Photochem. reactions and etching efficiencies of polycryst. Cu and GaAs(100) with Cl2 were studied in the spectral range of 105-300 nm by using synchrotron radiation and for Cl2 pressures between 10-6 and 10 mbar. Dissocn. of Cl2 leads in the case of Cu to the formation of CuClx films (typical thickness 104 nm) with an efficiency of .apprx.107 CuClx mols. per generated Cl atom at high Cl2 exposures. For low Cl2 exposures, anisotropic CuClx growth is obsd. in the irradiated area (typical thickness 50 nm) and the efficiency decreases with increasing photon energy. Anisotropic etching shows up for GaAs with typical etch depth of 100 nm in the irradiated area. The efficiency increases in general with photon energy, but exhibits in addn. strong variations within small photon energy ranges for example at 118-122 nm. The efficiency increases strongly with pressure in the range from 10-1 to 10 mbar. [on SciFinder (R)]