Light-induced reactions are generated with wavelength dispersed synchrotron radiation in the spectral range from 200 to 120 nm on polycrystalline Cu plates covered with a test mask in a Cl-2 pressure range from 10(-1) to 10(-7) mbar. AFM and SEM measurements of the topology of the resulting CuClx film reveal a selective reaction with addition of volcanoes (irradiated parts) and a non-selective reaction (shaded parts). The non-selective reaction originates from Cl-2 dissociation in the range from 140 to 120 nm, reaches quantum efficiencies around 200-1400, and each Cl atom generates from 2 x 10(6) up to 10(x) CuClx molecules. The selective reaction originates from dissociation of physisorbed Cl-2 molecules acid reaches quantum efficiencies between and 10 in two bands centered at 144 and 170 nm. The conversion of a passivating CuCl top layer to CuCl2 is identified as the amplifying process for both reactions from depth profiles of the composition obtained with AES, XPS, and sputtering. pressure and intensity studies reveal temperature-dependent diffusion lengths of several micrometers which are attributed to surface diffusion in the Cl-2 case and to bulk diffusion in grain boundaries for the Cu case, Cl-2 sticking coefficients exceed those involved in the dark reaction by more than the order of magnitude.