A novel approach to the enhancement of photochemical reaction yield in hydrogen containing clusters is outlined and applied to the process of the Cl$_2$ 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. Molecular dynamics simulations which properly sample the initial quantum state of the system show that this leads to more than a factor of two enhancement of the Cl2 yield both in the parent and deuterated clusters.