Ef®ciencies above unity in light-induced reaction of Cu with Cl2: excitation, ampli®cation, and diffusion processes

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 Cl2 pressure range from 10ÿ1 to 10ÿ7 mbar. AFM and SEM measurements of the topology of the resulting CuClx ®lm reveal a selective reaction with addition of volcanoes (irradiated parts) and a non-selective reaction (shaded parts). The non-selective reaction originates from Cl2 dissociation in the range from 140 to 120 nm, reaches quantum ef®ciencies around 200±1400, and each Cl atom generates from 2 106 up to 108 CuClx molecules. The selective reaction originates from dissociation of physisorbed Cl2 molecules and reaches quantum ef®ciencies between 2 and 10 in two bands centered at 144 and 170 nm. The conversion of a passivating CuCl top layer to CuCl2 is identi®ed as the amplifying process for both reactions from depth pro®les 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 Cl2 case and to bulk diffusion in grain boundaries for the Cu case. Cl2 sticking coef®cients exceed those involved in the dark reaction by more than the order of magnitude. # 2001 Elsevier Science B.V. All rights reserved.