivity, extending the periodic table in a third dimension, while they have recently been shown to catalyze a range of technologically significant gas-solid processes such as the growth of diamond films. Our nanoscopic approach to understanding the chemistry of these states employs both laboratory measurement and theoretical studies aimed at elucidating the potential energies and coupling terms that engender the formation of new species from the excited atom + target complex.
Molecular Beam Kinetics. We are currently focusing on the Group VIIIA chemical family, the excited noble gases, the simplest member being He*(1s2s), with plans to move into the transition metal ion groups. Our experimental approach uses crossed molecular beams, one beam of excited atoms A* created by electron impact, and one of a chosen ground-state target BC, to create isolated A*...BC complexes 
in a high-vacuum environment, as illustrated in the inset. At the present time, we are looking at ionizing (oxidative) reactions known collectively as Penning ionization; this dictates the kinds of detectors we use to observe the products of complex decay. These presently include a mass spectrometer and an electron energy analyzer for the detection of charged particles mounted in two separate beam apparatuses; we also state-select the excited-state beam with photons. We are able to resolve the angle and energy of excited and ionized collision products, as well as collect electron energy spectra directly from the complex. The theoretical analysis of these data can yield information on the potential energy of interaction and the coupling to the ionized states, to be described next.
Collision Theory and Modeling. We have been evolving the theory of these excited-state collisions, and implementing it with our own computer codes, as our experimental capabilities have progressed. We currently use a recently developed vibrationally adiabatic, infinite-order sudden theory to treat molecular scattering and Penning ionization in excited systems with molecular targets. This theory has been adapted to model nonreactive scattering of the excited atoms, reactive scattering of the molecular ions, and the molecular electron spectrum. We soon hope to do a multiproperty analysis in a few key systems, such as He* + H2, Ar, and N2, in order to derive well-determined estimates of potential energy curves and surfaces. These studies require carefully constructed models for these surfaces, which we are learning to acquire by semiempirical and ab initio methods, to be described next. We have learned that, for molecular systems, the lowest nonzero permanent electric moment (dipole or quadrupole) of the molecule plays a decisive role in the collisional outcomes.
Electronic Structure Theory. The interaction between the excited atom and a molecular target is usefully parsed into that involving the promoted electron (e.g. He 2s) and the core ion (He+). In addition the core ion must eventually accept an electron from the valence shell of the target, as the atom quenches its excitation in the course of reaction. We have recently initiated high-level ab initio studies of ion-molecule potential surfaces related to the excited systems we study experimentally, in an effort to delineate significant parts of the potentials we seek to determine. Our theoretical work on charge-transfer excited states may also lead to a model for valence molecular-to-core ion electron transfer. We have also developed and actively use a semiempirical pseudopotential method to obtain estimates of the excited-state surface itself. For both the collision theory and ab initio calculations, we employ a variety of computer resources from our own workstation to Cray supercomputers.
Awards
Fellow of the Alfred P. Sloan Foundation, 1975-79; Chancellor's Distinguished Teaching Award, 1987; Bellet Arts & Sciences Teaching Excellence Award, 2003
Selected Publications
"Angle-energy distributions of Penning ions in crossed molecular beams. IV. He*(2(1)S,2(3)S)+H-2 ->+He+H-2(+)+e(-)," Gulati, K.; Longley, E. J.; Dorko, M. J., Journal of Chemical Physics, 2004, 120, 8485-8493
"Energy dependence of the Penning ionization electron spectrum of Ne-*(3s P-3(2,0))+A," Jacobs, B. A.; Rice, W. A.; Siska, P. E., Journal of Chemical Physics, 2003, 118, 3124-3130
"Interactions of perfluorocarbon liquids and silicone oil as characterized by mass spectrometry," Friberg, T. R.; Siska, P. E.; Somayajula, K., Graefes Archive for Clinical and Experimental Ophthalmology, 2003, 241, 809-815
"Cold and ultracold ion-neutral inelastic collisions: Spin-orbit relaxation in He + Ne+," P. E. Siska, J. Chem. Phys., 2001, 115, 4527
"Comparison of quantum scattering on an ab initio potential surface with experimental total differential scattering measurements for Li+ + N2," P. E. Siska, J. Chem. Phys., 2001, 115, 1613
"Ab initio / spectroscopic interaction potential for He + Ne+," M. F. Falcetta, M. J. Dorko, and P. E. Siska, J. Chem. Phys., 2000, 113, 11044
"The interaction between He and H2+: anisotropy, bond length dependence, and hydrogen bonding," M.F. Falcetta and P.E. Siska, Mol. Phys., 1999, 97, 117
"Theoretical study of ion-molecule potentials for He+ and Li+ with N2," M. F. Falcetta and P. E. Siska, J. Chem. Phys., 1998, 109, 6615
"Theoretical characterization of long-range interactions in the Ne+(2P) + H2(1Sg+) charge-transfer states," M. F. Falcetta and P. E. Siska, Mol. Phys., 1998, 93, 229
"Dynamics of molecular Penning ionization: angle-energy distributions of N2+ from He*(21S) + N2," A. M. Mhaka and P. E. Siska, Chem. Phys. Lett., 1998, 282, 299