Ultracold molecule

Article By:

Naduvalath, Balakrishnan Department of Chemistry, University of Nevada, Las Vegas, Nevada.

Last reviewed:2013

DOI:https://doi.org/10.1036/1097-8542.YB130133

One of the long-standing goals in chemistry is to control and manipulate the outcome of chemical reactions to yield desired products. While some progress has been achieved in controlling product branching in photofragmentation of diatomic and triatomic molecules, the absolute control of bimolecular chemical reaction dynamics, even in simple atom-diatom exchange reactions (such as A + BC → AB + C or AC + B, where BC is the reactant molecule and AB and AC are possible product molecules), has yet to be realized. Precise control of molecular encounters and chemical reactions requires initial preparation of molecules in well-defined internal quantum states. However, at ordinary temperatures, molecules exist in a thermal population of internal quantum states, corresponding to different vibrational, rotational, and hyperfine levels. In typical diatomic molecules such as N₂ or O₂, the energy spacing between vibrational levels is on the order of a thousand kelvins (in this article, energy will be given as E/k_B expressed in kelvins, where E is the kinetic energy and k_B is the Boltzmann constant), the spacing between rotational energy levels is on the order of a few kelvins, and the spacing between hyperfine levels is a small fraction of a kelvin. The energy level separation becomes smaller for heavier molecules. To prepare molecules in specific internal quantum states, their translational temperature T (kinetic energy) must be reduced to significantly below 1 K so that the thermal energy k_B T is smaller than the tiniest energy separation between internal quantum states. Thus, to enable control of reaction dynamics, molecules must be cooled to temperatures lower than a millikelvin or microkelvin, depending on their mass and energy-level structure.

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