RESUMEN
The oxidation mechanism of hafnium overlayers on an Si(111) substrate [Hf-Si(111), including the outermost metallic Hf overlayers and interfacial Hf silicides (HfSi and HfSi4)] was investigated via high-resolution synchrotron radiation X-ray photoelectron spectroscopy (SR-XPS) of Hf 4f5/2,7/2, Si 2p1/2,3/2, and O 1s core levels. The atomic-scale interaction of O2 molecules with Hf-Si(111) is discussed by comparing the results obtained following thermal O2 exposures [translational energy (Et) ≈ 0.03 eV] with those obtained following supersonic O2 molecular beam (SOMB) irradiation (Et ≈ 2.2 eV). Metallic Hf and interfacial HfSi were immediately oxidized to HfO2 and Hf (sub)silicates (Hf-O-Si configurations) via trapping-mediated dissociative adsorption. Upon excessive SOMB irradiation, the other interfacial HfSi4 was oxidized via direct dissociation. When oxidation proceeded at the Si(111) substrate via excess SOMB irradiation, volatile Si atoms were emitted from the interfacial SiO2/Si-strained layers. When the volatile Si atoms were trapped in the overlayers, the HfO2 overlayers were converted into completely oxidized Hf silicate layers. However, when the volatile Si atoms passed through the HfO2 overlayers, they reacted with the impinging O2, and the outermost SiO2 deposition layers were formed on HfO2 (or Hf silicate) layers.
RESUMEN
Site-specific electron relaxations caused by Si:2p core-level photoionizations in F3SiCH2CH2Si(CH3)3 and Cl3SiCH2CH2Si(CH3)3 vapors have been studied by means of the photoelectron Auger electron coincidence spectroscopy. F3SiCH2CH2Si(CH3)3 shows almost 100% site-specificity in fragmentation caused by the Si:2p ionization. However, substitution of Cl for F of F3SiCH2CH2Si(CH3)3 considerably reduces the site-specificity at the Si atom bonded to three halogen atoms, with the site-specificity at the Si site bonded to three methyl groups remaining largely unchanged. The site-specificity reduction in Cl3SiCH2CH2Si(CH3)3 is considered to take place during the transient period between Si:L23VV Auger electron emission and the subsequent fragmentation. The reason for the reduction can be explained in terms of some differences between these two molecules in the L23VV Auger decay at the Si site bonded to the three halogen atoms.
RESUMEN
In an aim to create a "sharp" molecular knife, we have studied site-specific fragmentation caused by Si:2p core photoionization of bridged trihalosilyltrimethylsilyl molecules in the vapor phase. Highly site-specific bond dissociation has been found to occur around the core-ionized Si site in some of the molecules studied. The site specificity in fragmentation and the 2p binding energy difference between the two Si sites depend in similar ways on the intersite bridge and the electronegativities of the included halogen atoms. The present experimental and computational results show that for efficient "cutting" the following conditions for the two atomic sites to be separated by the knife should be satisfied. First, the sites should be located far from each other and connected by a chain of saturated bonds so that intersite electron migration can be reduced. Second, the chemical environments of the atomic sites should be as different as possible.
RESUMEN
We have developed a miniature electron ion coincidence (EICO) analyzer mounted on a conflat flange with an outer diameter of 114 mm. It consists of a cylindrical mirror analyzer (CMA), a time-of-flight ion mass spectrometer (TOF-MS), a commercially available linear motion feed through, and a tilt adjustment mechanism. Each sample surface was irradiated by synchrotron radiation, and the energies of emitted electrons were analyzed and detected by the CMA, while desorbed ions were collected by the TOF-MS in coincidence with the electrons. The performance of the EICO analyzer was tested by measuring the Auger-electron H+ photoion coincidence spectrum of condensed water at 4a1 <-- O 1s resonance.
RESUMEN
The rate constant and activation energy of the regeneration reaction of natural vitamin E by vitamin C were determined with a double-mixing stopped-flow spectrophotometer. The formation of vitamin C radical was observed in the absorption spectrum. The kinetic effect of methyl substitution on the aromatic ring of vitamin E radical indicates that partial charge-transfer plays a role in the reaction. Since a substantial deuterium kinetic isotope effect was not found, the tunneling effect may not play an important role under the present experimental conditions.
