On the Mechanism of Formation of Intratrack Yields of Water Radiolysis Products upon Irradiation with Fast Electrons and Positrons: 2. Concentration and Time Dependences of Yields

A mathematical model for the formation of main transient and final radiolysis products generated in tracks of fast electrons and positrons in water and aqueous solutions was constructed and described in terms of equations of inhomogeneous chemical kinetics in part 1 of this study. The model takes into account the reactions of a solute with epithermal electrons, thermal, and hydrated electrons; the ambipolar character of diffusion of charged intratrack particles; and new pathways of the formation of hydrogen and positronium due to the appearance of weakly bound states of electrons. In the present paper, the model was quantitatively fitted to experimental data on both time variation in the yields of radiolytic products (H₃O⁺, e _aq ⁻ , H, OH, OH⁻, H₂O₂) in pure water and the yields of hydrogen (H₂, H), hydrated electron (e _aq ⁻ ) and positronium (Ps) in various dilute and concentrated aqueous solutions.__________Translated from Khimiya Vysokikh Energii, Vol. 39, No. 5, 2005, pp. 330–338.Original Russian Text Copyright © 2005 by Stepanov, Byakov.     

On the mechanism of formation of intratrack yields of water radiolysis products upon irradiation with fast electrons and positrons: 1. Model formulation

A mathematical model is constructed for the formation of main transient (e _aq ^– , H, OH) and final (H₂, H₃O⁺, OH^–, Ps) radiolytic products formed in tracks of fast electrons and positrons in water and aqueous solutions. The model takes into account the occurrence of reactions of epithermal electrons, along with thermal and hydrated electrons, with a solute; the ambipolar character of diffusion of charged intratrack particles; and new pathways of the formation of hydrogen and positronium due to the appearance of weakly bound adducts of electrons with their scavengers, in addition to the traditional routes.__________Translated from Khimiya Vysokikh Energii, Vol. 39, No. 3, 2005, pp. 165–170.Original Russian Text Copyright © 2005 by Stepanov, Byakov.     

Vicinage effect in secondary electron emission from a clean (001) surface of SnTe induced by keV hydrogen clusters

The total secondary electron emission yields from a clean (001) surface of SnTe crystal are studied at the bombardment of hydrogen clusters (H⁺, H ₂ ⁺ and H ₃ ⁺ ions) with energies ranging from 7 to 12 keV/amu. The observed yield is not proportional to the number of protons in the cluster ion. The yields are successfully explained by applying the stopping powers of a homogeneous electron gas for the constituents of the clusters in keV energy region, which provide the vicinage effect.     

Modelling spur chemistry for alkaline and acidic water at high temperatures

The effect of pH and associated ionic strength on the primary yields in the radiolysis of pressurised water has been assessed by diffusion-kinetic calculations for temperatures in the range 100–300°C. Account has been taken for ionic strength I up to 0.1 mol kg⁻¹, assuming that the counter ions of H⁺ in acid solutions and of OH⁻ in base solutions have unit charge. In acid solutions, the H⁺ ions react with e⁻ _aq. The decrease in G(e⁻ _aq) and the increase in G(H) with decreasing pH becomes substantial for [H⁺] ≥ 1 × 10⁻⁴ m, but the primary yields of oxidising species are almost constant. In alkaline solutions, the OH⁻ anions affect the spur chemistry of radiation-generated protons and hydroxyl radicals for [OH⁻] ≥ 1 × 10⁻⁴ m. The scavenging of H atoms and hydrogen peroxide becomes significant for [OH⁻] ≥ 1 × 10⁻² m. The total yields G(OH) + G(O⁻) and G(H₂O₂) + G(HO₂ ⁻) are independent of base concentration below 0.01 m. In more alkaline solutions, G(OH) + G(O⁻) increases, whereas G(H₂O₂) + G(HO₂ ⁻) decreases with increasing [OH⁻]. Calculations showed the substantial yield of the reaction O⁻ + e⁻ _aq in 0.1 m base solution. Spur chemistry in alkaline hydrogenated water is not affected by the presence of H₂ if less than 0.001 m of hydrogen is added.     

A Mössbauer Tin-119 Emission Spectroscopy Study on Electron Scavenging in Frozen Solutions

The effects of impurity ions on the yield of valence states of tin after the internal conversion isomeric transition of ^119mSn in frozen alcohol solutions was studied. The ratios of rate constants for electron scavenging by the solutes H⁺, Cr⁺², and Cu⁺² were determined. Mössbauer emission spectra showed that, as the solute concentration increased, the yield of Sn⁺⁴ increased. The calculation of the electron scavenging rate constants in terms of various models (simple competitive reactions, the kinetics involving the epithermal electron, and the electron diffusion model) showed that metal ions have a higher electron scavenging ability. It was concluded that the presolvated electron takes part in reactions with the solutes.     

Hydrogen rearrangement in molecular ions of alkyl benzenes: Appearance potentials and substituent effects on the formation of [C7H8]+ ions

The mechanism of the formation of [C₇H₈]⁺ ions by hydrogen rearrangement in the molecular ions of 1-phenylpropane and 1,3-diphenylpropane has been investigated by looking at the effects of CH₃O and CF₃ substituents in the meta and para positions on the relative abundances of the corresponding ions and on the appearance energies. The formation of [C₇H₈]⁺ ions from 1,3-diphenylpropane is much enhanced at the expense of the formation of [C₇H₇]⁺ ions by benzylic cleavage, due to the localized activation of the migrating hydrogen atom by the γ phenyl group. A methoxy substituent in the 1,3-diphenylpropane, exerts a site-specific influence on the hydrogen rearrangement, which is much more distinct than in 1-phenylpropane and related 1-phenylalkanes, the rearrangement reaction being favoured by a meta methoxy group. The mass spectrum of 1-(3-methoxyphenyl)-3-(4-trideuteromethoxyphenyl)-propane shows that this effect is even stronger than the effect of para methoxy groups on the benzylic cleavage. From measurements of appearance potentials it is concluded that the substituent effect is not due to a stabilization of the [C₇H₇X]⁺ product ions. Whereas the [C₇H₇]⁺ ions are formed directly from molecular ions of 1-phenylpropane and 1,3-diphenylpropane, the [C₇H₈]⁺ ions arise by a two-step mechanism in which the s? complex type ion intermediate can either return to the molecular ion or fragment to [C₇H₈]⁺ by allylic bond cleavage. Obviously the formation of this s? complex type ion, is influenced by electron donating substituents in specific positions at the phenyl group. This is borne out by a calculation of the ΔH_f values of the various species by thermochemical data. Thus, the relative abundances of the fragment ions are determined by an isomerization equilibrium of the molecular ions, preceding the fragmentation reaction.     

Chemical constitution of 3d complexes and rate constants of ortho-para positronium conversion reactions

The positronium spin exchange (SE) reactions promoted by the 3d complexes Mn _aq ²⁺ , Co _aq ²⁺ , [Cr(NH₃)₆]³⁺, [Cr(NH₃)₅H₂O]³⁺, and [Cr(H₂O)₅Cl²⁺] were investigated at different temperatures in order to ascertain whether they were diffusion controlled like those promoted by Cr _aq ³⁺ , Fe _aq ²⁺ , Ni _aq ²⁺ and [Ni(NH₃)₆] _aq ²⁺ ions studied previously.It was found that the reactions are diffusion controlled and that the effective reaction radii deduced from the Smoluchowski equation are smaller than the compound radii calculated from bond lengths and angles. It was also found that the rate constants, and therefore the reaction radii, are correlated with the spin—orbit coupling of 3d unpaired electrons and ligand capabilities to cause expansion of the d-electron cloud (nephelauxetic effect). In particular the effective reaction radii of SE reactions promoted by 3d complexes increase as the electron delocalization from 3d atoms increases.Work supported by Ministero Università e Ricerca Scientifica e Tecnologica (M.U.R.S.T.) and by Consiglio Nazionale Ricerche (C.N.R.).     

AOT微乳液中通过水化电子产额调控合成BaSO_4纳米纤维

通过γ-辐照含有K2S2O8和BaCl2的二(2-乙基己基)琥珀酸酯磺酸钠(AOT)反相微乳液,将S2O82-通过辐射还原实现了SO42-的原位缓释,从而成功制备出BaSO4纳米纤维单晶,并进一步制得多层次的纳米纤维束结构.在此基础上,通过改变水与表面活性剂物质的量之比(ω值)、改变钡盐阴离子和在微乳液连续相添加芳香化合物等手段来调节水化电子(e-aq)产额,控制微乳液水池中S2O82-的还原和SO42-的缓释速率,成功实现了对BaSO4纳米粒子形貌的调控:随着ω值的增加或剂量率的增加,e-aq产额增加,从而加快了SO42-的释放,不利于BaSO4纳米纤维的生成;采用Ba(NO3)2为钡源时,NO3-能有效地降低e-aq产额和S2O82-的还原速率,因而在较高的剂量率和较高ω值下能得到BaSO4纳米纤维;在微乳液油相中加入甲苯来捕获油相中过量电子(e-oil),降低e-aq产额,从而在较高的剂量率下得到BaSO4纳米纤维.研究结果表明:通过e-aq产额调控纳米粒子形貌的机理在BaSO4纳米粒子的制备中得到很好体现.     

Characterization of an argon-hydrogen microwave discharge used as an atomic hydrogen source. Effect of hydrogen dilution on the atomic hydrogen production

This work is devoted to the study of an argon-hydrogen microwave plasma used as an atomic hydrogen source. Our attention has focused on the effect of the hydrogen dilution in argon on atomic hydrogen production. Diagnostics are performed either in the discharge or in the post-discharge using emission spectroscopy (actinometry) and mass spectrometry. The agreement between actinometry and mass spectrometry diagnostics proves that actinometry on the Ha(656.3 nm) and Hβ(486.1 nm) hydrogen Balmer lines can be used to measure the relative atomic hydrogen density within the microwave discharge. Results show that the atomic hydrogen density is maximum for a gas mixture corresponding to the partial pressure ratioP _{H 2}/P _Ar range between 1.5 and 2. The variation of atomic hydrogen density can be explained by a change of the dominant reactive mechanisms. At a low hydrogen partial pressure the dominant processes are the charge transfers with recombinations between Ar⁺ and H₂ which lead to ArH⁺ and H ₂ ⁺ ion formation. Both ions are dissociated in dissociative electron attachment processes. At a low argon partial pressure the electron temperature and the electron density decrease with increasing partial pressure ratio. The dominant mechanisms become direct reactions between charged particles (e, H⁺, H ₂ ⁺ , and H ₃ ⁺ ) or excited species H(n=2) with H₂ producing H atoms.     

On the postronium spin exchange reactions promoted by 3d aqua ions

In order to ascertain possible relationships between the rate constants, K_SE, of positronium, Ps, spin exchange /SE/ reactions promoted by 3d aqua ions and the electron configuration of the ion, the K_SE's of Mn _aq ²⁺ and Co _aq ²⁺ aqua ions were measured together with that of [Cr(NH₃)₆]³⁺. The K_SE's values obtained are discussed together with those of the Cr _aq ³⁺ , Fe _aq ²⁺ , Ni _aq ²⁺ and [Ni(NH₃)₆]²⁺ ions previously measured. It was found that Cr _aq ³⁺ and Mn _aq ²⁺ K_SE's are 1 M^–1ns^–1, while those of Fe _aq ²⁺ , Co _aq ²⁺ and Ni _aq ²⁺ ions are 2.5 M^–1ns^–1. Thus the K_SE values of 3d aqua ions do not depend solely on the number of their unpaired electrons as it was found for the 4f aqua ions. The trend observed parallels that of the electron delocalization from the metal atom; the delocalization occurs only with Fe _aq ²⁺ , Co _aq ²⁺ and Ni _aq ²⁺ ions, but not with the Cr _aq ³⁺ and Mn _aq ²⁺ aqua ions. The trend of the K_SE's of aqua, ammine and perhaps of chloro complexes parallels that of ligand capabilities to cause d-electron cloud expansion, and thus d-electron delocalization from metal atoms.     

Photo- and radiation-produced H-atoms in acid low temperature glasses containing Fe2+ ions

Frozen solutions of H₂SO₄ and acid 7 M NaClO₄ containing Fe²⁺ ions have been exposed to X rays and UV light and examined by EPR methods. It is concluded that the H atoms formed in UV-irradiated acid perchlorate matrices do not have mobile electrons as precursors. Thus no trapped electrons are found after UV irradiation even though these matrices provide efficient traps for electrons. The effect of the electron scavenger NO^?₃ is identical in UV- and X-irradiated matrices and largest in the H₂SO₄ matrix. Thus it seems that NO^?₃ acts on a similar H-atom precursor in the two cases, and that this precursor may be an excited CTTS state, rather than a mobile electron. The absence of allyl alcohol scavening effect on H atoms produced by UV light in the perchlorate matrix indicates that this H-atom scavenger does not interact with the CTTS state, and that the H atom is trapped in the vicinity of the Fe²⁺ ion where it was formed. The presence of small amounts of Fe²⁺ ions (< 10^?3 M) in the matrices causes a marked decrease in the spin—lattice relaxation time of the trapped hydrogen atoms as well as a decrease in the intensities of the satellite lines relative to the main hydrogen lines.     

Crystal structure,thermal behavior,and infrared absorption spectrum of cesium hydrogen selenite-selenious acid (12) CsHSeO3 · 2H2SeO3

The present work describes the crystal structure, thermal behavior, and infrared absorption spectrum of cesium hydrogen selenite-selenious acid (12), CsHSeO₃ · 2H₂SeO₃. This compound crystallizes in the monoclinic crystal system withP2₁c-C⁵_2b (Z = 4) as the space group. The unit cell dimensions are as follows:a = 8.9897(20), b = 8.5078(21), c = 12.6476(31)A˚, and β = 95.141(19)°. The crystal structure consists of discrete H₂SeO₃ molecules which are weakly hydrogen bonded to form layers which are further connected by (HSeO₃)⁻ ions with much stronger hydrogen bonds. The hydrogen atoms show no disorder within the hydrogen bonds. The Cs⁺ ions are coordinated to oxygens from both selenious acid molecules and hydrogen selenite ions. The thermal decomposition of CsHSeO₃ · 2H₂SeO₃ in air starts with incongruent melting due to rupture of hydrogen bonds at 310 K and is followed later by the formation of cesium diselenite phase. At higher temperatures (700 K) this compound decomposes with oxidation of selenium to yield cesium selenate. Both deformation and stretching vibrations of SeOH groups from both (HSeO₃)⁻ ions and H₂SeO₃ molecules can be found in the IR absorption spectrum of CsHSeO₃ · 2H₂SeO₃. This confirms the ordered position of hydrogen atoms in hydrogen bonds. The OH vibrations corresponding to hydrogen bonded species can be found also.     

Ring expansion and hydrogen scrambling in the molecular ion of 3-methylthiophene

The ratio [M ? D]/{[M-D] + [M ? H]} in the 70 eV mass spectra of six deuterated 3-methylthiophenes has been determined. From these values the mole fractions of the molecular ions that lose hydrogen atoms specifically from the various positions of the molecule were calculated, as well as the mole fraction in which the hydrogen atoms are fully scrambled before hydrogen elimination. It appears that hydrogen atoms are mainly lost from a fully scrambled [C₅H₆S]⁺· ion and from the α-position of the original molecular ion. A deuterium isotope effect of 1·60 to 1·72 was calculated for the hydrogen elimination. The reaction was also studied at low electron energies. In order to determine the degree of scrambling in the [C₅H₅S]⁺ ions, some decomposition reactions of this ion were investigated.     

Ionic Fragmentation Mechanisms of 2,2,2‐Trifluoroethanol Following Excitation with Synchrotron Radiation

Gaseous 2,2,2‐trifluoroethanol (TFE) is excited with synchrotron radiation between 10 and 1000 eV and the ejected electrons and positive ions are detected in coincidence. In the valence‐electron energy region, the most abundant species is CH₂OH⁺. Other fragments, including ions produced by atomic rearrangements, are also detected; the most abundant are COH⁺, CFH₂⁺ and CF₂H₂⁺. The energies of electronic transitions from C 1 s, O 1 s and F 1 s orbitals to vacant molecular orbitals are determined. A site‐specific C 1 s excitation is observed. The photofragmentation mechanisms after the excitation of core‐shell electrons are inferred from analysis of the shape and slope of the coincidence between two charged fragments in the bi‐dimensional coincidence spectra. The spectra are dominated by islands that correspond to the coincidence of H⁺ with several charged fragments. One of the most important channels leads to the formation of CH₂OH⁺ and CF₃⁺ in a concerted mechanism.     

Participation of halogen atoms in hydrogen transfer between remote positions in gas-phase ionized isomeric haloresorcinols

Mono, di- and trihaloresorcinols substituted by a halogen atom at position 2 exhibit a highly specific elimination of H₂O on electron impact ionization and under conditions of collisionally induced dissociation (CID). The high isomer specificity suggests the intermediacy of a hydrogen transfer from one of the hydroxy groups to the adjacent halogen atom. A subsequent hydrogen migration to the other hydroxy group readily explains the facile elimination of H₂O from the M^+· ions of these particular isomers. An analogous three-step hydrogen transfer has not been observed in 2,3-dihalo-l,4-hydroquinones. 4-Bromo- and 4-icdoresorcinol undergo elimination of the halogen atom followed by a very fast loss of CO under CID conditions, affording [M ? Hal]⁺ ions of low abundance and highly abundant[M ? Hal ? CO]⁺ ions. The elimination of CO is suppressed in the isomeric 5-haloresorcinols, resulting in very highly abundant [M ? Hal]⁺ ions. This behavior suggests that a ‘hidden hydrogen transfer’ accompanies the elimination of the halogen atom from the molecular ions of 4-haloresorcinols.     

Hydrogen‐bonded sheets in benzylmethylammonium hydrogen maleate

In the title compound, C₈H₁₂N⁺·C₄H₃O₄⁻, there is a short and almost linear but asymmetric O—H...O hydrogen bond in the anion. The ions are linked into C₂²(6) chains by two short and nearly linear N—H...O hydrogen bonds and the chains are further weakly linked into sheets by a single C—H...O hydrogen bond.     

Metastable ions in chemical ionization

The ion [C₃H₅]⁺ generated in a chemical ionization source by a variety of methods, including protonation and charge exchange, exhibits a metastable peak for H₂ loss which is two orders of magnitude weaker than that formed in an electron impact source. The stable [C₃H₅]⁺ ions generated by electron impact and chemical ionization undergo collision-induced dissociation to a comparable extent, both losing H₂ by only one of the two competitive mechanisms observed for metastable ions. In contrast to the behavior of [C₃H₅]⁺, the molecular ions of p-substituted nitrobenzene, generated by charge exchange at high source pressure, yield composite metastable peaks for NO loss which are very similar in shape and intensity to those generated by electron impact. The contrasting behavior of the metastable ions extracted from high pressure ion sources in the two systems may be due to differences in the efficiencies of quenching of the ionic states responsible for fragmentation as metastable ions. It is noteworthy that the NO loss reactions require considerably lower activation energies than does the H₂ loss reaction.     

Mechanism of Producing Metallic Nanoparticles,with an Emphasis on Silver and Gold Nanoparticles,Using Bottom-Up Methods

Bottom-up nanoparticle (NP) formation is assumed to begin with the reduction of the precursor metallic ions to form zero-valent atoms. Studies in which this assumption was made are reviewed. The standard reduction potential for the formation of aqueous metallic atoms—E⁰(Mⁿ⁺_aq/M⁰_aq)—is significantly lower than the usual standard reduction potential for reducing metallic ions Mⁿ⁺ in aqueous solution to a metal in solid state. E⁰(Mⁿ⁺_aq/M⁰_solid). E⁰(Mⁿ⁺_aq/M⁰_aq) values are negative for many typical metals, including Ag and Au, for which E⁰(Mⁿ⁺_aq/M⁰_solid) is positive. Therefore, many common moderate reduction agents that do not have significantly high negative reduction standard potentials (e.g., hydrogen, carbon monoxide, citrate, hydroxylamine, formaldehyde, ascorbate, squartic acid, and BH₄⁻), and cannot reduce the metallic cations to zero-valent atoms, indicating that the mechanism of NP production should be reconsidered. Both AgNP and AuNP formations were found to be multi-step processes that begin with the formation of clusters constructed from a skeleton of M⁺-M⁺ (M = Ag or Au) bonds that is followed by the reduction of a cation M⁺ in the cluster to M⁰, to form M_n⁰ via the formation of NPs. The plausibility of M⁺-M⁺ formation is reviewed. Studies that suggest a revised mechanism for the formation of AgNPs and AuNPs are also reviewed.     

Microwave Interferometry of Chemically Active Plasma of RF Discharge in Mixtures Based on Fluorides of Silicon and Germanium

The free electron concentration in hydrogen and chemically active plasmas in H₂ + SiF₄ and H₂ + GeF₄ mixtures was measured by microwave interferometry. The investigations were carried out under conditions of a RF capacitive-coupled discharge at a pressure of 1 Torr. In hydrogen plasma the concentration of free plasma electrons is 1.5 ± 0.03 × 10¹² cm^?3. When the fluoride is added to the hydrogen plasma, the electron concentration is reduced to 1.1 ± 0.05 × 10¹² cm^?3 for SiF₄ and 9.8 ± 0.05 × 10¹¹ cm^?3 for GeF₄. It is suggested that the main mechanism responsible for reducing the concentration of free electrons is the mechanism of dissociative attachment of electrons to SiF₄ and GeF₄ molecules. The difference in the electron concentration for these mixtures is due to the difference in the electron-acceptor ability of the SiF₄ and GeF₄ molecules determined by the affinity for the electron.     

Photobleaching of radiation induced trapped electrons in neutral and alkaline glasses. Hydrogen atom formation

Photomobilization of trapped electrons in 7 M NaClO₄- and 9 M NaOH-glass gives rise to trapped hydrogen atoms. This is probably due to the reaction e^?_m + H₂O → H + OH^?. Experiments with an electron scavenger indicate that electrons are not precursors to radiolytically produced hydrogen atoms. It seems that the mobile electrons produced during photobleaching are not slowed down to thermal energy before they react to produce hydrogen atoms, since the yield of the latter species is strongly dependent on the wavelength of the bleaching light.