Continuous-time photon-stimulated desorption spectroscopy studies on soft x-ray-induced reactions of CF3Br adsorbed on Si(111)-7×7.

Continuous-time core-level photon-stimulated desorption (PSD) spectroscopy was used to study the soft x-ray-induced reactions of CF(3)Br molecules adsorbed on Si(111)-7×7 near the Si(2p) edge (98-110 eV). The monochromatic synchrotron radiation was employed as a soft x-ray light source in the photon-induced reactions and also as a probe for investigating the produced fluorination states of the bonding surface Si atom in the positive-ion PSD spectroscopy. Several different surface coverages were investigated. The PSD spectra from the low-CF(3)Br-covered surfaces show the production of surface SiF species, while those from the high-CF(3)Br-covered surfaces depict the formation of surface SiF, SiF(2), and SiF(3) species. The photolysis cross section of the submonolayer CF(3)Br-covered surface is determined as ∼4.3×10(-18) cm(2). A comparison with the results on CF(3)Cl/Si(111)-7×7 surface is discussed.

Monochromatic soft-x-ray-induced reactions of CF2Cl2 adsorbed on Si(111)-7 × 7 studied by continuous-time photon-stimulated desorption spectroscopy near the F(1s) edge.

Continuous-time core-level photon-stimulated desorption (PSD) spectroscopy was used to investigate the monochromatic soft x-ray photoreactions of CF(2)Cl(2) adsorbed on Si(111)-7 × 7 near the F(1s) edge (681-704 eV). Sequential F(+) PSD spectra were observed as a function of photon exposure at the CF(2)Cl(2)-covered surface (dose = 2.0 × 10(14) molecules cm(-2), ∼0.75 monolayer). The F(+) PSD and total electron yield (TEY) spectra of solid CF(2)Cl(2) near the F(1s) edge were also measured. Both F(+) PSD and TEY spectra depict three features in the energy range of 687-695 eV, and are assigned to the excitations of F(1s) to (13a(1) + 9b(2))[(C-Cl)(∗)], (7b(1) + 14a(1))[(C-F)∗] antibonding and 5p Rydberg orbitals, respectively. Following the Auger decay process, two holes are created in the C-F bonding orbitals producing the 2h1e final state which results in the F(+) desorption. This PSD mechanism, responsible for the F(+) PSD of solid CF(2)Cl(2), is used to explain the first F(+) PSD spectrum in the sequential F(+) PSD spectra. The variation of spectral shapes in the sequential F(+) PSD spectra shows the consumption of adsorbed CF(2)Cl(2) molecules and the production of surface SiF species as a function of photon exposure. The photolysis cross section of the adsorbed CF(2)Cl(2) molecules by photons with varying energy (681-704 eV) is deduced from the sequential F(+) PSD spectra and found to be ∼6.0 × 10(-18) cm(2).

Photon-exposure-dependent photon-stimulated desorption for obtaining photolysis cross section of molecules adsorbed on surface by monochromatic soft x-ray photons.

Photon-exposure-dependent positive- and negative-ion photon-stimulated desorption (PSD) was proposed to study the photoreactions and obtain the photolysis cross sections of molecules adsorbed on a single-crystal surface by monochromatic soft x-ray photons with energy near the core level of adsorbate. The changes in the F(+) and F(-) PSD ion yields were measured from CF(3)Cl molecules adsorbed on Si(111)-7x7 at 30 K (CF(3)Cl dose=0.3x10(15) molecules/cm(2), approximately 0.75 monolayer) during irradiation of monochromatic soft x-ray photons near the F(1s) edge. The PSD ion yield data show the following characteristics: (a) The dissociation of adsorbed CF(3)Cl molecules is due to a combination of direct photodissociation via excitation of F(1s) core level and substrate-mediated dissociation [dissociative attachment and dipolar dissociation induced by the photoelectrons emitting from the silicon substrate]. (b) the F(+) ion desorption is associated with the bond breaking of the surface CF(3)Cl, CF(2)Cl, CFCl, and SiF species. (c) the F(-) yield is mainly due to DA and DD of the adsorbed CF(3)Cl molecules. (d) The surface SiF is formed by reaction of the surface Si atom with the neutral fluorine atom, F(+), or F(-) ion produced by scission of C-F bond of CF(3)Cl, CF(2)Cl, or CFCl species. A kinetic model was proposed for the explanation of the photolysis of this submonolayer CF(3)Cl-covered surface. Based on this model and the variation rates of the F(+)F(-) signals during fixed-energy monochromatic photon bombardment at 690.2 and 692.6 eV [near the F(1s) edge], the photolysis cross section was deduced as a function of energy.

Soft x-ray photoreactions of CF3Cl adsorbed on Si(111)-7x7 studied by continuous-time photon-stimulated desorption spectroscopy near F(1s) edge.

The continuous-time core-level photon-stimulated desorption (PSD) spectroscopy was employed to monitor the monochromatic soft x-ray-induced reactions of CF3Cl adsorbed on Si(111)-7x7 near the F(1s) edge (681-704 eV). Sequential F+ PSD spectra were measured as a function of photon exposure at the CF3Cl-covered surface (dose=0.3x10(15) molecules/cm2, approximately 0.75 ML). The F+ PSD and total electron yield (TEY) spectra of molecular solid CF3Cl near the F(1s) edge were also measured. Both F+ PSD and TEY spectra show two features at the energy positions of 690.2 and 692.6 eV, and are attributed to the excitations of F(1s) to 11a1[(C-Cl)*] and (8e+12a1)[(C-F)*] antibonding orbitals, respectively. Following Auger decay, two holes are created in the F(2p) lone pair and/or C-F bonding orbitals forming the 2h1e final state which leads to the F+ desorption. This PSD mechanism, which is responsible for the F+ PSD of solid CF3Cl, is employed to interpret the first F+ PSD spectrum in the sequential F+ PSD spectra. The variation of spectrum shapes in the sequential F+ PSD spectra indicates the dissipation of adsorbed CF3Cl molecules and the formation of surface SiF species as a function of photon exposure. From the sequential F+ PSD spectra the photolysis cross section of the adsorbed CF3Cl molecules by photons with varying energy (681-704 eV) is determined to be approximately 1.0x10(-17) cm2.

Photon-stimulated desorption of F(-) ions from CF(3)Cl adsorbed on Si(111)-7x7.

We report the photon-stimulated desorption of negative ions induced by direct dipolar dissociation and dissociative electron attachment. The photon-stimulated desorption of F(-) ions from CF(3)Cl physisorbed on a Si(111)-7x7 surface at 30 K in the photon energy range 12-35 eV was studied. The F(-) ion yield exhibits four resonances, at 12.8, 16.2, 19.5, and 22.3 eV, quite unlike the gas phase photodissociation cross section. The intensities of these resonances depend strongly on the CF(3)Cl coverage in a manner which varies from peak to peak. The resonances at 19.5 and 22.3 eV, which have a significant enhancement in the monolayer regime, are due to electron mediated dipolar dissociation of adsorbed CF(3)Cl molecules. The enhancement is attributed to surface electron attachment following molecular excitation. A significant enhancement in the monolayer regime has also been observed for the resonances at 12.8 and 16.2 eV. These two resonances are ascribable to a combination of electron mediated dipolar dissociation and dissociative electron attachment driven by photoelectrons generated in the neighboring molecules.

In vitro effects of citric acid application techniques on dentin surfaces.

The present study evaluated the in vitro effects of different application techniques of citric acid on dentin root surfaces. Ten freshly extracted, periodontally involved teeth were obtained and 4 dentin slabs, approximately 4 x 6 x 2 mm, were obtained from the roots of each tooth, for a total of 40 slabs. These slabs were identified by tooth and preserved in 1:1 anhydrous glycerol/absolute alcohol solution. Citric acid pH 1 was applied to 32 of the slabs for 5 minutes with one of 4 different techniques: 1) immersion; 2) placed with a saturated cotton pellet with no rubbing; 3) placed and burnished with a saturated cotton pellet; or 4) applied with a camel hair brush. The remaining 8 dentin slabs were used as negative control specimens, root-planed and non-acid treated. Following the various treatments, the slabs were fixed, dehydrated, critical point dried, and coated for scanning electron microscopic (SEM) evaluation. Scanning photomicrographs were obtained at 2,000, 6,000, and 40,000 magnifications. The surface characteristics of the treated dentin slabs were evaluated descriptively regarding the degree of fiber exposure; the number of exposed tubules and the surface area occupied by tubule orifices were also measured. Friedman's 2-way analysis for block designs was employed. Results demonstrated that root-planed, non-acid treated specimens had an amorphous, irregular surface which corresponded to a smear layer.(ABSTRACT TRUNCATED AT 250 WORDS)