Spur Reactions Research Articles

Scavenging the Water Cation in Concentrated Acidic Solutions.

Picosecond pulse radiolysis techniques were used to observe the kinetics of the SO4(•-), H2PO4(•), Cl2(•-), and Br2(•-) species formed in the fast oxidation of concentrated and highly acidic solutions of SO4(2-), PO4(3-), Cl(-), and Br(-). Experimental results were compared with model predictions to gain insight into the possible mechanisms occurring on the fast time scales. Simple kinetics involving the oxidizing OH(•) radical formed by radiolytic water decomposition could not account for the observed yields at the very short times (within the electron pulse ∼7 ps). Diffusion-kinetic simulations of the spur reactions induced by the incident electrons show that additional oxidation of the solutes must occur at very short times and involves their direct ionization along with scavenging of the highly oxidizing H2O(•+) radical formed in the initial ionization of the water medium. The fraction of H2O(•+) radicals scavenged varies as 0.26, 0.68, 0.92, and 0.97 for PO4(3-), SO4(2-), Cl(-), and Br(-) solutions, respectively. These studies represent the first semiquantitative estimation of the H2O(•+) radicals scavenging fractions for such a wide range of solutes.

Spur Reactions Observed by Picosecond Pulse Radiolysis in Highly Concentrated Bromide Aqueous Solutions

The formation of the well-known product Br3(-), observed in the steady-state radiolysis of highly concentrated Br(-) aqueous solutions, has now been directly observed at ultrashort times corresponding to the relaxation of the spur. The transient absorption induced by picosecond pulse radiolysis of 6 M Br(-) aqueous solution was probed simultaneously at 260 nm with the third harmonic laser wave and from 350 to 750 nm with a supercontinuum generated by the fundamental laser wave. This approach allows several transient radiolytic species to be followed in parallel, particularly the solvated electron, BrOH(-•), Br2(-•), and Br3(-). The kinetics measured within 4 ns at 260 and 370 nm clearly exhibit that the decay of Br2(•-) is correlated with the formation of Br3(-). In highly concentrated Br(-) solutions, the OH(•) radical is fully replaced by Br2(•-), and the spur kinetics of OH(•) radical in pure water is comparable with that of Br2(-•). Model calculations indicate that the main OH(•) radical combination product H2O2 in pure water has formation kinetics similar to that of Br3(-) in 6 M Br(-) solutions. Moreover, they point out that oxidation of Br(-) occurs within the electron pulse both by direct energy absorption and by scavenging of the water radical cation, H2O(•+).

Competition Reactions of H2O•+ Radical in Concentrated Cl– Aqueous Solutions: Picosecond Pulse Radiolysis Study

Picosecond pulse-probe radiolysis measurements of highly concentrated Cl(-) aqueous solutions are used to probe the oxidation mechanism of the Cl(-). The transient absorption spectra are measured from 340 to 710 nm in the picosecond range for the ultrafast electron pulse radiolysis of halide solutions at different concentrations up to 8 M. The amount of Cl(2)(•-) formation within the electron pulse increases notably with increasing Cl(-) concentration. Kinetic measurements reveal that the direct ionization of Cl(-) cannot solely explain the significant amount of fast Cl(2)(•-) formation within the electron pulse. The results suggest that Cl(-) reacts with the precursor of the OH(•) radical, i.e., H(2)O(•+) radical, to form Cl(•) atom within the electron pulse and the Cl(•) atom reacts subsequently with Cl(-) to form Cl(2)(•-) on very short time scales. The proton transfer reaction between H(2)O(•+) and the water molecule competes with the electron transfer reaction between Cl(-) and H(2)O(•+). Molecular dynamics simulations show that number of water molecules in close proximity decreases with increasing concentration of the salt (NaCl), confirming that for highly concentrated solutions the proton transfer reaction between H(2)O(•+) and a water molecule becomes less efficient. Diffusion-kinetic simulations of spur reactions including the direct ionization of Cl(-) and hole scavenging by Cl(-) show that up to 30% of the H(2)O(•+) produced by the irradiation could be scavenged for solutions containing 5.5 M Cl(-). This process decreases the yield of OH(•) radical in solution on the picosecond time scale. The experimental results for the same concentration of Cl(-) at a given absorbed dose show that the radiation energy absorbed by counterions is transferred to Cl(-) or water molecules and the effect of the countercation such as Li(+), K(+), Na(+), and Mg(2+) on the oxidation yield of Cl(-) is negligible.

Time-dependent yield of the hydrated electron in subcritical and supercritical water studied by ultrafast pulse radiolysis and Monte-Carlo simulation

Fast kinetics and time-dependent yields of the hydrated electron (e(-)(aq)) in pure water under conditions of high temperature and pressure up to the supercritical region were investigated by picosecond and nanosecond pulse radiolysis experiments. More significant decays at short times followed by plateau components at longer times were observed with increasing temperature, suggesting faster spur reaction processes. In supercritical water, it was also found that the e(-)(aq) yields strongly depend on the pressure (density). Comparison of these measurements with Monte-Carlo computer simulations allowed us to identify spur reactions of e(-)(aq) that occur predominantly at high temperatures and also to provide new key information on certain spur model parameters. In particular, the experimental time-dependent e(-)(aq) yields were best reproduced if the electron thermalization distance decreases with increasing temperature. This "shrinkage" of spur sizes at high temperatures was attributed to an increase in the scattering cross sections of subexcitation electrons, likely originating from a decrease in the degree of structural order of water molecules as the temperature is increased.

Picosecond Pulse Radiolysis of Direct and Indirect Radiolytic Effects in Highly Concentrated Halide Aqueous Solutions

Recently we measured the amount of the single product, Br(3)(-), of steady-state radiolysis of highly concentrated Br(-) aqueous solutions, and we showed the effect of the direct ionization of Br(-) on the yield of Br(3)(-). Here, we report the first picosecond pulse-probe radiolysis measurements of ionization of highly concentrated Br(-) and Cl(-) aqueous solutions to describe the oxidation mechanism of the halide anions. The transient absorption spectra are reported from 350 to 750 nm on the picosecond range for halide solutions at different concentrations. In the highly concentrated halide solutions, we observed that, due to the presence of Na(+), the absorption band of the solvated electron is shifted to shorter wavelengths, but its decay, taking place during the spur reactions, is not affected within the first 4 ns. The kinetic measurements in the UV reveal the direct ionization of halide ions. The analysis of pulse-probe measurements show that after the electron pulse, the main reactions in solutions containing 1 M of Cl(-) and 2 M of Br(-) are the formation of ClOH(-•) and BrOH(-•), respectively. In contrast, in highly concentrated halide solutions, containing 5 M of Cl(-) and 6 M of Br(-), mainly Cl(2)(-•) and Br(2)(-•) are formed within the electron pulse without formation of ClOH(-•) and BrOH(-•). The results suggest that, not only Br(-) and Cl(-) are directly ionized into Br(•) and Cl(•) by the electron pulse, the halide atoms can also be rapidly generated through the reactions initiated by excitation and ionization of water, such as the prompt oxidation by the hole, H(2)O(+•), generated in the coordination sphere of the anion.

Hydrated electron decay measurements with picosecond pulse radiolysis at elevated temperatures up to 350 °C

The present work reports the first pulse-probe absorbance measurements of the hydrated electron at picosecond range and at elevated temperature. The decay of the hydrated electron, from room temperature up to 350 °C, is observed at 790 nm using a laser triggered picosecond electron accelerator. With increasing temperature, the decay of the hydrated electron becomes less and less pronounced. At 350 °C, almost a flat signal from 100 ps to 2.75 ns is observed, suggesting an efficient escape of hydrated electrons from spur reactions.

Direct observation of elementary radical events: low- and high-energy radiation femtochemistry in solutions

The fundamental importance of understanding the primary effects of ionizing radiations in liquid phase and solutions is emphasized in fields such as electron transfer reactions, radical chemistry and radiobiology. With the advent of ultrashort optical pulses and powerful laser systems (TW lasers), ultrafast spectroscopic investigations might conjecture the direct observation of primary events induced by low-energy (photons) and high-energy (relativistic electrons) radiations. The different points discussed in this paper concern the investigation of short-time solvent caging effects on elementary radical reactions in homogeneous liquid phase and nascent spurs.

Pulse radiolysis of alkanes: a time-resolved EPR study—part I. Alkyl radicals

Time-resolved EPR was applied to detect short-lived alkyl radicals in pulse radiolysis of liquid alkanes. Two problems were addressed: (i) the mechanism of radical formation and (ii) the mechanism of chemically-induced spin polarization in these radicals. (i) The ratio of yields of penultimate and interior radicals in n-alkanes at the instant of their generation was found to be ≈ 1.25 times greater than the statistical quantity. This higher-than-statistical production of penultimate radicals indicates that the proton transfer reaction involving excited radical cations must be a prevailing route of radical generation. The relative yields of hydrogen abstraction and fragmentation for various branched alkanes are estimated. It is concluded that the fragmentation occurs prior to the formation of radicals in an excited precursor species. (ii) The analysis of spin-echo kinetics in n-alkanes suggests that the alkyl radicals gain the emissive polarization in spur reactions. This initial polarization increases with shortening of the aliphatic chain. We suggest that the origin of this polarization is the ST − mechanism operating in the pairs of alkyl radicals and hydrogen atoms generated in dissociation of excited alkane molecules. It is also found that a long-chain structure of alkyl radicals results in much higher rate of Heisenberg spin exchange relative to the recombination rate (up to 30 times). That suggests prominent steric effects in recombination or the occurrence of through-chain electron exchange. The significance of these results in the context of cross-linking in polyethylene and higher paraffins is discussed.

Radiolysis of water at elevated temperatures—II. Irradiation with γ-rays and fast neutrons up to 250°C

Determination of G-values of water decomposition products in neutral water formed by irradiation with γ-rays and fast neutrons at elevated temperatures up to 250°C was carried out by a combination of the NaNO 2, acetone + methanol, and HClO 4 + methanol solutions, which had been established at room temperature. From the measurement of the H 2 yields for these solutions, the G H2, G H2 + G H, and G eaq− + G H + G H2, respectively, have been determined. Consequently, the G H2, G H and G eaq− were obtained. By using a mass balance relation, the value for water decomposition, G −H2O, was calculated. The yields of H 2O 2, formaldehyde and ethyleneglycol produced as a function of dose in the NaNO 2 and HClO 4 + methanol solutions were chosen to derive the G OH and G H2O2 by fitting the simulated calculation to the experimental results. In the γ-radiolysis, the G-values obtained in the present experiment are in good agreement with recently reported ones except for the G OH and G H2O2. The G equ− for γ-radiolysis is in a good agreement with the reported values up to 200°C, but slightly lower at 250°C. The water decomposition gives a similar pattern with G eaq− yields. The G OH does not change significantly with temperature, which is contrary to the evaluation by other groups and the G H2O2 tends to increase with temperature. In the fast neutron radiolysis, although a similar change of the G-values with temperature was observed, the higher molecular products of G H2 and G H2O2 and the lower radical yields of G H and especially G eaq− were determined. It was made clear that the LET effect still remains even at elevated temperatures. Concerning the experimental results, it is suggested that the spur size would expand at higher temperatures and the fraction of the intra spur reactions decreases. Consequently, the amount of radicals diffusing out from the spur increases. To confirm this assumption, an additional measurement of H 2 yields in the 10 −3 mol · kg −1 HClO 4 solutions with different concentrations of methanol irradiated with γ-rays and fast neutrons was also made.

Stochastic analysis of the asymptotic kinetics of multi-particle spurs

Analyses are presented of the long-time asymptotic kinetics of spur reactions in the absence of scavenger, and of the asymptotic concentration dependence of the scavenging yield. The results show no dependence on the initial spatial distribution of the particles apart from that inherent in the escape yield. However, the asymptotics are shown to provide further information about the probability distribution of the numbers of particles escaping, and hence to provide an indirect probe of the initial number distribution.

Diffusion and CIDEP of H and D atoms in solid H 2O, D 2O and isotopic mixtures

Hydrogen and deuterium atoms have studied by pulsed EPR spectroscopy in polycrystalline samples of H 2O ice, D 2O ice, and various isotopic mixtures. At high temperature (−10°C) the pattern and the time-dependence of the EPR line intensities are similar to previous results for H and D in liquid water. Chemically induced dynamic electron polarization (CIDEP) is generated in second-order atom combination reactions. The CIDEP behavior was found to change over the temperature range studied (−5°C to 130°C), consistent with additional contributions from spur reactions, and at lower temperatures, geminate recombination of (D…OD) radical pairs. Transverse spin relaxation times were measured by the spin-echo technique, and interpreted in terms of translational motion of free atoms diffusing through the ice lattice. One surprising result is that D atoms diffuse faster than H atoms below 200 K. This is explained as a vibrational zero point energy effect, by applying transition state theory to a model in which the diffusing atom must pass through a tight “bottleneck” in the electronic potential surface, as it passes from one minimum energy site in the lattice to the next. The H and D spin relaxation rates were successfully simulated by means of a semiclassical potential which was constructed by pairwise addition of atom-atom contributions represented by modified Buckingham potential functions. Extension of the model to include tunneling resulted in little change to the fit of the H and D data. Although predictions of muonium diffusion rates using the same potential do not give quantitative agreement with published results from spin relaxation measurements, they do serve to illustrate the dominant effect of tunneling over a wide temperature range for that light atom.

Radiolysis of liquids with high static dielectric constant: An estimate of the total ionization yield, electron thermalization distance, and contribution of heterogeneous reactions

In a previous study, we found an exponential dependence of the free-ion yield (Gfi) on the static dielectric constant (εs) for a number of irradiated liquids with εs>10. On the basis of this study, we develop here a simple model by which we quantitatively estimate the total ionization yield (Gtot), the most probable electron thermalization distance (b), and the yield of solvated electrons that are removed by diffusion-controlled reactions during spur expansion (Gdif). Using solvated electron yields available in the literature, we get Gtot≂6.6 mol/100 eV (value nearly independent of the nature of the liquid) and b≂29 Å at 298 K. b is found not to depend appreciably on εs which indicates that the Coulomb attractive force between the ion and secondary electron is practically inefficient before electron thermalization occurs. The evaluation of Gdif teaches us that spur reactions have a profound influence in the fate of ion pairs formed during radiolysis of liquids of high εs values.

Muon and muonium chemistry: Numerical calculation of residual polarisation

A method for numerical calculation of the residual polarisation of diamagnetic muons and muonic radicals formed by reactions of muonium is described; it has the advantage that complex kinetic situations can be treated exactly. It is applied to fast reactions in solution for which the rate constant is time dependent and to competition between homogeneous and spur reactions.

Stochastic models of diffusion-controlled ionic reactions in radiation-induced spurs. 1. High-permittivity solvents

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTStochastic models of diffusion-controlled ionic reactions in radiation-induced spurs. 1. High-permittivity solventsPeter. Clifford, Nicholas J. B. Green, Michael J. Pilling, and Simon M. PimblottCite this: J. Phys. Chem. 1987, 91, 16, 4417–4422Publication Date (Print):July 1, 1987Publication History Published online1 May 2002Published inissue 1 July 1987https://doi.org/10.1021/j100300a043RIGHTS & PERMISSIONSArticle Views142Altmetric-Citations42LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (678 KB) Get e-Alerts

The use of positron lifetime measurements in a study of electron scavenging by a range of radiosensitizer compounds

Abstract Positron lifetime measurements were made in aqueous solutions of a number of radiosensitizer compounds in order to measure the o -Ps inhibition and quenching constants. By making measurements in the presence and in the absence of perchloric acid, the inhibiting effects on the o -Ps yield due to epithermal and trapped electrons could be determined on the basis of the spur model. Inhibition constants relating to the scavenging by the sensitizer molecule of epithermal electrons. ( K 1 ) and traped electrons, ( K 2 ) were found for eight basic nitroimidazoles. The results are also discussed in terms of electron trap depth which is here taken to be equivalent to the one-electron redox potential, E 1 7 . It is concluded that o -Ps lifetime measurements alone are not capable of determining the finer details of these spur reactions. Estimates of the respective yields of RS - and OH . species in oxygen and R—NO 2 compounds show that these are not able to explain the large differences in sensitizing efficiency.

Yields of Solvated Electrons at 30 Picoseconds in Water and Alcohols

Abstract Direct measurements of the yield of hydrated electrons in aqueous solutions and solvated electrons in alcohols were carried out using a stroboscopic picosecond pulse radiolysis system. The yield of solvated electrons was obtained by simultaneous consideration of the pulse shape and radiation doses. The yield at 30 ps after irradiation, G(eaq−)30ps, was determined to be 4.8±0.3 products per 100 eV as compared to the G(eaq−)100ns of 2.7. Experimental values, including the present ones, are higher by 0.3 than the theoretical ones calculated using the spur diffusion model in the time period from 3×10−11 to 10−8 s. Nevertheless, both results show a quite similar time dependence, i.e., fast decay of hydrated electrons in spur reactions. The G-values of solvated electrons at 30 ps were also determined in alcohols. The G-value of solvated electron is 3.4±0.3 in both methanol and ethanol, 2.0±0.3 in 1-propanol and 2.3±0.3 in ethylene glycol.

Muon spin relaxation and positron annihilation in cyclohexane related to spur structures in radiation chemistry

Recent developments in the study of fast processes in radiation chemistry in our laboratory, the pico-second pulse radiolysis, single photon counting technique etc., are briefly mentioned with special emphasis on the study of hydrocarbon liquids. A very close correlation between positronium formation and spur reactions in radiation chemistry is also demonstrated with our recent study of the electron scavenger effects on Ps formation in cyclohexane as an example. Several problems in muonium chemistry are pointed out in the light of these investigations.

Spin correlation effects in radiolysis

Magnetic field and magnetic isotope effects demonstrate the importance of spin correlation in geminate ion recombination in hydrocarbons. The role of these studies in identifying mechanisms and their extension to low temperatures are discussed. Calculations are presented which show that field effects may be observed by pulse radiolysis at times less than one nanosecond. In contrast, correlation effects on reactions of neutral radicals are speculative at present. The relatively low rate constant of hydrogen atom recombination may be due to spin limitations: if so, re-encounters should lead to field-dependent changes in the relative yields of ortho- and pyra-hydrogen from the non-geminate reaction. Reactions in spurs should lead to further effects but these are likely to be small.

Radiation chemistry of high-temperature (300–410 °C) water. Part 1.—Reducing products from gamma radiolysis

Using gamma rays, and dilute solutions of radical scavengers, the G values of the primary reducing products from water, namely H2, H and e–aq, were found to be 2.0, 0.3 and 0.4 at 400 °C, density 0.45 kg dm–3, in marked contrast to the G values 0.4, 0.6 and 2.7, respectively, for liquid water at 25 °C. At 300 °C, density 0.75 kg dm–3, GH2 was similar to the value at 400 °C.The high G value for hydrogen at 400 °C, coupled with low values for GH and Ge–aq suggests that, in contrast to reactions at room temperature, only a small fraction of the hydrogen is formed by the reaction together of the species H and e–aq in spur reactions. The molecular hydrogen is thought to be formed together with oxygen atoms mainly in unimolecular decomposition processes, e.g. H2O*= H2+ O.The low dielectric constant of 8 at 400 °C compared with 80 at 25 °C is considered to be an important factor in determining the low G value for scavengeable electrons.The low overall G value for water molecules ultimately decomposed at 400 °C and density 0.45 kg dm–3 suggests a greater efficiency for the deactivation of excited molecules at that condition compared with liquid state at 25 °C.A reaction mechanism is proposed which accounts for the observed product against dose curves when computer simulation of these is carried out.

Positron spur reactions with excess electrons and anions in liquid organic mixtures of electron acceptors

By means of the positron lifetime technique we have measured positronium (Ps) yields in mixtures of nonpolar liquids with various electron scavengers which bind the electron fairly weakly (1–2 eV) in stable anions. The results are discussed with reference to recent excess electron works, and new experiments on anions and excess electrons are proposed. The minimum of the Ps yield versus CS2 concentration curves caused by partly delocalization of electrons on several scavenger molecules, which was observed previously in saturated aliphatic hydrocarbons occurred also in the saturated cyclic hydrocarbon cyclohexane, but did not appear in the aromatic benzene. This might be explained by the weak electron acceptor property of aromatics. In the Ps yield versus SF6 concentration curve in hexane a similar minimum appeared as in the CS2 case, probably by the same reason. By adding 0.8 M CS2 to the system the minimum was shifted towards the lower concentrations, which might be explained by some overlap of the energy levels on CS2 and SF6. Antiinhibition effect of C6F6 and CS2 was studied in 0.05 M CCl4/hexane solutions. In the CS2 case first a small minimum appeared in the Ps yield versus CS2 concentration curve. Alcohol clusters did not show antiinhibition in 0.05 M CCl4/neopentane system. Weak electron scavengers (C6F6, naphthalene, biphenyl, benzene) which generally act as antiinhibitors were added to pure non-polar solvents (hexane, isooctane, cyclohexane) and caused Ps enhancement. This can be attributed to “antirecombination” effect with a similar mechanism as that of the antiinhibition, but some contribution from positive ion scavenging cannot be excluded either. The increase of the Ps yield in hexane as a function of C6F6 or naphthalene concentration was much steeper if 0.8 M CS2 was added to the system, indicating that antirecombination and antiinhibition took place simultaneously. On addition of hexane, isooctane, CS2 or 0.8 M CS2/hexane to pure C6F6 the Ps yield drastically decreased. This correlates well with the drastic decrease of the electron mobility by adding similar additives to C6F6 observed recently.