In a step forward for using laser pulses to precisely control which bonds are made and which are broken in a chemical reaction, researchers have used tailored laser light to steer photoassociation of magnesium atoms to form Mg2(Phys. Rev. Lett. 2015, DOI: 10.1103/physrevlett.114.233003). Such coherent control of photodissociation reactions is well established, but it has been difficult to achieve for photoassociation reactions. To produce Mg2, a team led by Zohar Amitay of Technion—Israel Institute of Technology and Christiane P. Koch of Germany’s University of Kassel targeted magnesium atoms at 1,000 K with laser pulses in which the electric field of the pulse oscillates faster as the pulse progresses. The magnesium atoms absorb two photons to form Mg2, which then absorbs a third photon to reach an excited state. The excited-state structure subsequently emits ultraviolet light as it decays to two individual atoms. The structure of the laser pulse affects vibrational transitions in an intermediate state in a way that enhances that UV emission, signaling photoassociation. The next step is to control a reaction that yields stable products, Koch says.