A new way to control of 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. On the other hand, 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.