Cold chemistry with two atoms

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Science  25 May 2018:
Vol. 360, Issue 6391, pp. 855-856
DOI: 10.1126/science.aat7917

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For centuries, chemists have written equations representing chemical reactions by using symbols for atoms and molecules; for example, 2H2O + 2Na → 2NaOH + H2. This short notation shows only four reacting particles, but even in a classroom demonstration where a small piece of sodium is dropped in water, the total number of reactants will be on the order of Avogadro's number (∼6 × 1023). On page 900 of this issue, Liu et al. (1) instead study a chemical reaction taking place between a minimal number of participants. In their experiment, exactly two atoms collide, absorb a photon, and form a molecule in the excited state. And this time, the reaction equation, Na + Cs → NaCs* (where the asterisk denotes an excited molecule), describes exactly the process that takes place in the laboratory.