Cold chemistry with two atoms

Two atoms react to form a molecule in an optical “beaker” For centuries, chemists have written equations representing chemical reactions by using symbols for atoms and molecules; for example, 2H 2 O + 2Na → 2NaOH + H 2 . 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 × 10 23 ). 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.