Screening effect of metal, semiconductor and insulator surfaces on the photo-fragmentation of physisorbed molecules

Photon-stimulated desorption of fragment ions from physisorbed CCl4 on metallic Cu(100), semiconducting Si(100) and insulating SiO2 surfaces has been investigated using synchrotron soft X-ray around the Cl K-edge. Only atomic Cl+ ions were desorbed from CCl4/Cu(100) and CCl4/Si(100) following the Cl K-edge excitation. In contrast, molecular CCl3+ ions were desorbed as well from CCl4/SiO2. The lack of molecular-ion desorption from Cu(100) and Si(100) is interpreted as the quenching of the excited states due to charge-transfer screening by the substrate. The desorption of molecular species observed only for CCl4/SiO2 is interpreted not as the direct core-to-valence resonant excitation but as Auger-stimulated secondary electrons due to the weak screening effect of the insulator surface. (C) 1999 Elsevier Science B.V. All rights reserved.