Synchrotron Radiation Studies of Solid Surfaces and Interfaces.

The use of synchrotron radiation as a light source for the study of surfaces and interfaces is demonstrated. Two surface science techniques, soft X-ray photoelectron spectroscopy (SXPS) and photon-stimulated desorption (PSD), are used in the experiments, both of which use synchrotron radiation as a tunable, high-resolution light source, were used. First, the adsorption of I_2 on Si(111)-7 x 7 is studied with SXPS. The results indicate that I_2 adsorbs dissociatively, forming a mixture of SiI, SiI_2 and SiI_3 moieties, of which SiI dominates. The adsorption proceeds first by attachment of I to the dangling bonds of the 7 x 7 unit cell and then by the breaking of the Si-Si bonds between the adatoms and the second-layer atoms, producing SiI_2 and SiI_3^ecies via the sequential addition of I at Si adatom sites. The results also indicate that substrate Si-Si bonds are broken by the reaction with I_2. Next, SXPS is used to study the coadsorption of K and PF_3 on Ru(0001) at 90 and 300 K. In the absence of K, PF_3 adsorbs molecularly at both temperatures. In the presence of K, PF_3 chemisorbs dissociatively as well as molecularly. The dissociative chemisorption continues until all the K are reacted and have become KF. Further exposure to PF_3 results in the molecular adsorption. At 300 K, there is evidence that a fraction of the incoming PF_3 molecules react with the adsorbed KF and F to form KPF _6. This species in not observed at 90 K, however, indicating that adsorption kinetics of the PF_3 molecules are different at the two temperatures, as some of the chemical reactions channels that occur at 300 K are eliminated at 90 K resulting in different concentrations of surface species. Finally, the radiation damage in CaF_2 films grown epitaxially on Si(111) is studied by SXPS and PSD. When irradiated with photons of sufficient energy, F-centers defects form in CaF_2 films. The electronic transitions responsible for the formation of F-centers at the surfaces of these films have been identified via PSD. Results for films ranging from submonolayer to bulk CaF_2 indicate that excitations of the Ca 3p core level result in F^+ desorption, and hence, in F-center formation, while excitations of the F 2s do not. It is proposed that this difference is due to nuclear motion that occurs prior to the de-excitation of the core hole.