ADSORPTION, DESORPTION AND PHOTOCHEMISTRY OF SMALL MOLECULES ON CLEAN AND CHEMICALLY MODIFIED RU(001) SURFACES

Abstract

Two UHV systems equipped with mass spectrometers, LEED and HREELS were set up in this project. A base pressure of 2 x 10-10 Torr range can be readily achieved for both systems. Some experiments on the adsorption of small molecules were conducted on clean and chemically modified Ru(001) to investigate the 'through metal' and lateral interactions between coadsobates. Thermal and photo-induced desorption and decomposition of metal carbonyls (Iron and Molybdnenum) were also carried out. On S/Ru(001), two new weakly bound hydrogen adsorption states at 135 and 265 K were observed which are attributed to the electronic interaction of S-adatoms with nearby ruthenium sites. On the other hand, pre-adsorbed N-atoms only suppress the adsorption of hydrogen with each N-adatom blocking four hydrogen adsorption sites. These differences can be qualitatively attributed to the weaker interaction of N-atoms with Ru(001). In the studies involving decomposition of ammonia on O- and S-covered Ru(001): on O-covered surface, a new sharp nitrogen desorption peak was observed at 640 K, growing in intensity with oxygen coverage, which can be qualitatively explained by the repulsive interactions between the adsorbed nitrogen and oxygen atoms. Our work farther revealed that sulfur adatoms significantly inhibit the decomposition or ammonia. The adsorption or N2O on clean, oxygen, deuterium and carbon monoxide modified Ru(001) surfaces was investigated. Interestingly, on partially O-covered Ru(001) the enhanced adsorption or N2O was observed. Whereas, on D- and CO-precovered surfaces, the N2O adsorption is destabilised. These results can be accounted for by considering the ‘through-metal' and the lateral interactions between coadsorbates. The thermal and photo-induced desorption and decomposition of Fe(CO)5 and Mo(CO)6 on clean and chemically modified Ru(001) surfaces were studied. Both carbonyls adsorb dissociatively on clean Ru(001) forming CO and M(CO)X (M = Fe or Mo) fragments. Thus CO thermal desorption spectra (TDS) of both carbonyls on clean surface showed peaks attributed to molecularly adsorbed carbonyls (mono- and multilayers) as well as the surface stabilized decomposition products (Mx(CO)y species). In the interaction of Fe(CO)5 with H-adatoms studied by TDS, the migration of H-adatoms into the multilayers of Fe(CO)5 was proposed and a model was derived to explain this phenomenon. The photochemical studies of Fe(CO)5 at mono- and multi-layer molecular coverages were carried out by UV - irradiation at various wavelengths (290 - 450 nm). Irradiation at wavelengths > 370 nm resulted in photodesorption, while photodecomposition showed significant contribution at shorter wavelengths. Chemical modifications of the surface with O- and S-adatoms strongly influence the adsorption of Mo(CO)6 by both stabilizing the first molecular layer of Mo(CO)6 and by suppressing the decomposition. Photochemical studies were conducted for one monolayer of molecular Mo(CO)6 adsorbed on the clean as well as the O- and CO-saturated Ru(001) surfaces by UV irradiation at various wavelengths (290 - 390 nm). The calculated total photochemical cross sections for both carbonyls closely follow their UV absorption spectra in gaseous phase. This suggests that the photoreaction is mainly due to the direct absorption of UV-photons by the adsorbed molecule. An UHV chamber was specially designed and constructed for the HREELS system. The HREELS was set up and tuned. the resolution of the spectrometer was found to be around 30 - 40 cm-1 (full width at half-maximum).