The surface photochemistry of NO₂ on ultrathin Ag(111) films (5-60 nm) on Si(100) substrates has been studied. NO₂, forming N₂O₄ on the surface, dissociates to release NO and NO₂ into the gas phase with translational energies exceeding the equivalent of the sample temperature. An increase of the photodesorption cross section is observed for 266 nm light when the film thickness is decreased below 30 nm despite the fact that the optical absorptivity decreases. For 4.4 nm film thickness this increase is about threefold. The data are consistent with a similar effect for 355 nm light. The reduced film thickness has no significant influence on the average translation energy of the desorbing molecules or the branching into the different channels. The increased photodesorption cross section is interpreted to result from photon absorption in the Si substrate producing electrons with no or little momenta parallel to the surface at energies where this is not allowed in Ag. It is suggested that these electrons penetrate through the Ag film despite the gap in the surface projected band structure.