A novel approach to the enhancement of photochemical reaction yield in hydrogen-containing clusters isoutlined and applied to the process of the Cl2 molecule formation from a UV-photolyzed Cl···HCl species.The control mechanism consists of a far-IR preexcitation of the large-amplitude hydrogenic bending (librational)mode prior to the HCl photodissociation. Molecular dynamics simulations that properly sample the initialquantum state of the system show that this leads to more than a factor of 2 enhancement of the Cl2 yield bothin the parent and deuterated clusters.