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## Interações de íons sulfato com sais de alumínio em soluções ácidas. Estudos básicos e aplicações ambientais

Silva, Renato Dantas Rocha da
Tipo: Tese de Doutorado Formato: application/pdf
POR
Relevância na Pesquisa
36.07%
O presente trabalho teve o objetivo de estudar os aspectos químicos básicos envolvidos nas interações (co-precipitação e adsorção) entre íons sulfato e espécies de alumínio em soluções aquosas em meio ácido e aplicar esta técnica de imobilização do ânion, no tratamento de efluentes da indústria do carvão. Os estudos foram desenvolvidos com o uso de cloreto de polialumínio comercial (CPA) e soluções de cloreto de alumínio (AlCl3.6H2O - reagente de pureza analítica) em diferentes valores de pH e sob diferentes razões molares entre os íons de alumínio e íons sulfato (α). Os mecanismos dessas interações físicas e químicas (adsorção e co-precipitação) foram investigados a partir da distribuição das espécies monoméricas, poliméricas e hidróxidos de alumínio em solução (especiação de Ala, Alb e Alc, respectivamente) realizada pelo método Ferron e da caracterização dos precipitados gerados por técnicas de espalhamento dinâmico de luz, difração a laser, microscopia eletrônica de varredura (MEV), energy dispersive X-ray (EDS), difração de raio-X e fluorescência de raio-X. Os resultados mostraram que elevados teores de espécies Alb e Alc foram gerados em soluções moderadamente ácidas...

## Energy transfer and frequency upconversion involving triads of Pr3+ ions in (Pr3+, Gd3+) doped fluoroindate glass

Rátiva, Diego J.; Araújo, Cid B. de; Messaddeq, Younes
Tipo: Artigo de Revista Científica
ENG
Relevância na Pesquisa
45.96%
Blue and ultraviolet luminescence in (Pr3+, Gd3+) doped fluoroindate glass is studied for excitation in the red region (≈590 nm). Frequency upconversion (UC) is observed due to energy transfer (ET) among three Pr3+ ions initially excited to the D21 state corresponding to the ET process D21 + D21 + D21 → S01 + H53 + H53. Additionally, UC luminescence from states P 72 6 and I 72 6 of Gd3+ is observed for an excitation wavelength resonant with transitions of the Pr3+ ions. The characterization of the luminescence signals allowed to determine ET rate among the Pr3+ ions and provides evidence of interconfigurational ET between Gd3+ and Pr3+ ions. © 2006 American Institute of Physics.

## Identification of molecular species of simple lipids by normal phase liquid chromatography-positive electrospray tandem mass spectrometry, and application of developed methods in comprehensive analysis of low erucic acid rapeseed oil lipids

Kalo, P. J.; Ollilainen, V.; Rocha, J.M.; Malcata, F.X.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
36.15%
Mono-, di- and triacylglycerol (MAG, DAG, TAG), sterol ester (SE), free sterol (S) and free fatty acid (FFA) standards were analyzed in the presence of ammonium ions and ammonia by flow injection MS2 and MS3, and by normal phase-liquid chromatography (NP-LC) MS2 positive electrospray ionization (ESI) mass spectrometry (MS). The MS data recorded for ammonium adducts ([M+NH4]+) of TAGs, DAGs, and MAGs were consistent with stepwise fragmentation mechanisms. In the first step, ammonium ion in [M+NH4]+ donates proton to acylglycerol and ammonia is released. In the second step, FFA is cleaved from protonated TAG, water from protonated 1,3-DAG and MAG, both FFA and water from protonated 1,2-DAG, hence leading to formation of [DAG]+ ion from TAG and 1,3-DAG, [DAG]+ and [MAG]+ ions from 1,2-DAG, and [MAG]+ ion from MAG. In the third step, [DAG]+ ion of TAG is fragmented to yield [Acyl]+, [Acyl + 74]+, [DAG−74]+ ions, [DAG] ion of 1,3-DAG to [Acyl]+ ions, and [MAG]+ ion of MAG to protonated FAs, which are decomposed to water and [Acyl]+ ions in the fourth step. A stepwise mechanism for fragmentation of FFA was also evident from MS2 and MS3 data. Molecular species of low erucic acid rapeseed oil simple lipids were identified from characteristic ions produced in the NP-LC–ESI-MS2 of [M+NH4]+ ions. The percentage composition of the molecular species of each lipid class was calculated from integrated extracted ion chromatograms of [(M+NH4)]+ ions of SE...

## Influence of metal ions on pellet morphology and polygalacturonase synthesis by Aspergillus niger 3T5B8

Couri,Sônia; Pinto,Gustavo Adolfo Saavedra; Senna,Lilian Ferreira de; Martelli,Hebe Labarthe
Tipo: Artigo de Revista Científica Formato: text/html
Relevância na Pesquisa
36.09%
The effects of cations addition on pellet morphology and polygalacturonase (PG) synthesis by Aspergillus niger 3T5B8 were studied and compared with a control system. Fe(II), Cu(II), Zn(II) and Mn(II) were added to the fermentation medium separately, and also as combined groups of cations. The addition of Fe2+ and/or Zn2+ ions was significantly positive to the enzyme production. A positive effect in the biomass content, however, was only obtained when the same metal ions were added separately. On the other hand, Cu2+ and Mn2+ ions had almost no effect on these parameters. The morphology of the pellets was studied by image processing techniques. Small pellets with small cores were usually obtained when Fe2+ and Zn2+ ions were individually or collectively added to the medium. The pellets produced in media containing Fe2+ or Zn2+ ions were compact, while the ones produced in a medium containing both cations were considered diffuse. Autolysis of the core was observed for large control pellets, due to the deficient nutrient transfer to the interior of the pellet. The pellets obtained in a medium containing both Fe2+ and Zn2+ ions were high enzyme producers, probably due to a loose morphology, induced by the presence of combined groups of metal ions in the medium...

## Fragmentation of oxime and silyl oxime ether odd-electron positive ions by the McLafferty rearrangement: new insights on structural factors that promote α,β fragmentation

Laulhé, Sébastien; Bogdanov, Bogdan; Johannes, Leah M.; Gutierrez, Osvaldo; Harrison, Jason G.; Tantillo, Dean J.; Zhang, Xiang; Nantz, Michael H.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
46.03%
The McLafferty rearrangement is an extensively studied fragmentation reaction for the odd-electron positive ions from a diverse range of functional groups and molecules. Here, we present experimental and theoretical results of 12 model compounds that were synthesized and investigated by GC-TOF MS and density functional theory calculations. These compounds consisted of three main groups: carbonyls, oximes and silyl oxime ethers. In all electron ionization mass spectra, the fragment ions that could be attributed to the occurrence of a McLafferty rearrangement were observed. For t-butyldimethylsilyl oxime ethers with oxygen in a β-position, the McLafferty rearrangement was accompanied by loss of the t-butyl radical. The various mass spectra showed that the McLafferty rearrangement is relatively enhanced compared with other primary fragmentation reactions by the following factors: oxime versus carbonyl, oxygen versus methylene at the β-position and ketone versus aldehyde. Calculations predict that the stepwise mechanism is favored over the concerted mechanism for all but one compound. For carbonyl compounds, C–C bond breaking was the rate-determining step. However, for both the oximes and t-butyldimethylsilyl oxime ethers with oxygen at the β-position...

## Photofragmentation studies of semiconductor positive cluster ions

Zhang, Qingling
ENG
Relevância na Pesquisa
36.19%
Laser photofragmentation of Si, Ge, and GaAs positive cluster ions prepared by laser vaporization and supersonic beam expansion has been investigated in a tandem time-of-flight mass spectrometer. Si$sb{ m n}sp +$ up to size 80, Ge$sb{ m n}sp +$ and Ga$sb{ m x}$As$sb{ m y}sp +$ up to a total of 31 atoms have been studied. Ga$sb{ m x}$As$sb{ m y}sp +$ fragment in a nearly uniform pattern probably via a step-loss of atoms pattern with odd fragments being more prominent than their even neighbors. Both Si$sb{ m n}sp +$ and Ge$sb{ m n}sp +$ of all sizes show as one fragmentation channel loss of one atom to produce Si$sb{n-1}sp +$ or Ge$sb{n-1}sp +$. In addition, Si$sb{ m n}sp +$ and Ge$sb{ m n}sp +$ in the size range n = 10-30 photofragment to produce positive ions of about one half the mass of the parent. For Ge$sb{ m n}sp +$ as n becomes greater than 30, this fragmentation becomes a pattern in which positive ions containing 10 (or 7) fewer atoms are the principal products at low laser fluence. Increased fluence seems to fragment these daughters in the same way as they would behave as primary ions. The Si$sb{ m n}sp +$ ions of greater than 30 atoms do not show this 10 or 7 atom neutral loss. Both Si$sb{ m n}sp +$ and Ge$sb{ m n}sp +$ for n $>$ 30 show an additional channel in which small positive ion fragments in the n = 6 to 11 size range are produced. This channel requires high fluence and is shown to involve at least two 6.4 eV (ArF) photons for Si$sb{60}sp +$. The absence of intermediate fragments in this channel suggests that these large cluster ions break down into several clusters because the positive charge would be expected to stay with the larger fragment in a fission into two clusters as the ionization potentials of the neutral clusters decrease with size. A reflectron tandem time-of-flight mass spectrometer has been designed and constructed. The resultant high resolution makes the apparatus a highly efficient instrument for fragmentation and spectroscopic studies.

## PHOTODISSOCIATION AND PHOTODETACHMENT STUDIES OF TRANSITION METAL CLUSTER IONS

ZHENG, LAN-SUN
Tipo: Thesis; Text Formato: application/pdf
ENG
Relevância na Pesquisa
36.12%
A cold cluster ion source has been developed within our lab. This source is capable of generating both positive and negative cluster ions which are cooled in the supersonic expansion and suitable for spectroscopic studies. A tandem time-of-flight apparatus was built for mass selecting a particular cluster ion and then performing a second mass analysis after it has been hit by laser photons. Using these techniques, photodissociation and photodetachment studies have been performed on the transition metal cluster ions with and without chemisorbed species attached. The binding energies and electron affinities of these clusters were measured as a function of cluster size and composition. Laser photodetachment studies have been performed on copper, silver and some other metal cluster negative ions. The electron affinity of copper clusters shows a sharp even/odd alternation superimposed on a steady monotonic increase with cluster size. Of the various netative cluster ions studied, only Ag(,x)('-) displayed photodissociation process as well as photodetachment. The photodissociation process of metal cluster positive ions with and without ligands attached has been studied to determine binding energies. The binding energies of bare cobalt and niobium positive cluster ions are found to be roughly constant with cluster size. By monitoring fragment yield as a function of dissociation laser wavelength...

## Molecular anions in circumstellar envelopes, interstellar clouds and planetary atmospheres: quantum dynamics of formation and evolution

Carelli, Fabio
EN
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36.05%
Nowadays, it is a well known fact that most of the matter in our Solar System, in our Galaxy and, probably, within the whole Universe, exists in the form of ionized particles. For decades astronomers and astrophysicists believed that only positively charged ions were worthy of relevance in drawing the networks for possible chemical reactions in the interstellar medium, as well as in modeling the physical conditions in most of astrophysical environments. Thus, negative ions (and especially molecular negative ions) received minor attention until their possible existence was observationally confirmed (discovery of the first interstellar anion, C6H-), about thirty years after the first physically reasonable proposal on their actual detection was theoretically surmised by E.Herbst (1981). From a purely theoretical point of view, negatively charged ions play a peculiar role as they can be formed in large quantities in the gas phase by attachment of low (1.5 eV < E < 10 eV) and very low-energy (E < 1.5 eV) free electrons, while sometimes such a formation process can occur even close to zero eV. In an astrophysical context, roughly speaking, their role should be then found in their involvement in the charge balance as well as in the chemical evolution of the considered environment: depending on their amount and on the global gas density...

## Variational Monte Carlo study of core-valence separation schemes for first-row atoms and positive ions /

Staroverov, Viktor N.
Fonte: Brock University Publicador: Brock University
Tipo: Electronic Thesis or Dissertation
ENG
Relevância na Pesquisa
45.88%
All-electron partitioning of wave functions into products ^core^vai of core and valence parts in orbital space results in the loss of core-valence antisymmetry, uncorrelation of motion of core and valence electrons, and core-valence overlap. These effects are studied with the variational Monte Carlo method using appropriately designed wave functions for the first-row atoms and positive ions. It is shown that the loss of antisymmetry with respect to interchange of core and valence electrons is a dominant effect which increases rapidly through the row, while the effect of core-valence uncorrelation is generally smaller. Orthogonality of the core and valence parts partially substitutes the exclusion principle and is absolutely necessary for meaningful calculations with partitioned wave functions. Core-valence overlap may lead to nonsensical values of the total energy. It has been found that even relatively crude core-valence partitioned wave functions generally can estimate ionization potentials with better accuracy than that of the traditional, non-partitioned ones, provided that they achieve maximum separation (independence) of core and valence shells accompanied by high internal flexibility of ^core and Wvai- Our best core-valence partitioned wave function of that kind estimates the IP's with an accuracy comparable to the most accurate theoretical determinations in the literature.

## Anion emission from water molecules colliding with positive ions: Identification of binary and many-body processes

Chesnel, J. -Y.; Juhász, Z.; Lattouf, E.; Tanis, J. A.; Huber, B. A.; Bene, E.; Kovács, S. T. S.; Herczku, P.; Méry, A.; Poully, J. -C.; Rangama, J.; Sulik, B.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
45.95%
It is shown that negative ions are ejected from gas-phase water molecules when bombarded with positive ions at keV energies typical of solar-wind velocities. This finding is relevant for studies of planetary and cometary atmospheres, as well as for radiolysis and radiobiology. Emission of both H- and heavier (O- and OH-) anions, with a larger yield for H-, was observed in 6.6-keV 16O+ + H2O collisions. The ex-perimental setup allowed separate identification of anions formed in collisions with many-body dynamics from those created in hard, binary collisions. Most of the ani-ons are emitted with low kinetic energy due to many-body processes. Model calcu-lations show that both nucleus-nucleus interactions and electronic excitations con-tribute to the observed large anion emission yield.; Comment: 5 pages, 4 figures

## Measurements of positive ions and air-earth current density at Maitri, Antarctica

Siingh, Devendraa; Pant, V; Kamra, A K
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
46.14%
Simultaneous measurements of the small-, intermediate- and large- positive ions and air earth current density made at a coastal station, Maitri at Antarctica during January to February 2005, are reported. Although, small and large positive ion concentrations do not show any systematic diurnal variations, variations in them are almost similar to each other. On the other hand, variations in intermediate positive ion concentrations are independent of variations in the small/large positive ions and exhibit a diurnal variation which is similar to that in atmospheric temperature on fair weather days with a maximum during the day and minimum during the night hours. No such diurnal variation in intermediate positive ion concentration is observed on cloudy days when variations in them are also similar to those insmall/large positive ion concentrations. Magnitude of diurnal variation in intermediate positive ion concentration on fair weather days increases with the lowering of atmospheric temperature in this season. Scavenging of ions by snowfall and trapping of Alha - rays from the ground radioactivity by a thin layer of snow on ground, is demonstrated from observations. Variations in intermediate positive ion concentration are explained on the basis of the formation of new particles by the photolytic nucleation process.; Comment: 38 pages...

## Ground states of the atoms H, He,...., Ne and their singly positive ions in strong magnetic fields: The high field regime

Ivanov, M. V.; Schmelcher, P.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
46.05%
The electronic structure of the ground and some excited states of neutral atoms with the nuclear charge numbers $1\leq Z \leq 10$ and their single positive ions are investigated by means of our 2D mesh Hartree-Fock method for strong magnetic fields $0.5\leq \gamma \leq 10000$. For $\gamma=10000$ the ground state configurations of all the atoms and ions considered are given by fully spin-polarized configurations of single-electron orbitals with magnetic quantum numbers ranging from m=0 to m=-N+1 where N is the number of the electrons. Focusing on the fully spin polarized situation we provide critical values of the magnetic field strength for which crossovers with respect to the spatial symmetries of the ground state take place. It is found that the neutral atoms and singly charged positive ions with $2\leq Z \leq 5$ have one fully spin-polarized ground state configuration whereas for $6\leq Z \leq 10$ one intermediate fully spin-polarized configuration with an orbital of 2p_0 type occurs.; Comment: to appear in Physical Review A

## Solvation of positive ions in water: The dominant role of water-water interaction

Krekeler, Christian; Site, Luigi Delle
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
45.95%
Local polarization effects, induced by mono and divalent positive ions in water, influence (and in turn are influenced by) the large scale structural properties of the solvent. Experiments can only distinguish this process of interplay in a generic qualitative way. Instead, first principles quantum calculations can address the question at both electronic and atomistic scale, accounting for electronic polarization as well as geometrical conformations. For this reason we study the extension of the scales' interconnection by means of first principle Car-Parrinello molecular dynamics applied to systems of different size. In this way we identify the general aspects dominating the physics of the first solvation shell and their connection to the effects related to the formation of the outer shells and eventually the bulk. We show that while the influence of the ions is extended to the first shell only, the water-water interaction is instead playing a dominant role even within the first shell independently from the size or the charge of the ion.; Comment: 4 pages 3 figures (color)

## Formation of Positive and Negative Ions on Rhenium, Oxygenated Tungsten, Hafnium, Lanthanum Hexaboride, and Thoriated Tungsten Surfaces

Persky, Avigdor; Greene, Edwad F.; Kuppermann, Aron
Fonte: American Institute of Physics Publicador: American Institute of Physics
Tipo: Article; PeerReviewed Formato: application/pdf
Relevância na Pesquisa
36.05%
Yields are reported for the formation of positive and negative ions from molecular beams striking various surfaces. Negative ions are formed from beams of several halogen compounds and tetracynoethylene on Re, Hf, thoriated W, and LaB6 with efficiencies ranging up to a maximum of 2.6 × 10^−2 for KCl on thoriated W (activated). This filament may be useful in detectors for some molecular-beam studies. The efficiencies for the formation of positive ions from K and potassium halides are generally in agreement with the Saha–Langmuir model. They are near unity on oxygenated W, Re, and thoriated W (deactivated by heating to deplete the thorium) but much lower (2.5–6.6×10^−3) on thoriated tungsten (activated).

## Ionization of gases by positive ions

Sutton, Richard M.
Fonte: Instituto de Tecnologia da Califórnia Publicador: Instituto de Tecnologia da Califórnia
Tipo: Article; PeerReviewed Formato: application/pdf
Relevância na Pesquisa
46.11%
With a tube of simple design, positive ions of potassium are driven by controllable accelerating potentials up to 750 volts into neon and argon at various pressures. Small collecting potentials are adjusted in such a way as to eliminate practically all secondary effects due to the ions, enabling the collection of any electrons liberated by ionization in the gases. Pressure ranges between 0.05 and 1 mm give definite variation of ionization with pressure. Curves are given showing the variation of ionization with pressure and the number of new ions formed per positive ion per cm path reduced to 1 mm pressure. Ionization by this process does not offer a sharp initial point, but a measurable amount is present in neon at 100 volts, and in argon between 100 and 150 volts. The efficiency of the effect is decidedly less than for electron bombardment in these gases; it is several fold greater in argon than in neon. In neither gas has the efficiency reached a maximum at 750 volts, although there is evidence in the case of argon that the maximum efficiency may be found at a slightly higher accelerating potential. The pressure range within which the ionization effect shows itself is higher than should be expected from kinetic theory, which seems to indicate the presence of long mean free paths for the positive ions in accord with results of other observers using magnetic analysis apparatus.

## Ionization of helium by potassium positive ions

Sutton, Richard M.; Mouzon, J. Carlisle
Fonte: Instituto de Tecnologia da Califórnia Publicador: Instituto de Tecnologia da Califórnia
Tipo: Article; PeerReviewed Formato: application/pdf
Relevância na Pesquisa
46.08%
A continuation of previous attempts to ionize gases by positive ions has been extended to helium using potassium positive ions as high as 750 volts accelerating potential. At pressures between 0.01 and 0.1 mm there is definite evidence of ionization of helium above 150 or 200 volts accelerating potential. The effect is much smaller than in the gases previously reported, but it may be distinguished from the secondary emission of electrons from the metal parts under action of the positive ions. Great difficulty was encountered in changes of intensity of ionization due to impurities in the helium. Some of the uncertainties in previous work have been removed by changing the relative positions of electrodes.

## The secondary emission from a nickel surface due to slow positive ion bombardment

Klein, A. L.
Fonte: Instituto de Tecnologia da Califórnia Publicador: Instituto de Tecnologia da Califórnia
Tipo: Article; PeerReviewed Formato: application/pdf
Relevância na Pesquisa
36.08%
The apparatus was arranged so that positive ions from a heated Mo strip coated with aluminum phosphate, were accelerated radially through slots in shields A and B, and a wider slot in an electrode C to the target T, from which the secondary emission to the electrode C was measured. The parts A, B, C and T were all concentric nickel cylinders, symmetrically placed about the emitter. If the potential of the emitter is made zero and the accelerating potential V1, when the collector is at zero potential only reflected positives are collected. As the potential is increased, secondaries also reach it, the net negative charge increasing until a saturating potential 2V1 is reached, when all negatives and no positives are collected. The percentage saturated secondary electron emission was found to increase approximately as the square of the primary ion energy, reaching 22 percent of the primary ion current, for primary ions of energies corresponding to 380 volts. The curves show that large numbers of reflected positive ions have energies between zero and two volts, and there is also a group of reflected positive ions with energies approximately 0.9 of the primary energy.

## Positive Ion Flux from a Low-Pressure Electronegative Discharge

Sheridan, T; Boswell, Roderick
Fonte: Institute of Physics Publishing Publicador: Institute of Physics Publishing
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
56.11%
We compute the flux of positive ions exiting a low-pressure, planar, electronegative discharge as a function of the negative ion concentration and temperature. The positive ions are modelled as a cold, collisionless fluid, while both the electron and negative ion densities obey Boltzmann relations. For the plasma approximation, the plasma edge potential is double-valued when the negative ions are sufficiently cold. When strict charge neutrality is relaxed, spatial space-charge oscillations are observed at the edge of the plasma when the flux associated with the low (in absolute value) potential solution is less than that of the high potential solution. However, the flux is always well defined and varies continuously with the negative ion concentration. We demonstrate that the correct solution for the plasma approximation is that having the greater flux.

## Kinetic model for a low-pressure discharge with negative ions

Chabert, Pascal; Sheridan, T
Fonte: Institute of Physics Publishing Publicador: Institute of Physics Publishing
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
46.15%
A computational model of a low-pressure discharge having a negative-ion component is developed. Many existing models for this type of discharge consider collisionless positive ions and two negative species each obeying Boltzmann relations. Our aim is to relax the Boltzmann-negative-ion assumption and we use a more realistic model with kinetic positive ions, kinetic negative ions and Boltzmann electrons. Positive and negative ions are created uniformly in the discharge at a constant rate, and lost either to the walls or via volume recombination. This model is solved using a hybrid simulation with particle-in-cell (PIC) ions. The negative-ion distribution function is found to have cold and hot components of nearly equal densities for which Te/T-cold≈100 (the creation temperature) and Te/T-hot≈5-20. The computed positive ion flux exiting the discharge agrees approximately with those calculated from Boltzmann-negative-ion models when the negative-ion temperature is accounted for correctly. It has been predicted that three electronegative discharge structures can exist: uniform, stratified and double-layer stratified. All three structures are observed in our model. In particular, a double-layer stratified discharge is observed when the effective negative-ion temperature is sufficiently low...

## Wall Effects on the Chemistry in a Pulsed Oxygen/Silane Radiofrequency Helicon Plasma

Charles, Christine
Fonte: Institute of Physics Publishing Publicador: Institute of Physics Publishing
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
46.1%
A pulsed oxygen/silane radiofrequency (13.56 MHz) plasma is created in a helicon diffusion reactor used for silicon dioxide deposition. An energy selective mass spectrometer is used to measure the time-averaged ion energy distribution function of the four predominant positive ions (H3+, H3O+, SiH3+, and SiOH+) and the positive ion mass spectra during the breakdown phase and the steady-state phase of the pulse. Charging of the silica-covered chamber wall by ∼55 V is observed both for continuous and pulsed excitations. All positive ions with mass greater than 62 (as well as H2O+ and SiOH+) are mostly detected during the plasma breakdown phase. This could result from some physical sputtering of the chamber walls at the beginning of each pulse (when the plasma potential and the ion bombardment energy reach their maximum) or from an increase in the density of heavy radicals during the post-discharge (as a result of the electron density and temperature decrease) which can be ionized during plasma breakdown of the oncoming pulse. During the plasma breakdown an ion bombardment energy of ∼70 V is measured which is larger than the physical sputtering threshold (25 V) of thermally grown SiO2 (P-etch ∼ 2 Å s-1) in an oxygen plasma. Consequently...