Ultracold Chemistry

The development of techniques for cooling and trapping of a wide variety of atomic and molecular samples in recent years has created exciting opportunities for probing and controlling atomic and molecular encounters with unprecedented precision. The experimental methods such as photoassociation spectroscopy magnetic tuning of Feshbach resonances, buffer-gas cooling, and Stark deceleration have been developed and applied to a variety of molecular systems to create cold and ultracold molecules with thermal and nonthermal vibrational populations. Since energy level spacing between rotational and vibrational levels in molecules is much larger than the trap depths exothermic collisions leading to rotational, vibrational energy transfer, spin-exchange, or chemical reactions can lead to trap loss. We have been interested in understanding mechanisms of energy transfer in atom-molecule and molecule-molecule systems at cold and ultracold temperatures and how they depend on the internal rovibrational energy levels of the molecules. Calculations have shown that rovibrational relaxation can be highly efficient and specific at ultralow energies and they may depend strongly on the initial rovibrational levels of the molecule. Calculations of H-H2, He-H2, He-CO, He-O2 and H2-H2 systems have shown that rate coefficients for inelastic collisions become finite on the limit of zero-temperature, in agreement with Wigner's law. Our calculations of F+H2, F+HD, Li+HF, and Cl+HD reactions have also demonstrated that barrier reactions may occur very efficiently at ultracold temperatures due to quantum tunneling. A number of recent studies on barrierless reactions involving alkali metal systems such as Li+Li2, K+K2, and our own recent study of the O+OH reaction, have shown that barrierless reactions also occur with large rate coefficient at ultracold temperatures. Our ongoing work in this field is aimed at understanding mechanistic details of chemical reactions at cold and ultracold temperatures and how long-range intermolecular forces influence reactivity at temperatures close to absolute zero. Recent publications in this area are listed here.

Selected Publications

F+H2rxn

Figure 1. Product HF vibrational level resolved reaction cross sections for the F+H2 → HF+H reaction. At low energies, the cross section varies inversely as the velocity in accordance with Wigner's law. Link to paper


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H2-H2

Figure 2. The most favorable transition involves ΔE ≈ 0 and Δjab = 0. Link to paper


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