Electron Electron Energy Loss Spectroscopy Research Articles

Dissociative adsorption and thermal evolution of carbon tetrachloride on Si(1 1 1)7 × 7

The adsorption of CCl 4 on Si(1 1 1)7 × 7 at room temperature has been characterized by electronic and vibrational electron energy loss spectroscopy (EELS). CCl 4 was found to adsorb dissociatively on Si(1 1 1)7 × 7 with the formation of Si Cl bond and adsorbed CCl x species. At high exposures, CCl 4 appears to more readily undergo a greater degree of dechlorination. Using a Si 16H 18 cluster to model only the adatom–restatom site, our density functional theory calculations suggest two plausible adstructures involving the dissociated Cl and CCl 3 fragments. The wavenumbers of the observed EELS features are found to be in general accord with the corresponding calculated wavenumbers. Thermal evolution of the adsorbed fragments has been followed by both vibrational and electronic EELS as a function of the sample annealing temperature. A new EELS feature at 920–960 cm −1 attributed to SiC film or alloy formation is found to emerge at 573 K, while the Si Cl stretch at 550 cm −1, corresponding to dissociated surface Cl, is removed upon annealing to ∼883 K. Furthermore, the electronic EELS spectra reveal an energy loss at 9.3 eV that can be assigned to a single-electron transition from the Cl(p z ) bonding state to the Cl(p z ) antibonding state produced by the Si Cl bond.

Bimolecular surface photochemistry: Mechanisms of CO oxidation on Pt(111) at 85 K

Results from a photoinduced bimolecular surface reaction are presented. The reaction, occurring from CO coadsorbed with O2 on Pt(111) at 85 K, is O2+CO+hν→O+CO2. Surface analysis techniques employed include electron energy loss spectroscopy (EELS), thermal desorption spectroscopy (TDS), photon-induced desorption spectroscopy (PID), and low energy electron diffraction (LEED). The incident power, photon energy, and polarization dependences of the photochemical processes, O2 photodesorption and CO2 photoproduction, were characterized, with the cross section for both processes being 3×10−19 cm2 at 240 nm. Electronic EELS studies were performed to acquire information on the electronic structure of O2 on Pt(111). The experimental results are compared to predictions of models describing direct dipole excitation of the O2–Pt system and substrate mediated hot carrier mechanisms. Reaction mechanisms involving photogenerated hot O atoms or excited O2 molecules on the surface are considered. The implications of this work on surface reaction dynamics are discussed.

Correlation of electronic and optical transitions: Mo(CO) 6 adsorbed on clean and K-preadsorbed Si(111)7 × 7

Electron energy loss spectroscopy (EELS) is used to measure the electronic structure of adsorbed Mo(CO) 6 molecules on clean and potassium-preadsorbed Si(111)7 × 7 at 82 K. The electronic structure of physisorbed MoCCO) 6 is found to be not significantly (within 0.1 eV) perturbed from that of free molecules. The implication of electronic EELS measurements to surface photochemistry is discussed in this paper.

Correlation of electronic and optical transitions: Mo(CO) 6 adsorbed on clean and K-preadsorbed Si(111) 7 × 7

Electron energy loss spectroscopy (EELS) is used to measure the electronic structure of adsorbed Mo(CO) 6 molecules on clean and potassium-preadsorbed Si(111)7 × 7 at 82 K. The electronic structure of physisorbed Mo(CO) 6 is found to be not significantly (within 0.1 eV) perturbed from that of free molecules. The implication of electronic EELS measurements to surface photochemistry is discussed in this paper.

Resonant photodissociation of Mo(CO)6 adsorbed on graphite and Ag(111)

The adsorption and photodissociation of Mo(CO)6 on the basal plane of graphite and Ag(111) are studied by photoinduced desorption, high-resolution electron-energy-loss spectroscopy, electronic electron-energy-loss spectroscopy (EELS), and thermal desorption spectroscopy. Mo(CO)6 is found to absorb in pure molecular form, without dissociation, on each surface at 85–90 K. Electronic EEL spectra confirm that the electronic structure of the molecule remains relatively unperturbed on the surface. Similar to the gas phase, electronic transitions of the molecule, including the ligand-field transition and the metal-to-ligand charge transfer, were observed for absorbed Mo(CO)6. Upon low-power UV irradiation (λ<360 nm), the adsorbed molecules readily photodissociate and release CO. The mechanism of photodissociation on each surface is separately identified to be direct photoelectronic excitation of the adsorbed Mo(CO)6 by careful photon power and wavelength-dependence studies. Linear power dependence was found in each case, indicating the initial excitation is due to the absorption of a single photon. Resonances in the photodissociation spectra of the molecules were observed at ∼290 nm and 325 nm, due to the metal-to-ligand charge transfer and the ligand-field transition, respectively. For graphite, the relative photoyield was observed to closely match the Mo(CO)6 absorption spectrum. In contrast, a relative enhancement in the photoyield was observed for Ag(111) at ∼325 nm due to the increase of the surface electric field associated with the onset of the d band to the Fermi-level transition in this wavelength region. In addition, small quantities of the fragments from Mo(CO)6 were observed as a result of bombardment of the surface by low-energy (20 eV) electrons. The photodissociation yields are very sensitive to the adsorbate coverage. For a coverage of about 0.25 monolayer or less, there is no observable photodissociation for Mo(CO)6 on Ag. On both Ag and graphite, the photodissociation yield increases as the coverage approaches and exceeds one monolayer; photodissociation competes efficiently with relaxation into the substrate.

Auger electron spectroscopy and electron energy loss spectroscopy studies of the formation of silicon nitride by implanting low energy nitrogen ions into silicon

Thin ( d ⩽ 10 nm) films of almost stoichiometric silicon nitride were prepared by implanting nitrogen ions with energies below 5 keV into silicon with subsequent thermal annealing. A combination of Auger electron spectroscopy (AES) and electron energy loss spectroscopy (ELS) was used to determine the chemical composition and to investigate the chemical bonding states of the thin films. The spectra are in good agreement with those obtained for Si 3N 4 produced by chemical vapour deposition. The use of AES and electron ELS allowed the stepwise formation of the SiN bonds to be investigated for the first time, and hence further information about the electronic structure of Si 3N 4 was obtained. Depth-concentration profiles for nitrogen are presented.