A novel approach to the control of photochemical reaction yield in hydrogen containing clusters is outlined and applied to the process of the Cl₂ molecule formation from a UV photolyzed Cl...HCl species. The control mechanism consists in a far-IR preexcitation of the large amplitude hydrogenic bending (librational) mode prior to the HCl photodissociation, leading to more than a factor of two enhancement of the Cl₂ yield both in the parent and deuterated cluster.