The Shafer-Ray Atomic and
Molecular Physics Laboratory
The University of Oklahoma
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The Shafer-Ray Atomic and
Molecular Physics Laboratory The University of Oklahoma
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Research Description Last updated, November 20, 2003 Introduction My current research interests cover four distinct areas of Atomic and Molecular Physics. One area of research is Chemical Reaction Dynamics. Here I study chemical reactivity as a scattering problem, investigating the exact manner reactants collide to form new chemical species. A second area of interest is Spectroscopic Field Measurements. Here I use molecules and highly excited atoms to probe electric and magnetic fields. The techniques I develop are designed to meet a growing need for field control in both the electronics industry and modern physics experiments. A third area is in the emerging field of Ultracold Molecular Physics. Here I seek to produce and trap molecules at temperatures well below 1 K. Ultracold molecules may be used for precise spectroscopy, quantum computing, and navigational systems. My fourth area of interest is Spectroscopy Beyond the Standard Model. Here features in the precise quantum-beat spectroscopy of ultracold diatomic molecules are probed for effects that cannot be explained by the most widely accepted theory of particle interactions, i.e. the Standard Model. In the following sections, each of these areas are described. Elementary Chemical Reaction Dynamics Funding sources: National Science Foundation, Petroleum Research Fund
Much of our understanding of chemical reactions comes from statistical models in which the rate of a reaction is predicted from the concentration and temperature of the reactants, as well as the energy and entropy of the transition state. Underlying this extraordinarily successful semi-empirical theory of reactions is the rigorous physics of relativistic quantum mechanics that governs the dynamics of the nuclei and electrons as they interact. In the words of Dirac[1], The underlying physical laws necessary for the mathematical theory of a large part of physics and the whole of chemistry are thus completely known, and the difficulty is only that the exact application of these laws leads to equations much too complicated to be soluble. With the advent of highly sophisticated numerical algorithms and breathtaking computer speed Science is making inroads into the family of problems that Dirac would might have labeled as insoluble. Arguably the most successful of these endeavors is the calculation of the structure of an isolated molecule. However, my interest in a fundamental understanding of bimolecular reaction dynamics presents a much greater challenge. Calculations of scattering processes are difficult because of the complex interaction of a large number of quantum states, called open channels, that must be considered. Despite huge challenges, simple reactions such as H+H2, H+F2 and Li+Li2 are now well understood from first principles and more complex reactions such as H+O2, and O+O2 are likely to be completely understood in the very near future. As molecular spectroscopy provides the most rigorous test of electronic structure calculations, state-to-state differential cross sections provide the most rigorous test of theoretical models of elementary reaction dynamics. A state-to-state differential cross section gives the probability that two reactants will form products as a function of how fast the reactants approach each other, the initial quantum states of the reactants, the final quantum states of the products, and the center of mass scattering angle of the reaction. Such measurements are extremely challenging and have motivated important advances in molecular beam technology, lasers, and molecular detection. I was involved in the first such measurements that were made on the F+H2 and H+D2 reactions in the early 90’s. At the University of Oklahoma I have improved the collision energy resolution of scattering measurements by approximately one order of magnitude[2,3]. I have also developed new spectroscopic probes of the OH radical[4] and designed a new cylindrically symmetric beam machine to attempt to measure the state-to-state differential cross section of the H+O2 reaction. Many of the hot-atom reactions we study are central to the chemistry of combustion and the upper atmosphere. The complex chemistry of the upper atmosphere may be influenced by the scattering dynamics of individual bimolecular reactions. Consider, for example, the scattering of a hot oxygen atom, produced by the ultraviolet photo-dissociation of an O2 molecule in the upper atmosphere. When the oxygen atom is forward scattered by an O2 molecule the physics is similar to the scattering of two adjacent billiard balls by a fast-moving cue ball. If the product oxygen molecule is forward scattered, it obtains a large fraction of the kinetic energy of the hot oxygen atom. If, however, the molecule is back scattered in the center of mass frame it receives little or no kinetic energy from the hot oxygen atom. I hypothesize that an isotopic dependence on the probability of the differential cross section may be responsible for the observed anomalies in the distribution of ozone isotopes in the upper atmosphere. Because of widespread interest in this anomaly, O+O2 scattering is on the short list of processes we wish to study.
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Spectroscopic Measurements of Electric Fields funding: NATO Science for Peace Program
Laser-induced fluorescence of atoms and molecules is well known to be highly sensitive to electric fields1. This sensitivity is commonly used to probe the electric field within discharge plasmas2. My laboratory, in collaboration with the Institute for Molecular Spectroscopy in Riga Latvia, has demonstrated that Stark-induced alignment phenomena can be used to create direct images of electric fields. One technique I have developed will improve electric field measurements in discharge plasmas by allowing for the direct creation of images and/or videos of the field3. Another technique allows for direct imaging of electric fields surrounding integrated circuits switching on a picosecond time scale4. In a newly started experimental effort at Brookhaven National Laboratories, methods to measure the electric dipole moment of the muon and deuteron are being developed. One strict requirement of these methods is the creation of a region in space with a very uniform electric field. Specifically, it has been established that a variation of ΔE=100μV/cm could lead to systematic errors in the proposed measurement. I have begun to develop techniques involving both H atoms and polar diatomic molecules that could be sensitive to these extremely small variations in electric fields.
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funding: ONR
The 1998 and 2001 Nobel Prizes in physics both involved the cooling of gases of alkali atoms to "ultracold" temperatures (so T<<1mK.) I am involved in research to try to create gases of trapped molecules at similar temperatures. There are many reasons that the Atomic and Molecular Physics community seeks sources of cold molecules1: One is to explore scattering dynamics at extremely low energies. Another is to carry out precision spectroscopy and measure lifetimes of metastable states. A third is to explore the use of trapped polar molecules for quantum computing2. A major motivation for my group is the production of trapped heavy diatomic molecules that might exhibit physics beyond the Standard Model (see Spectroscopy Beyond the Standard Model.) Our strategy is to create a very weak but continuous source of cold molecules that sublimate off a cold surface3. The ultracold leading edge of the velocity distribution is "skimmed" using either an electric or magnetic guide. These cold selected molecules are guided into a trap where they are optically pumped into a dark state, causing them to "stick" to the trap. By waiting for approximately 100 seconds, an appreciable number density of ultracold molecules should be be created.
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Spectroscopy Beyond the Standard Model funding: National Research Council
Symmetry dictates that states of an atom or molecule with total angular momentum M≠0 along the axis of an electric field will exhibit a two-fold degeneracy between states differing only in the sign of of M. A time-reversal asymmetry could break this degeneracy. The Standard Model of Physics indicates that if such a time-reversal asymmetry exists, it should be much too small to measure1. Almost every other theory, and most notably Super Symmetric Theories, indicate that time-reversal asymmetry should lead to an observable energy difference between two otherwise degenerate ±M states2. This time reversal asymmetry may be attributed to an non-zero electric dipole moment of the electron. The current limit3,4 on the electric dipole moment (EDM) of the electron is 1.6×10-27 e cm. Surpassing this limit will not be easy, as even proving that the electron dipole moment must be less than the current limit was an impressive feat of modern experimental physics. At the current limit, an electron would gain 17 times more potential energy by moving 1 angstrom up the earth's gravitational field than by flipping its spin in a strong laboratory field of 105volts/cm. Sensitivity to the electron's dipole moment therefore requires the study of a system that both exaggerates its effect and is amenable to precision measurement. The current limit has been achieved by Commins and coworkers by taking advantage of the electronic structure of the Tl atom. As was first pointed out by Sandars5, a relativistic atom may have energy levels that are altered by a EDM by an energy substantially greater than expected from 2deE of a free electron. The fact that the energy difference between the F=1, M=±1 fine structure states of 205Tl in an electric field has been calculated6 to be -585(2deE) is one reason for the success of the Commins experiment. A second reason is the fact that the F=1,M=±1 states of Tl are completely degenerate in the absence of an applied field. This allows a precise quantum beat measurement of the electron's EDM. In the case of the Tl experiment, a Ramsey atomic beam resonance experiment was done to obtain sensitivity to extremely small energy differences between the F=1, M=±1 states. To move significantly beyond the Tl experiment, it has been proposed that molecular systems be used. Molecular systems provide large internal electric fields that can further exaggerate the effect of an electron EDM. Electronic structure calculations on the PbF radical show it to have the largest known ratio of enhancement of the EDM-induced M=±1 splitting with respect to the more mundane B-field induced splitting7. Even with this large enhancement, the background magnetic field will have to be kept well below a micro Gauss and it will take several seconds of phase accumulation to observe a splitting between otherwise degenerate levels of PbF. Our strategy will be to trap PbF molecules in a biased Stark trap8 and perform an optical quantum beat experiments to measure the EDM-induced energy splitting.
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