Pulsed reactive crossed beam laser ablation of La0.6Ca0.4CoO3 using O: Where does the oxygen come from?

The composition of thin perovskite films, especially the oxygen content, is a crucial parameter which influences many physical properties, such as conductivity and catalytic activity. Films produced by pulsed laser deposition are normally annealed in an oxygen atmosphere after deposition to achieve a desired oxygen content. In pulsed reactive crossed beam laser ablation, no annealing step is necessary, but a fundamental question regarding this deposition technique is still open: where does the oxygen in the films come from?There are three possibilities, i.e. from the target, from the gas background, or from the gas pulse. To answer this question two experiments were performed: ¹⁸O₂ was used during the deposition process as background gas with ¹⁶O anions in the target and ¹⁶O₂ gas pulse, and a ¹⁸O₂ gas pulse with ¹⁶O from the target and background. These experiments revealed that the quantification of the oxygen origin is only possible, when no oxygen exchange occurs at the deposition temperature. The films are characterized after deposition by elastic recoil detection analysis (ERDA) to determine the ¹⁶O/¹⁸O ratio. Experiments with different oxidizing species in the gas pulse (N₂O and O₂) confirm that the oxidizing potential (N₂O > O₂) as well as the number of molecules are important.     

Oxygen-exchange reaction between O2 and NO coadsorbed on a Pt(111) surface: reactivity of molecularly adsorbed oxygen

The oxygen-exchange reaction between N¹⁶O and ¹⁸O₂ coadsorbed on Pt(111) has been studied by temperature-programmed desorption (TPD). Reaction products of N¹⁸O and ¹⁸O¹⁶O are desorbed from Pt(111) initially saturated with ¹⁸O₂ at 94 K followed by exposure of N¹⁶O. Three distinct desorption peaks are observed in N¹⁸O TPD spectra at 145, 310, and 340 K, and two peaks in ¹⁸O¹⁶O at 155 K and between 600 and 1000 K. In contrast, the exchange reaction is greatly suppressed when oxygen molecules are replaced with oxygen adatoms at three-fold hollow sites of Pt(111). These results strongly suggest that adsorbed oxygen molecules are responsible for the exchange reaction. NO₂ or NO₃ is postulated as a reaction intermediate. However, since desorption signals corresponding to these species are not detected, the oxygen-exchanged products are not due to the cracking processes of the higher order nitrogen oxides in the mass spectrometer. Thus, the reaction proceeds via the intermediate that is dissociated during the elevation of surface temperature.     

Human Respiration Under the Condition of Severe Anemia Investigated with the Aid of Fractionation Effects of Oxygen Isotopes

Abstract

The dependence of overall fractionation effect of respiration between ¹⁶O₂ and ¹⁶O¹⁸O (δ_IU) on varying levels of hemoglobin concentration within blood is investigated. Experiments are carried out on 6 healthy humans and 6 patients suffering from various degrees of chronic anemia.

δ_IU-values of anemic patients are decreased towards 0‰ meaning that within anemia rates of ¹⁶O_2? and ¹⁶O¹⁸O-transport are aligned together more than they are under healthy conditions.

On the basis of a resistance model of respiration, it is concluded that within anemia the entire oxygen transport system is more limited by blood flow than it is in healthy humans. This limitation starts to influence respiration far above a hemoglobin concentration of 8 g/100 ml.     

Behavior of oxygen in zinc oxide films through thermal annealing and its effect on sheet resistance

Behavior of oxygen in sputtering deposited ZnO films through thermal annealing and its effect on sheet resistance of the films were investigated. The crystallinities of the ZnO film were improved by post-deposition annealing in vacuum. However, the sheet resistance of ZnO film was dramatically decreased after post-deposition annealing in vacuum at more than 300 °C, while O₂ desorbed from the film. The oxygen vacancies which acted as donors were formed by the thermal annealing in vacuum. The sheet resistance of the films was recovered by annealing in oxygen ambient. In this paper, ¹⁸O₂ gas as an oxygen isotope was used as the annealing ambient in order to distinguish from ¹⁶O, which was constituent atom of the ZnO films. SIMS analysis revealed that ¹⁸O diffused into the ZnO film from the top surface by ¹⁸O₂ annealing. Therefore oxygen vacancies formed by the post-deposition annealing in vacuum could be compensated by the annealing in oxygen ambient.     

A diode laser spectrometer for symmetry selective detection of ozone isotopomers

A tunable diode laser absorption spectrometer operating in the 10 μm range is described, which for the first time permits simultaneous detection and quantification of the five naturally most abundant ozone isotopomers: ¹⁶O₃, ¹⁶O¹⁶O¹⁸O, ¹⁶O¹⁸O¹⁶O, ¹⁶O¹⁶O¹⁷O, and ¹⁶O¹⁷O¹⁶O. Ozone samples of 25 μmole size are analysed with an estimated accuracy of 6‰ (2σ). This level of accuracy is demonstrated by comparing spectroscopically determined isotopologue enrichments of ¹⁶O₂ ¹⁷O and ¹⁶O₂ ¹⁸O with mass spectrometer measurements. Samples for the comparison were generated from natural oxygen in an electric discharge under two different pressure conditions. The precision obtained is sufficient to study both the isotopic and the symmetry dependence of the unique oxygen isotope anomaly in ozone formation, which shows isotopomer specific fractionation values well in the 10% range. PACS 42.60.By; 42.55.Px; 33.20.Ea     

High sensitivity CRDS of the aΔg−XΣg band of oxygen near 1.27 μm: Extended observations, quadrupole transitions, hot bands and minor isotopologues

The CW-Cavity Ring Down Spectroscopy (CW-CRDS) technique has been used to record the high sensitivity absorption spectrum of the a¹Δ_g-X³Σ_g⁻ band of oxygen near 1.27 μm. The spectra were obtained between 7640 and 7917 cm⁻¹ with “natural” oxygen and with a sample highly enriched in ¹⁸O. The absolute intensities of 376 and 643 oxygen transitions were measured in the two spectra. They include the a¹Δ_g−X³Σ_g⁻ (0-0) bands of ¹⁶O₂, ¹⁶O¹⁸O, ¹⁶O¹⁷O, ¹⁷O¹⁸O and ¹⁸O₂. The (0-0) bands of ¹⁶O₂ and ¹⁸O₂ show weak quadrupole transitions with line intensities ranging from 1×10⁻³⁰ to 1.9×10⁻²⁸ cm/molecule. They are accompanied by the a¹Δ_g−X³Σ_g⁻ (1-1) hot bands, which are reported for the first time. The line profiles of the transitions of the ¹⁶O¹⁷O and ¹⁷O¹⁸O isotopologues were observed to be broadened due to an unresolved magnetic hyperfine structure. Accurate spectroscopic parameters of the different energy levels involved in the observed bands were derived from a global fit of the observed line positions, combined with microwave and Raman measurements available in the literature.     

The Utilization of 18O Labelled Potassium Phosphate in Studies Concerning Fertilizer Phosphorus Assimilation by Plants

The utilization of ¹⁸O labelled potassium phosphate KH₂P¹⁸O₄, instead of radiophosphorus, ³²P, labelled phosphate in biochemical and ecological studies concerning fertilizer phosphorus assimilation by plants is presented.

The 30–60 mg dry plant samples were heated under 10^?2 mm Hg vacuum in Pyrex ampoules to form carbon dioxide (C¹⁶O¹⁸O) for ¹⁸O mass spectromietric analyses. Potassium phosphate labelled with 1.36 ¹⁸O atom per cent is sufficient to obtain a significant increase of the ¹⁸O plant content, the added ¹⁸O quantity in 0.5 kg soil being 1.4 × 10^?2 g ¹⁸O in this case.     

An isotopic labeling study of the diffusion mechanism during oxidation of Si(100) in water vapor by successive oxidation

The diffusion mechanism during the wet oxidation of Si(100) at 1373 K was investigated by successive oxidations finally containing isotopic water. SiO₂ was first thermally grown on Si in non-labeled oxidizing ambient (dry O₂ or H₂O) followed by isotopic water (H₂¹⁸O) to trace ¹⁸O species in SiO₂. The distributions of ¹⁶O and ¹⁸O in the oxide film were analyzed by means of secondary ion mass spectroscopy (SIMS). SIMS depth profiles show that there was a wide overlap of both isotopes (¹⁸O and ¹⁶O) throughout the SiO₂ layer, no matter whether the first oxidation step was carried out in dry O₂ or H₂O, and the concentration gradient of ¹⁸O decreased with increasing oxidation time at the second oxidation step by H₂¹⁸O. The results suggest that the diffusion mechanism in SiO₂ during water vapor oxidation is exchange diffusion; H₂O related oxidizing species diffuse through the network with significant exchange with the pre-existing oxygen in it.     

Oxygen permeation in bismuth-based materials. Part II: Characterisation of oxygen transfer in bismuth erbium oxide and bismuth calcium oxide ceramic

The ¹⁸O/¹⁶O Isotope Exchange Depth Profile technique was applied to BE25 ((Bi₂O₃)_0.75(Er₂O₃)_0.25) and BICAO ((Bi₂O₃)_0.73–(CaO)_0.27) dense ceramics to characterise the oxygen transfer in these materials. Pure ceramics, ceramics coated with silver on the surface and 40% silver cermets were investigated. Slow oxygen surface exchange kinetics were confirmed for the pure ceramics whilst the kinetics were improved when silver was painted on the surface. However oxygen permeation fluxes were not improved when a silver paste was painted onto the membrane surface. Permeation was also limited by electronic conduction which is likely to be more limiting than oxygen exchange at the surface of these ceramics. In contrast, fast kinetics of oxygen surface exchange were revealed for cermets.     

The oxidation of H2CO on a copper(110) surface

The oxidation of H₂C¹⁶O by adsorbed ¹⁸O was studied on an Cu(110) sample by temperature programmed reaction spectroscopy. Formaldehyde exchanged its oxygen with surface ¹⁸O upon adsorption to yield H₂C¹⁸O_(a) and ¹⁶O_(a). Formaldehyde was also oxidized by surface ¹⁶O and ¹⁸O atoms to H₂COO which subsequently released one of the hydrogen atoms to form HCOO. The evolution of H₂ from the Cu(110) surface was desorption limited, and the low pre-exponential factor for the recombination of the surface hydrogen atoms suggested stringent requirement on the trajectories of the colliding partners. The formate was very stable and dissociated at elevated temperatures to simultaneously yield H₂ and CO₂. The surface concentration of ¹⁸O exerted a pronounced affect on the activity of the oxidation of formaldehyde on Cu(110).     

Regime Shift Signatures from Stable Oxygen Isotopic Records of Otoliths of Atlantic Cod (Gadus morhua)

Abstract

The sudden collapse of Atlantic cod (Gadus morhua) may relate to ocean climate, or regime shifts as demonstrated in production of Pacific salmon. This paper reports the results of stable oxygen isotope ratio analyses (¹⁸O/¹⁶O or δ¹⁸O_A) from 91 otoliths of cod over a period of about 20 years. Seasonal δ¹⁸O_A variations of individual otoliths started at an initial value of about ?0.5 to 0‰ VPDB, and then reached a stable level in the range of +2.5 to +3.5‰ VPDB after 4–5 years. The initial low values correspond to the natal sources of mature cod, while the higher δ¹⁸O_A values represent the water conditions before the cod was caught. This pattern of δ¹⁸O_A variation was observed over the life history of all cod examined. Furthermore, the calculated isotopic temperatures agreed with those obtained from summer bottom trawl survey, indicating that δ¹⁸O_A of otoliths could be used as a thermometer in determining the ambient seawater temperature where the cod lived. Comparison of long-term δ¹⁸O_A records and biological and meteorological observations suggested that decadal-scale ecosystem changes did occur in the late 1970s and early 1990s in Atlantic Canada, comparable to regime shifts occurred in the North Pacific.     

A Modified Technique for the Preparation of SO2 from Sulphates and Sulphides for Sulphur Isotope Analyses

Abstract

A modified technique for the conversion of sulphates and sulphides to SO₂ with the mixture of V₂O₅—SiO₂ for sulphur isotopic analyses is described. This technique is more suitable for routine analysis of large number of samples. Modification of the reaction vessel and using manifold inlet system allows to analyse up to 24 samples every day. The modified technique assures the complete yield of SO₂, consistent oxygen isotope composition of the SO₂ gas and reproducibility of δ³⁴S measurements being within 0.10‰. It is observed, however, oxygen in SO₂ produced from sulphides differs in δ¹⁸O with respect to that produced from sulphates.     

In situ measurement of CO2 and water vapour isotopic compositions at a forest site using mid-infrared laser absorption spectroscopy

We conducted continuous, high time-resolution measurements of CO₂ and water vapour isotopologues (¹⁶O¹²C¹⁶O, ¹⁶O¹³C¹⁶O and ¹⁸O¹²C¹⁶O for CO₂, and H₂¹⁸O for water vapour) in a red pine forest at the foot of Mt. Fuji for 9 days from the end of July 2010 using in situ absorption laser spectroscopy. The δ¹⁸O values in water vapour were estimated using the δ²H–δ¹⁸O relationship. At a scale of several days, the temporal variations in δ¹⁸O-CO₂ and δ¹⁸O-H₂O are similar. The orders of the daily Keeling plots are almost identical. A possible reason for the similar behaviour of δ¹⁸O-CO₂ and δ¹⁸O-H₂O is considered to be that the air masses with different water vapour isotopic ratios moved into the forest, and changed the atmosphere of the forest. A significant correlation was observed between δ¹⁸O-CO₂ and δ¹³C-CO₂ values at nighttime (r²≈0.9) due to mixing between soil (and/or leaf) respiration and tropospheric CO₂. The ratios of the discrimination coefficients (Δ_a/Δ) for oxygen (Δ_a) and carbon (Δ) isotopes during photosynthesis were estimated in the range of 0.7–1.2 from the daytime correlations between δ¹⁸O-CO₂ and δ¹³C-CO₂ values.     

Raman scattering from phonons and electronic excitations in UO2 with different oxygen isotopes

Comparison of the phonons and electronic excitations of UO₂ with different oxygen isotopes (¹⁶O, ¹⁸O), two purely electronic excitations at 4162 and 5778 cm⁻¹ have been evidenced by their limited shift, when ¹⁶O is replaced by ¹⁸O. The bands observed at 570, 1144 and 2300 cm⁻¹ in U¹⁶O₂ shift to 545, 1091 and 2190 cm⁻¹ for U¹⁸O₂. The wavenumbers and the isotope shifts of these bands exhibit multiple relationships. It supports the assignments of the 1LO (~570 cm⁻¹) and 2LO (~1144 cm⁻¹) phonons for U¹⁶O₂. Both bands exhibit marked decrease for increasing O/U ratio. The observed phenomenon is regarded as a good estimation of the stoichiometry of a uranium oxide. In addition, by analyzing the intensity ratios of the 2LO to 1LO phonons for various UO₂ samples, the 2LO band is assigned to the Fröhlich interaction. Defect‐induced and/or deformation potential mechanisms play a dominant role for the 1LO band. Copyright © 2015 John Wiley & Sons, Ltd.     

Interactions between SO2 and NO on the surface of stepped Pt(3 3 2)

Interactions between S¹⁸O₂ and NO on the surface of stepped Pt(3 3 2) were studied using Fourier transform infra red reflection-absorption spectroscopy (FTIR-RAS) combined with thermal desorption spectroscopy (TDS). Adsorbed S¹⁸O₂ does not seem to have a preference for step sites on Pt(3 3 2). As such, the presence of S¹⁸O₂ molecules following exposures of ?1.6 L does not significantly block the subsequent adsorption of NO (?0.8 L) on these step sites. Adsorbed S¹⁸O₂ molecules undergo dissociation (S¹⁸O₂(a) → S¹⁸O(a) + ¹⁸O(a)) as the surface temperature is increased to 250 K and above, but the resultant ¹⁸O(a) further reacts with sulfur oxides (S¹⁸O₂(a) and S¹⁸O(a)) to form S¹⁸O_x (x > 2) species at ∼400 K and above. The S¹⁸O_x species desorb as S¹⁸O₂. Even though the presence of co-adsorbed S¹⁸O₂ suppresses NO dissociation and subsequent N₂ production, this effect is not significantly enhanced with increasing the exposures of S¹⁸O₂ in the range ?1.6 L; N₂ desorption is still detectable at an exposure of 1.6 L S¹⁸O₂, at which a considerable amount of S¹⁸O₂ desorption is detected.     

Sulphur and Oxygen Isotopes in Sulphates in Natural Waters: (2) Deep-Waters from Horizons Below Baltic Sea Floor

Abstract

In this paper we consider deep waters from horizons located under the Baltic-Sea bottom. The samples were taken twice from wells bored in Hel peninsula. The δ¹⁸O and δD values of these waters show their glacial origin since they have significantly low values (δ¹⁸O from ?14 to ?11 whereas the modern groundwaters in Gdańsk area have ?10%₀). In contrast to surface waters the sulphates dissolved in these deep waters have rather high and uniform δ¹⁸O values which are correlated with δ¹⁸O of H₂O but not correlated with δ³⁴S. The isotope patterns may be explained assuming that the waters are very old, in which the slow process of oxygen isotope exchange at low temperatures, but extended in time, could enrich the sulphate in heavier oxygen isotopes.     

Measurements of O-Enriched Ozone Isotopomer Abundances Using High-Resolution Fourier Transform Far-IR Spectroscopy

The distribution of ozone isotopomers in ozone mixtures produced by electric discharge in mixtures of ¹⁶O₂ and ¹⁸O₂ at 77 K was measured by high-resolution FTIR spectroscopy. It was of key importance to assess not only the total amount of isotopomers of a certain mass but also the relative amounts of corresponding asymmetric and symmetric ozone species of the same mass given as the ratios [¹⁶O¹⁶O¹⁸O]/[¹⁶O¹⁸O¹⁶O] and [¹⁶O¹⁸O¹⁸O]/[¹⁸O¹⁶O¹⁸O]. For many purposes both ratios have been assumed to have the statistical value 2.00. Pure rotational spectra in the far-IR region (30–100 cm⁻¹) were recorded for three different ¹⁸O-enriched ozone mixtures, all at 0.00185 cm⁻¹ resolution. All the spectra were corrected for thermal emission. Linestrengths for individual lines in a particular spectrum were measured by means of a fitting technique taking into account contributions from all other lines in the spectrum. For this purpose theoretical linestrengths for all six ozone species containing ¹⁶O and ¹⁸O obtained from a quantum-number-dependent dipole operator were used. The ratios between observed and theoretical linestrengths were used to determine the abundances of individual isotopomers in a particular ozone mixture. For one of the ozone samples the abundances of all six ozone species were determined within 1% relative uncertainty. For the three ozone mixtures studied, the ratio between asymmetric and symmetric species of mono-¹⁸O ozone were determined to 1.99(2), 2.01(2), and 2.10(6). The ratio between asymmetric and symmetric species of di-¹⁸O ozone were determined to 2.51(4), 2.42(10), and 2.46(3).     

Luminescence of solids excited by adsorbed oxygen atoms

The luminescence excited upon the discharge of molecular gases O₂, N₂O, and CO onto a Zn₂SiO₄-Mn crystalline phosphor surface prefilled with oxygen atoms is studied. An interpretation of this phenomenon in terms of the model of acceleration of the surface heterogeneous recombination of oxygen atoms in the appearing adsorption layer of oxygen-containing molecules O₂, N₂O, and CO by the exchange-association mechanism is given.     

Oscillator strengths for the Schumann-Runge bands of 16O18O

Oscillator strenghts were measured for the (2-0)–(15-0) Schumann-Runge bands of ¹⁶O¹⁸O. Individual rotational lines were studied at a resolution of ≈0.05Å. Band oscillator strenghts decrease with increasing rotation at a rate intermediate between those for ¹⁶O₂ and ¹⁸O₂. Our measurements are in good agreement with oscillator strenghts calculated using potential curves and dipole moments derived from ¹⁶O₂ measurements, and support conclusions that ¹⁶O¹⁸O plays only a minor role in the photodissociation of atmospheric O₂. Dipole moments deduced from our oscillator strenghts agree well with those obtained for ¹⁶O₂, ¹⁸O₂, as well as with recent ab initio and semi-empirical determinations.