Metastable Oxygen Research Articles

Critical effects in optical response due to charge transfer vibronic excitons and their structure in perovskite-like systems

A mechanism for bilinear interaction between high-frequency light-induced electronic polarization and low-frequency soft lattice polarization is proposed. It is based on the fluctuations of the charge transfer connected with charge transfer vibronic excitons (CTVE). This bilinear mechanism leads to the appearance of the critical peculiarities of the absolute diffraction efficiency of transient gratings near the ferroelectric phase transition. A semi-empirical Hartree–Fock INDO method was used for the evaluation of the energy parameters and the equilibrium displacements for the CTVE in KTaO 3. This numerical study did confirm the proposed CTVE-model. It was shown that the CTVE-phase (with correlated CTVE in each cell) is characterized by new phonon spectra with a new set of soft lattice modes. It was also shown that metastable oxygen vacancy states are of dipole type in the CTVE-phase.

Adsorption process of metastable molecular oxygen on a Si(111)-(7×7)surface

The adsorption process of metastable molecular oxygen on a Si~111!-(737) surface is studied by ultraviolet photoelectron spectroscopy. At 120 K, three metastable states at 2.1, 3.8, and 5.1 eV grow up simultaneously following the decrease of the back-bond state as the dosage increases. After annealing at 600 K, we observed an unusual state attributed to the dangling bond. These results and the symmetries of the molecular orbitals lead to a conclusion that the metastable molecular species is adsorbed on top of an adatom stabilized by a coadsorbed atomic oxygen into the back bond. @S0163-1829~99!51636-6#

Etching polyimide with a nonequilibrium atmospheric-pressure plasma jet

An atmospheric-pressure plasma jet has been used to etch polyimide films at 1.0–8.0±0.2 μm/min at 760 Torr and between 50 and 250 °C. The plasma was produced by flowing helium and oxygen between two concentric electrodes, with the inner one coupled to 13.56 MHz rf power and the outer one grounded. The etch rate increased with the O2 partial pressure, the rf power and the substrate temperature. The apparent activation energy for etching was 0.16 eV. Langmuir-probe measurements revealed that the ion densities were between 1×1010 and 1×1011 cm−3, 5 mm from the end of the powered electrode. Biasing the substrate had no effect on the rate. Ozone, singlet sigma metastable oxygen (b 1Σg+), and singlet delta metastable oxygen (a 1Δg) were detected in the plasma by emission spectroscopy. More ozone was produced in the effluent through the recombination of O atoms with O2. Based on the production rate of O3, the concentration of O atoms 6 mm from the powered electrode was estimated to be ∼7×1014 cm−3 at 6.6 Torr O2 and 200 W power. It is proposed that O atoms are the principal reactive species involved in etching polyimide.

Efficient Production of O- by Dissociative Attachment of Slow Electrons to Highly Excited Metastable Oxygen Molecules

In this paper, the mechanism for efficient production of O- in the afterglow of a low-pressure and high-density oxygen plasma has been described. We proposed a new production process, the dissociative attachment of slow electrons to highly excited metastable oxygen molecules O2 (A3 Σu+, A'3Δu, c1Σu-). The electron attachment frequency has been obtained from the experimental results, and was significantly high for the electron temperature of less than 2 eV. The cross section, σDA, for this process was evaluated by a quantum mechanical approach. σDA at 0.1 eV was larger by two-orders of magnitude than the peak value at 6.7 eV for the ground state O2 (X3Σg+). The enhancement of the electron attachment frequency at low temperature can be explained by dissociative attachment of slow electrons to O2 (A3Σu+, A'3Δu, c1Σu-).

Absolute Reactive Cross Section for H Atom Formation in the Reaction of Translationally Energetic O(1D) Atoms with Methane

The dynamics of the H atom formation channel in the reaction of metastable oxygen atoms O(1D) with methane were studied in the gas phase using the pulsed laser photolysis/vacuum-UV laser-induced fluorescence “pump-and-probe” technique. Translationally energetic O(1D) atoms with an average collision energy of 37 kJ/mol in the O(1D)−CH4 center-of-mass system were generated by laser photolysis of N2O at 193 nm. Nascent H atoms produced in reactive collisions of O(1D) with room-temperature CH4 molecules were detected with sub-Doppler resolution via (2p2P ← 1s2S) laser-induced fluorescence. An absolute reaction cross section of σH(37 kJ/mol) = 1.4 ± 0.5 A2 was determined for the H atom formation channel by means of a calibration method. From the H atom Doppler profiles measured under single-collision conditions the product translational energy release was determined.

The theory of positive glow corona

A theory for the current and light pulses of positive glow corona from a point in air is presented; this phenomenon was first observed as an apparently continuous glow by Michael Faraday. Results are obtained, in concentric sphere geometry, for air at atmospheric pressure, by solving the continuity equations for electrons, positive ions, negative ions and metastable oxygen molecules, coupled with Poisson's equation. A series of `saw-toothed' current pulses of period about is predicted with a DC current level. Accompanying the current peaks are discrete pulses of light 30 ns wide. Successive `shells' of positive ions, from successive current pulses, carry 96% of the mean current. The mean current - voltage relationship has the classic square-law form. The seed electrons required for successive pulses are detached from negative ions by metastable oxygen molecules. Photo-ionization is crucial for the discharge at the anode and for the formation of negative ions throughout the gap. The pulse frequency varies with applied voltage and is found to be approximately proportional to the positive-ion mobility. The surface electric field at the central electrode remains close to Peek's onset field. The origin of onset streamers is explained and sub-microsecond voltage pulses are found to produce streamers. The results for concentric-cylinder electrodes are described briefly.

Magnetic Analysis of Supersonic Beams of Atomic Oxygen, Nitrogen, and Chlorine Generated from a Radio‐Frequency Discharge

AbstractSupersonic beams of oxygen, nitrogen, and chlorine atoms and of metastable oxygen and nitrogen molecules produced from a high‐pressure radio‐frequency discharge beam source have been characterized by coupling velocity selection with magnetic analysis in the transmission mode. The present work leads to the determination of the relative populations of the electronic states of the species in the produced beams, showing that estimates of the populations from plasma temperatures or final translational temperatures could bring on incorrect conclusions.

Identification of the adsorption sites of molecular oxygen on Si(111) using XPS

Most of the oxygen adsorb dissociatively on Si, however there is also a significant amount of metastable molecular oxygen chemisorbed on Si. The adsorption site/configuration of these molecular oxygen species is still a controversial subject. New XPS results for oxygen adsorption on Si(111) 7×7 (150K) are presented. They reveal four distinct oxygen components; one of the metastable components has never been reported before. We tentatively identify them as: i) stable oxide (Si-O-Si bridges)(~532 eV), ii) diffusing oxygen atom (~533 eV) in silicon oxide and iii) metastable molecular oxygen species (~527.6 eV and ~530.6 eV). The latter have been attributed to peroxy radical which is defined as a diatomic oxygen bonded to a single Si adatom. Our results allow us to distinguish the two main configurations of peroxy radical: paul-para and grif. Both of them possess a lifetime of ~ 180 min.

The reactivity of molecular and atomic oxygen in oxygen-exchange reaction between NO and O2 coadsorbed on a Pt(111) surface

Surface reactions between N16O and 18O2 coadsorbed on Pt(111) have been studied by temperature-programmed desorption (TPD), low energy electron diffraction and infrared reflection absorption spectroscopy (IRAS). When the surface covered with N16O and 18O2 is annealed, 18O16O desorbs at 155 K and N18O at 145, 310, and 340 K. In addition, a new absorption line at 1658 cm−1 due to N18O appears above 265 K. However, there is no indication of oxidation products of NO in the TPD and IRAS measurements. Thus, only oxygen-exchange reactions take place on the coadsorbed surface. Reaction yields and mechanisms of the oxygen-exchange strongly depend on the adsorption states of oxygen. At ∼145 K, molecularly adsorbed oxygen and N16O (νNO=∼1930 cm−1) directly interacting with the oxygen molecule are involved in the exchange reaction. Above 270 K the exchange reaction takes place between atop N16O (νNO=∼1720 cm−1) and the metastable oxygen adatoms that do not possess a long-range order. On the contrary, the oxygen-exchange reaction is greatly suppressed when NO is coadsorbed on the surface with well-ordered p(2×2) oxygen atoms.

Chemical and structural analysis of aluminium-oxygen coatings obtained by d.c. magnetron reactive sputtering: some features of the process

For a total working pressure of about 1 Pa, stable d.c. magnetron reactive sputtering conditions have been found which enable AlO coatings in the range 0–60 at.% O to be prepared with a high growth rate. The structure of the deposits is a metastable oxygen supersaturated f.c.c. solid solution over the range 0–28 at.% O and an oxide-type amorphous phase over the range 50–60 at.% O. At intermediate compositions, it consists of a mixture of these two solid solutions. The physical quality of optically transparent stoichiometric alumina films can be significantly improved by medium frequency electrical pulses (a few tenths of a kilohertz) which decrease by several orders of magnitude the number of microarcs. For a total working pressure of about 0.3 Pa, a new process-stabilizing method using superimposed low frequency pulses (a few hertz) enables the time control of the reactive sputtering process by suppressing the hysteresis effect occurring when the total working pressure or the pumping speed is lowered.

Reactive Plasma Species in the Modification of Wool Fibre

Physical and chemical changes to the surface properties of wool fibre caused by exposure to RF generated plasma have been studied. Comparison is made between the changes for a wide range of plasma conditions and of the experimentally measured concentrations of the relevant neutral and charged particle species to elucidate those principally responsible and the likely mechanism. Results show that the measured increase in surface energy (from hydrophobic to hydrophilic) is well correlated with the integrated flux to the fibre of atomic oxygen, regardless of its origin. Charged particles appear to have little effect beyond sputtering, while the role of metastable excited oxygen molecules seems to be not statistically significant. Changes in surface chemistry shown by XPS are consistent with direct interaction of reactive � on the C-C and C-H bonds of the lipid layer which forms the outer (",,3 nm thick) layer of the natural fibre. Similar but weaker effects sometimes seen with discharges in argon appear to be due to the presence of oxygen as an impurity.

Trapping of metastable oxygen species at Cu(111) surfaces

The coadsorption of ammonia–nitric oxide and ammonia–dioxygen mixtures at a Cu(111) surface has been studied at temperatures between 170 and 480 K using X-ray photoelectron (XP) and vibrational electron energy loss (VEEL) spectroscopies. At low temperatures (T⩽, 250 K) further evidence has been obtained for the formation of two oxygen states, characterised by O1s binding energies of 531.3 and 528.8 eV and the assignment of the low binding energy state to a metastable atomic species has been confirmed. It is suggested that this state corresponds to small Cu–O transients trapped by the coadsorbed ammonia and prevented from coalescing into larger copper oxide is lands. At 298 K, coadsorption results in a highly efficient oxydehydrogenation of the ammonia. The rate of this reaction, which is shown to be independent of the substrate temperature, is very close to the rate of dioxygen bond cleavage at Cu(111).

High-resolution study of the emission system of 80Se2 in the near infrared

The [Formula: see text] system of 80Se2 has been observed from a discharge flow system where [Formula: see text] molecules were excited by electronic energy transfer from metastable oxygen, O2(a1Δg), to ground state Se2. Emission spectra of the 0–0, 0–1, 0–2, 1–1, 1–2, 1–3, and 2–3 bands have been recorded at high resolution with a Bomem Fourier-transform spectrometer. The rotational structures of the bands are determined by magnetic dipole selection rules. Detailed analysis has yielded accurate molecular constants for the [Formula: see text] and X21g states of 80Se2.

Thermal stability of Li xCoO 2, Li xNiO 2 and λ-MnO 2 and consequences for the safety of Li-ion cells

LiCoO 2, LiNiO 2 and LiMn 2O 4 are all stable in air to high temperature. By contrast, Li x CoO 2, Li x NiO 2 and Li x Mn 2O 4 ( x<1) are metastable and liberate oxygen when they are heated in air or in inert gas. The temperature at which oxygen evolution occurs depends on x and on the material. Using thermal gravimetric analysis and mass spectrometry, we have studied the thermal decomposition of these materials in inert gas. We find that the nickel materials are least stable, the manganese compounds are most stable, and that the cobalt compounds show intermediate behaviour. These results have important consequences for the safety of Li-ion cells, and suggest that cells using LiMn 2O 4 as the cathode should be safer than those using LiNiO 2 or LiCoO 2.

Cause of terrestrial nightglow continuum

Since the nightglow’s green continuum was discovered over 40 years ago the theory that it arises from the NO 2 air afterglow has been generally accepted so that its intensity is regarded as a useful measure of the abundance of nitric oxide in the lower thermosphere. This theory is shown to be untenable. Observational evidence points unambiguously to a novel source: metastable oxygen molecules colliding with ambient gas molecules and thereby forming complexes that dissociate by allowed radiative transitions.

A synchrotron radiation study of NO and oxygen on Rh(110)

Measurements are reported of the surface core level shifts of the clean Rh(110) surface, the clean metastable (1 × 2) surface, and NO and oxygen covered surfaces. The shifts are discussed in terms of a Born-Haber cycle formalism, and also within an initial state model of charge transfer after adsorption. Thermodynamic data are derived and it is shown that the heat of adsorption of NO is higher on Rh than on Pd. In addition, the valence level electronic structure of NO adlayers has been examined by Cooper minimum spectroscopy, and the molecular orbitals of NO, adsorbed oxygen and adsorbed nitrogen are reported. Near edge X-ray absorption spectra (NEXAFS) of molecular NO are reported, and the molecule is found to be adsorbed with its axis normal to the surface.

The dissociation of molecular iodine by metastable oxygen—II. Populations of reservoir states through laser excitation spectra

The dissociation of molecular iodine by metastable oxygen has been studied through the determination of the population of iodine electronic states involved. It is shown that three main mechanisms must be considered as responsible for this dissociation. Two of these correspond to the consumption of two molecules of O 2 ( 1Δ g) per I 2 molecule, and the third one is a three-step process with the consumption of three O 2 ( 1Δ g) molecules per I 2. The measured consumption of O 2 ( 1Δ g) molecules per iodine molecule has been experimentally determined to be 2 ⩽ n ⩽ 6, and this inflated O 2 ( 1Δ g) consumption is explained by energy loss via vibrational relaxation in the ground state of iodine.

Collisional excitation of metastable oxygen O( 1D) atoms through the B3∑ −u channel of O 2

Pectroscopic data on the shifts and widths of the energy levels of molecular oxygen have been used in the empirical construction of a diabatic potential matrix that characterizes the interactions of the B 3∑ − u state with the 5Π u, 2 3∑ + u, 3Π u and 1Π u states. The diabatic potential matrix is u theory formulation to calculate the cross-sections for the excitation of O( 1D) atoms in collisions of two O( 3P) atoms. Total cross-sections are obtained by adding the excitation from the 3Π g , channel. The rate coefficient for quenching of O( 1D) by O( 3P) is evaluated as a function of temperature. The values conflict with a recent analysis of the emission of the oxygen red line in the upper atmosphere.

Coverage dependent interaction of N2O and O2 with Si(001)2×1 as monitored by the O KLL Auger intensity ratio

The adsorption behavior of O2 on Si(001)2×1 at 100 K sample temperature has been studied by measuring the intensity ratio of the KL1L1 and KL2,3L2,3 O Auger transitions α as a function of the fractional oxygen coverage in an attempt to solve a longstanding discussion whether this reaction also involves molecular chemisorption. Recent cluster calculations provided evidence that the bond distance between Si and O and between the two oxygen atoms in the O2 molecule is a key parameter affecting the magnitude of α. By referring to the room temperature adsorption of N2O on the same surface we have found evidence for the coadsorption of a metastable molecular oxygen species at advanced adsorption stages, a result which is essentially in agreement with previous experimental predictions and theoretical calculations.

Theory of electrical breakdown in air-the role of metastable oxygen molecules

A theory is given of electrical breakdown in air for non-uniform electric fields. It is proposed that space-charge effects tend to make the electric field in the streamer column uniform at a value of E/N for which effective ionization and attachment rates are equal (E is the electric field and N the gas number density). It is also proposed that a1 Delta g metastable states of molecular oxygen, produced in the pre-breakdown corona and streamer processes, have a dominant role in determining E/N in the streamer channel, because of their ability to detach electrons from O2- ions and thus reduce the effective attachment coefficient. Assuming an effective value of alpha /N=10-22 cm2 ( alpha being the ionization coefficient) at E/N=20 Td, predictions of breakdown fields in dry air and humid air are in fair agreement with experimental results for rod-rod breakdown with electrode spacings of from 0.5 m to 2.5 m. The effect of the metastables is to reduce the breakdown field applicable to plane parallel electrodes by a factor of about 6. It is predicted that 1% water vapour in air increases breakdown fields by about 11%. A breakdown criterion is derived defining the critical E/N for breakdown in terms of electron transport coefficients and metastable rate constants.