Librational control of reactions in large clusters

A way to control the yield of reactions in clusters is presented and demonstrated on the formation of the Cl2 molecule from a photodissociated HCl molecule trapped on a Cl(Ar)12 cluster using molecular dynamics simulations. The key to the method is the large-amplitude librational motion of the hydrogen atom prior to photolysis. Starting from the ground librational state practically no product is formed. However, turning on the control mechanism by optical or thermal preexcitation of the system to the first excited librational state leads to a 70% Cl2 product yield.