Atomic, Molecular, and Chemical Physics

The Atomic, Molecular, and Chemical Physics group focuses on interactions of atoms, molecules, electrons and photons at low temperatures and low energies. Our programs include both experimental and theoretical projects, many of which entail collaborations within and outside the Department. Combining expertise in several areas of atomic, molecular, and optical physics with an emphasis on chemical physics, our group offers a breadth and range rarely found in either Chemistry or Physics Departments. Recent additions to our group include Eric Abraham and Neil Shafer-Ray. Eric's experimental program studies the physics of atoms and molecules at ultracold temperatures, including Bose-Einstein condensation and atomic clocks. Neil's experimental program investigates highly controlled scattering of atoms and molecules.

Current specific research areas include chemical reaction dynamics (theory and experiment), the physics of ultra-cold atoms and molecules, including Bose-Einstein condensation (theory and experiment), creation of more accurate and precise atomic clocks (theory and experiment), collisions involving highly excited states of atoms and molecules including Rydberg states (theory and experiment), low-energy elastic and threshold inelastic scattering of charged particles (theory), orientation and alignment effects (theory and experiment), the determination of potential energy surfaces (theory and experiment), and the study of atoms in magnetic and optical fields (theory and experiment). These programs benefit from ongoing collaborations between experimentalists and theoreticians here at OU, around the country, and around the world.

Our experimental facilities include a scattering apparatus for the study of energy transfer and chemically reactive collisions. A strong emphasis of the experimental program is on the use of laser probes to study the dynamics of reactions and energy transfer processes, novel optical traps to study ultracold atoms, and techniques to cool molecules into the ultracold regime. These experiments complement ongoing theoretical work in electron-molecule collisions, near-resonant energy transfer of Rydberg atoms, dimensional perturbation theory, doubly-excited states of atoms, atoms in magnetic fields, and Bose-Einstein condensation. Our computational facilities include an extensive network of powerful computer workstations, which are freely shared among members of the Department, and an SP-2 super computer. These facilities are used in several experimental contexts and in theoretical research such as ongoing study of processes fundamental to combustion and of CPU-intensive reactive scattering processes.

Since its inception, our group has been regularly funded by such sources as the National Science Foundation, the American Chemical Society, the Office of Naval Research, and the Air Force Office of Scientific Research. We also pursue collaborations with local industries. In addition, various members of the group participate in long-term collaborations with scientists from Italy, Australia, Switzerland, Canada, Germany, Israel, Latvia, the United Kingdom, and various laboratories and universities in the U.S. A highlight of the program is a regular, intensive program of visits and colloquia by outside members of the atomic, molecular, and chemical physics community, including our many collaborators.

Eric Abraham
Assistant Professor
B.A. 1991 St. Olaf College
Ph.D. 1996 Rice University

The goal of our research program is to investigate Bose-Einstein condensation of atomic gases, to develop new atom interferometric techniques, to create more accurate and precise atomic clocks, and to trap and study ultracold molecules. We use a variety of lasers and magnetic fields that can cool atoms to a range of temperatures colder than anything else in the known universe (between 10 nanoKelvin and 100 microKelvin.) At these temperatures, the wave-like nature of atoms is enhanced allowing studies of the exotic, quantum-mechanical nature of matter.

Over 70 years ago, Albert Einstein predicted that a gas of non-interacting particles could undergo a phase transition, collecting a macroscopic number of particles into the same quantum state. The gas must be cooled to where the de Broglie wavelengths of the individual atoms overlap. This concept of Bose-Einstein condensation has since been an integral part of the understanding of strongly interacting systems such as superfluids and superconductors. However, BEC in dilute atomic gasses more accurately approximates Einstein's original prediction for non-interacting particles.

Atom interferometry also exploits the wave-like characteristics of matter. In the past few years the field of atom optics has established techniques to manipulate the de Broglie wave of atoms just as light is controlled with conventional optics. Atom interferometers have emerged as powerful tools for fundamental physics measurements (e.g. fundamental constants) and possible technological applications (e.g. rotation sensors for navigation.)

Another area of investigation with practical applications is to improve current atomic clocks. Atomic clocks are important to a wide range of scientific research (astronomy and tests of general relativity) as well as commercial and governmental applications (communications, global positioning systems, and deep space navigation.) While laser cooling and trapping techniques have produced a revolution in atomic physics, it is limited to a few atoms. We hope to extend ultracold trapping techniques to molecules.

This work is a solid combination of both optical and atomic physics. We use semi-conductor diode lasers with the latest in optic, fiber optic, acoustic-optic and electro-optic technology. The experiments use Rubidium atoms and take place in ultra-high vacuum at pressures as low as 10^-11 Torr, and utillize state-of-the-art, as well as home-built, microwave, rf, and DC electronics.

J. Tempere, J.T. Devreese, and E.R.I. Abraham, ``Vortices in Bose-Einstein condensates confined in a multiplly connected Laguerre-Gaussian optical trap'', submitted for publication (2000).

D. Mueller, D.Z. Anderson, E.A. Cornell, and E.R.I. Abraham, ``Guiding laser cooled atoms in hollow core fibers'', Phys. Rev. A 61, 033411 (2000).

E.R.I. Abraham, W.I. McAlexander, J.M. Gerton, R.G. Hulet, R Cote, and A. Dalgarno, ``Triplet s-Wave resonance in ⁶Li collisions and scattering lengths of ⁶Li and ⁷Li'', Phys. Rev. A 55, R3299 (1997).

Michael A. Morrison
David Ross Boyd Professor of Physics and General Education
Adjunct Professor of English
B.S. 1971 Rice University
Ph.D. 1976 Rice University

For over two decades, my research group has investigated a wide range of problems in quantum collision theory. Much of our work has explored low-energy scattering of electrons (and positrons) by diatomic molecules, exploring such diverse topics as non-local and many-body interactions such as exchange and correlation, new dynamical theories for near-threshold excitations, model potentials suitable for calculations on large, complex systems, and the determination of accurate cross sections needed for applications such as laser kinetic modeling and the study of planetary atmospheres. In addition, we have worked on multi-step laser excitation of atoms, orientation and alignment effects in electron-atom scattering, and collisions of Rydberg atoms with rare-gas atoms. Typically, we choose problems that entail a blend of formal mathematics and quantum mechanics, numerical algorithms and their computational implementation, and analysis of experimental data.

Our current research includes theoretical studies of the extension of density functional theory to bound-free correlation effects in continuum states in electron scattering (with Robert K. Nesbet of the IBM Almaden Research Labs), of dissociative attachment (with Gregory A. Parker), of the generalization beyond conventional Boltzmann theory of the description of transport of swarms of electrons through atomic and molecular target gases (with Rob Robson of James Cook University and Bruce Mason), and joint experimental/theoretical studies of Rydberg-atom-rare-gas scattering (with Neil Shafer-Ray), of e-CO₂ scattering (with Stephen Buckman of the Australian National University), and of laser cooling and trapping (with Eric Abraham).

Vital to our program are our vigorous, continuing collaborations with experimental and theoretical physicists at a variety of institutions, including the Australian National University, the University of Kentucky, the Joint Institute for Laboratory Astrophysics, IBM Research Laboratories, and the Los Alamos National Laboratory. These projects often entail visits of members of our group to other institutions and vice versa.

Michael A. Morrison, Eric G. Layton, and Gregory A. Parker, ``Rydberg electron interferometry,'' Physical Review Letters 84, 1415 (2000).

Neil E. Shafer-Ray, Michael A. Morrison, and Gregory A. Parker, ``A classical ensemble model of three-body collisions in the point contact approximation and application to alignment effects in near-resonant energy transfer collisions of He atoms with Rydberg Ca atoms,'' Journal of Chemical Physics 113, 4274 (2000).

Stephane Mazevet, Michael A. Morrison, Olen Boydstun and R. K. Nesbet, ``Approximate Treatments of Vibrational Dynamics in Low-energy Electron-Molecule Scattering,'' Journal of Physics B 32, 1269, (1999).

Michael A. Morrison, ``Understanding Quantum Physics: A User's Manual'', (Prentice-Hall, Englewood Cliffs, NJ, 1990), 668 pp.

Gregory A. Parker
Professor
B.S. 1973 Brigham Young University
Ph.D. 1976 Brigham Young University

Chemical reactions dynamics is challenging intriguing and at the forefront of chemical physics. I am interested in accurately solving the time-dependent and time-independent quantal Schrödinger equation for reactive and nonreactive processes.

Over the past decade our research has contributed significantly to the current understanding of reactive scattering. One of the first things found in early one-dimensional reactive scattering calculations were quantum resonances (long-lived collision complexes) that can dramatically affect the reaction probabilities. With new methods developed by ourselves and others, it is now possible to do calculations for triatomic systems of real physical interest. It is becoming clear that quantum resonances dominate many if not most systems in the full three-dimensional space. These quantum resonances are system-specific and very sensitive to the potential energy surface and any approximations made, so the only way to really understand them is via accurate quantum dynamics.

Some of the systems of particular interest to us are:

$$F+H_2 \rightleftharpoons H+FH$$

$$e^++H \rightleftharpoons p^++Ps$$

$$He+H_2^+ \rightleftharpoons HeH^++H$$

$$Li+FH \rightleftharpoons LiF+H$$

$$H+O_2 \rightleftharpoons O+OH$$

$$H+Ne_2 \rightleftharpoons H+Ne+Ne$$

As the last reaction suggests we are currently developing methods for treating three-body recombination processes and collision induced dissociation.

I enjoy interactive collaborations with the experimental group of Professor Neil Shafer-Ray. We can calculate center-of-mass and laboratory cross sections for direct comparison with their experimental observations. Since the interplay between experiment and theory is mutually beneficial it is a real opportunity to have an excellent experimental group in our department with which to collaborate. We also have productive collaborations with theorists at Los Alamos National Laboratory, University of Houston and the University of Perugia, Italy and of course here at OU with Professor Michael A. Morrison.

S. S. Iyengar, G. A. Parker, D. J. Kouri, and D. K. Hoffman ``Symmetry Adapted Distributed Approximating Functionals: Theory and Application to the ro-vibrational states of H₃⁺.'' J. Chem. Phys., 110, 10283-10298, (1999).

S. S. Iyengar, D. J. Kouri, G. A. Parker and D. K. Hoffman ``Estimating the Upper and Lower Bounds for the Eigenvalues of any Matrix.'' Theor. Chem. Acc. 103, 507-517 (2000).

M. A. Morrison, E. G. Layton, and G. A. Parker, ``Rydberg Electron Interferometry'' Phys. Rev. Lett. 84, 1415-1418 (2000).

Antonio Laganá, Alessandro Bolloni, Stefano Crocchianti and Gregory A. Parker, ``On the Effect of Increasing the Total Angular Momentum on Li+HF Reactivity.'' Chem. Phys. Lett. 324, 466-474 (2000).

G. A. Parker, Mark Keil, Michael A. Morrison and Stefano Crocchianti, ``Quantum Reactive Scattering in Three Dimensions. Using Tangent-Sphere Coordinates to Smoothly Transform from the Hyperspherical to Jacobi Regions.'' J. Chem. Phys. 113, 957-970 (2000).

Neil Shafer-Ray
Assistant Professor
B.S. 1986 M.I.T.
Ph.D. 1990 Columbia

Our group has recently devised, constructed, and demonstrated an apparatus that explores reactive scattering dynamics with a new level of precision and control. Specifically, we have the ability to measure state-to-state and scattering-angle dependent cross sections of reactive scattering processes. Our apparatus is unique in its ability to continuously tune the collision energy with milli-electron volt resolution. We are using this apparatus to search for highly structured energy-dependent cross sections that would result from long-lived collision complexes (dynamical resonances) occuring in chemical reactions. Our group is also interested in near-resonance energy transfer that occurs when an excited Rydberg atom collides with a rare-gas atom. Recently Michael Morrison and coworkers have created quantum models of this process that show strongly structured energy-and-alignment dependent cross sections. We are currently planning to measure the energy-dependent cross section for the process $He(18d)+Xe \to He(19p)$+Xe to test these exciting predictions.

N. Shafer-Ray, M. Morrison, and G. Parker, ``A Classical Ensemble Model of Three-Body Collisions in the Point Contact Approximation and Application to Alignment Effects in Near-Resonant Energy Transfer Collisions of He atoms with Rydberg Ca Atoms'',Journal of Physical Chemistry, 113, (2000).

Marcis Auzinsh, Revin Ferber, O. Nikolayeva, Neil Shafer-Ray and Maris Tamanis, ``Influence of the Stark effect on the fluorescence polarization of $X^{1}\Sigma \rightarrow B^{1}\Pi$-state-laser-excited NaRb: Application to the direct imaging of electric fields'', submitted, Physical Review A, (2000).

K. Dharmansena, S. Kennedy, G. Mu, and N. Shafer-Ray, ``A Method to Obtain meV-Collision-Energy Resolution in scattering studies: Application to the $H+D_2\to HD(v=0,J)+D (\theta_{rel}<82\deg$ reaction at E_rel=1.275eV'', Chemical Physics, 244, 449 (1999).

N.E. Shafer-Ray, R.N. Zare, ``Measurement of Rapidly Varying Electric Fields Through Parity Oscillations in the Rydberg States of Hydrogenic Atoms'', Applied Physics Letters 69, 3749 (1996).

Deborah K. Watson
Professor
B.S. 1972 Allegheny College
Ph.D. 1977 Harvard

My group is engaged in the study of fundamental quantum mechanical questions for both simple atomic systems such as helium and most recently for Bose-Einstein condensates. Specifically, we are trying to address these questions using a method called dimensional perturbation theory, in which the Schrödinger equation is solved in an arbitrary number of dimensions. Our philosophy stems from the notion that, just as the two-dimensional world is easier to understand from the perspective of three dimensions, so we believe that we can gain insight into our three-dimensional world using the perspective of higher dimensions. We are presently pursuing several studies, including a detailed look at states of helium as a function of dimension D including the group-theoretic basis for inter-dimensional degeneracies, a study of diamagnetic hydrogen including Rydberg states, and an analysis of properties of Bose-Einstein condensates using trap parameters that approximate current experimental conditions at various laboratories. Our Bose-Einstein work is exploring ways to go beyond the mean field approximation, known as the Gross-Pitaevskii equation, to bring in many-body effects.

Dimensional perturbation theory has thus far been the source of some surprising insight into the dynamics of few-body systems, including electron geometry, classification of doubly-excited states, patterns in helium spectra, and should provide a unique vantage point from which to analyze Bose-Einstein condensation.

B. A. McKinney and D. K. Watson, ``Semiclassical perturbation theory for two electrons in a D-dimensional quantum dot,'' Phys. Rev. B 61, 4958 (2000).

J. R. Walkup, M. Dunn and D. K. Watson, ``Local Optimization of the Summation of Divergent Power Series,'' J. Math. Phys. 41 56814 (2000).

D.K. Watson and B.A. McKinney, ``An Improved Large-N Limit for Bose-Einstein Condensates from Perturbation Theory,'' Phys. Rev. A. 59, 4091 (1999).

J.R. Walkup, M. Dunn, T.C. Germann, and D.K. Watson, ``Avoided Crossings of Diamagnetic Hydrogen as Functions of Magnetic Field Strength and Angular Momentum,'' Phys. Rev. A, 58, 4668 (1998).

D.K. Watson, M. Dunn, T.C. Germann, D.R. Herschbach, D.Z. Goodson, and J.R. Walkup, ``Dimensional Expansions for Atomic Systems'', New Methods in Quantum Theory, edited by C.A. Tsipis, V.S. Popov, D.R. Herschbach, and J. Avery, NATO Conference Book 8, Kluwar Academic, Dordrecht Holland, p. 83.