[en] The reaction between SiH₄ and UF₆ under thermal gas phase and photon initiated conditions in cryogenic matrices has been investigated. The gas phase thermal reaction begins at 130--140⁰ C producing HF, SiH₃F, and a solid uranium fluoride product. When the UF₆ is matrix isolated within SiH₄ at 12 K and exposed to low power infrared radiation of 25 mW cm^-2 using laser emissions resonant with the ν₃ band of UF₆, a reaction also occurs. This reaction produces SiH₃F, UF₅, and UF₄ as products. The same reaction is also catalyzed using a broad band incoherent source with a photon flux density of 10 μW cm^-2. The activation energy for the photoreaction (1.8 kcal mol^-1) is considerably lower than the thermal activation barrier for the gas phase reaction. These experiments are part of the bases for uncovering a process of photochemistry of reactants in fixed relative configurations--single photon absorption reaction chemistry in the solid state (SPARCSS). This process is a manifestation of a previously unrecognized, general matrix phenomenon in which spatial configurations dramatically affect potential energy surfaces for reaction