High Resolution Electron Energy Loss Spectroscopy Data Research Articles

Oxygen chemisorption on Cr(110) I. Dissociative adsorption

The initial stages of oxygen chemisorption on Cr(110) at 300 K have been investigated using high resolution electron energy loss spectroscopy (HREELS), electron stimulated desorption ion angular distributions (ESDIAD), low energy electron diffraction (LEED), and Auger electron spectroscopy (AES). Dissociative chemisorption occurs with near unit sticking probability, leading to an ordered p(4 × 2)O overlayer at θo ⋍18. A model with atomic oxygen located in the two-fold symmetric hollow sites is proposed. The previously reported c(3 × 1)O overlayer was not observed in this study. A disordered oxygen adlayer is found as θo increases. ESDIAD and HREELS data suggest that chemisorbed oxygen atoms occupy inequivalent local binding sites for θ > 0.25; subsurface oxygen is not found under these mild oxidation conditions.

Vibrational study of the SiO2/Si interface by high resolution electron energy loss spectroscopy

By using high resolution electron energy loss spectroscopy (HREELS) the surface phonon spectra of SiO2(0001) and of thick amorphous SiO2 layers have been measured. The spectra recorded on the crystalline surface are characterized by three first order peaks at 498, 798, and 1176 cm−1. The HREELS data could be interpreted by the dielectric theory applied to anisotropic material, and the optical constants were compared to infrared spectroscopy values. HREELS spectra as a function of oxide thickness could be obtained from oxidized Si(100) wafers by sputter erosion. Peak positions shifted to lower frequencies and intensities decreased regularly with decreasing oxide thickness. At the interface a transition layer of about 25 Å was observed and carbon contamination was identified. The dielectric theory applied to a thin homogeneous supported film cannot account for the observed frequency shift.

A high-resolution electron energy loss spectroscopy study of the surface structure of benzene adsorbed on the rhodium(111) crystal face

Benzene adsorption on the Rh(111) crystal surface has been studied by HREELS (high-resolution electron energy loss spectroscopy), LEED (electron diffraction), and TPD (temperature programmed desorption). The vibrational spectra indicate that benzene adsorbs molecularly at 300 K and is ..pi..-bonded to the surface with the ring plane parallel to the surface plane. Recent dynamic LEED calculations together with the angle-dependent HREELS studies reported here establish a C/sub 3v/(sigma/sub d/) bonding symmetry for the c(2..sqrt..3 x 4)rect-C/sub 6/H/sub 6/ structure. Several other ordered benzene overlayers can be formed between 300 and 400 K depending on the benzene coverage. No large changes occur in the chemisorption bonding mode or geometry coincident with the two-dimensional ordering phase transitions in this temperature range. The vibrational spectra show that two molecular adsorption sites can be populated. Benzene adsorption is only partially reversible; less than 20% of the adsorbed benzene desorbs molecularly upon heating. The remaining benzene irreversibly decomposes, evolving hydrogen and leaving a carbon-covered surface. The TPD and HREELS data on Rh(111) and other single-crystal surfaces show correlations between the metal-benzene bond strength, the work function of the clean surface, and the frequency shifts of some of the molecular benzene vibrational modes. 44 references, 10 figures,more » 4 tables.« less

The metal-to-semiconductor transition in ternary ruthenium (IV) oxides: a study by electron spectroscopy

The pyrochlore ruthenium (IV) oxides Pb2Ru2O7-y, Bi2Ru2O7, Y2Ru2O7 and the perovskites SrRuO3, CaRuO3 have been studied by ultraviolet and X-ray photoelectron spectroscopy (UPS and XPS) and by high-resolution electron-energy-loss spectroscopy (HREELS). The presence of a Fermi edge in UPS confirms that, with the exception of Y2Ru2O7, these materials are metallic; the density of states at the Fermi energy g( epsilon F) decreases in the sequence Pb2Ru2O7-y>Bi2Ru2O7>SrRuO3>CaRuO3. The probability of reaching final states in XPS where a Ru:3d core hole is screened by conduction electrons decreases in the same sequence, and for Y2Ru2O7 only unscreened final states are observed. In parallel with these trends, the conduction-electron plasmon loss found in HREELS moves to lower energy with decreasing g( epsilon F); plasmon excitation is not evident in the energy-loss spectrum of semiconducting Y2Ru2O7. The results are consistent with a progressive decrease in the transfer-energy (resonance) integral that determines the Ru:4d bandwidth in the series Pb2Ru2O7-y>Bi2Ru2O7>SrRuO3>CaRuO3>Y2Ru2O7, and with a transition to non-metallic behaviour due to correlation-induced electron localisation in Y2Ru2O7. A technique for the calculation of the width of photoemission lines in UPS from HREELS data is presented.