Effect Of Electron Scavengers Research Articles

Radiation-induced excitation of luminescent organic solutes in aqueous media

Studies of the radioluminescence of various aromatic substances as well as of purines and pyrimidines are reviewed. Most experiments were on polar solutions and glasses between 77 and 300 K. The luminescence was studied during X-irradiation and subsequent to it. Thus, both fluorescence and phosphorescence were observed together with a slow “afterglow” and thermoluminescence. The radioluminescence of these systems arose almost exclusively from the solute. Luminescence caused by solvent to solute energy transfer was negligible. However, in molecules containing aliphatic groups some of the excitation of these groups appeared to be transferred to the triplet manifold of the chromophore. The results were compared with corresponding data obtained upon exposure with non-ionizing U.V. light under identical conditions. The emission spectra and the phosphorescence decay kinetics demonstrated that X-irradiation produces the same radiative levels as does U.V. exposure. At low temperatures the phosphorescence to fluorescence ratio was considerably enhanced with the ionizing radiation, the main reason apparently being that a large fraction of the radioluminescence at low temperatures is due to excitation resulting from geminate recombination between solute cations and electrons. On the basis of the temperature dependence of the radioluminescence of indole dissolved in ethylene glycol-water mixtures, as well as the effect of electron scavengers on this luminescence, it was estimated that at 77 K 80–85 per cent of the solute luminescence is due to recombination, the remainder being excitation directly from the ground state. The triplet to singlet ratio (T/S) for the recombination process below 150 K was 1·2, whereas for the direct excitation T S = 0·2 . The lifetime of the ion pair prior to recombination was between 2·5 × 10 −8 and 5 × 10 −5s, indicating that the electrons recombine from shallow traps or from charge transfer to solvent-like states rather than from the “free” state. Excitation by recombination fell rapidly above 150 K so that in liquid solution above 240 K direct excitation was the only mode of excitation.

Mechanism of High-energy-induced Luminescence of Solid 2,3-Dimethylbutane Containing Toluene at 77 K

Abstract The luminescence from 2,3-dimethylbutane (23DMB) containing a small amount of toluene (To) has been observed in the solid phase at 77 K during γ-irradiation or by ns pulse irradiation with X-rays. The emission spectrum from 23DMB–To (2 mol%) during γ-irradiation at 77 K consists of the fluorescence from the singletexcited toluene and the phosphorescence from the triplet-excited toluene. Both the singlet- and triplet-excited toluenes are formed by the energy transfer from the irradiated 23DMB to toluene. When nitrous oxide or propylene is added to the 23DMB–To mixture, the emission spectrum is changed and becomes very similar to that from the 23DMB–To mixture during illumination with UV lights at 77 K. The spectrum, which disappears upon the addition of nitrous oxide or propylene, consists mainly of phosphorescence. It is concluded that the triplet-excited toluene may be formed by the migration of a hole and an electron. The effects of electron scavengers, such as nitrous oxide and carbon dioxide, or hole scavengers, such as propylene or tetramethylethylene, on the fluorescence have been studied by nsec-pulse irradiation. Since the fluorescence is not appreciably affected by the addition of these scavengers, the singlet-excited toluene may be formed by the excitation transfer.

Effect of Electron Scavenger on the γ-Radiolysis of Gaseous Propane at High Densities

Hydrogen formation from γ-radiolysis of gaseous propane at 120 ± 1 °C was studied as the function of density in the range from 0.1 to 0.5 g/ml. G(H2) gradually decreased with density from 8.0 at 0.12 g/ml towards the liquid-phase value of 6.4 at room temperature. G(H2) in the presence of SF6 showed similar decrease from 3.9 to 3.0. The decrease was discussed in comparison to the abrupt drop in G-values from the radiolysis of ammonia at high density (11). The separation distances of ion–electron pairs were estimated from the analysis of the electron scavenging reaction.

Energy Transfer in the γ-Irradiated Benzene Solutions of Carbonyl Sulfide, Nitrous Oxide, and Carbon Dioxide

Abstract The γ-radiolysis of benzene solutions containing carbonyl sulfide, nitrous oxide, and carbon dioxide has been investigated at room temperature and at 0°C. Carbonyl sulfide was shown to be an effective electron scavenger and to give carbon monoxide as the product. By comparing the solute concentration dependences of G(N2) and G(CO), it was concluded that the collision of secondary electrons with benzene does not efficiently produce the first excited singlet state, the G-value of which is less than 0.2 if it is produced at all. The concentration dependences of the G(CO) and G(phenol) from the benzene solution of carbon dioxide also suggested that the superexcited state or the highly-excited state of benzene is too short-lived to transfer its energy to the solute.

Concentrated electron scavenger effects on the yields of trapped species in .gamma.-irradiated alkaline glass

Concentrated electron scavenger effects on the yields of trapped species in .gamma.-irradiated alkaline glass

The radiation-induced formation of excited states of aromatic hydrocarbons in cyclohexane IV. Effect of electron scavengers

The effects of the electron scavengers nitrous oxide, nitromethane, carbon dioxide, sulphur hexafluoride and chloracetic acid on the yields of naphthalene excited states observed in pulse radiolysis of deaerated naphthalene-cyclohexane solutions have been investigated. The excited-state yields were reduced by all the solutes examined although they showed marked differences in their effects on the formation of singlet and triplet excited naphthalene. It is proposed that in the absence of a second solute naphthalene excited states are produced as a result of electron and positive charge scavenging by naphthalene followed by the ion-recombination reaction Naph + + Naph - -> b s (or b ' s ) 1 Naph* + b t (or b ' s ) 3 Naph*, where b s and b t are the number of singlet and triplet excited states of naphthalene formed initially per geminate ion-pair recombination and b ' s and b t are the corresponding efficiencies per free ion-pair recombination. The concentration dependences of the singlet yield G( 1 Naph)*, the triplet yield formed directly on ion recombination, G( 3 Naph*) r , and the total triplet yield, G( 3 Naph*) = G( 3 Naph*) r + ^ t G( 1 Naph*) reported in parts II and III of this series can be fitted to the expressions G( 1 Naph*) = b ' s G gi {α1/2_ [Naph]1/2 /1+α1/2_[Naph]1/2}{α1/2_ [Naph]1/2 /1+α1/2_[Naph]1/2}, G( 3 Naph*) = B ' t G fi + B t G gi >{α1/2_ [Naph]1/2 /1+α1/2_[Naph]1/2}{α1/2_ [Naph]1/2 /1+α1/2_[Naph]1/2}, using the following parameters: G fi = 0.15, G gi = 3.9, a_ = 3.291/mol, a + = 50001/mol, b 8 = 0.863, b ' s = 1.22, B t = b t + (fi t b s = 1.34 and B ' t = b ' t + (j) t b ' s = 1.89. Of the various competing electron scavengers investigated only the results of the nitromethane system at [CH 3 NO 2 ] > 10 -2 mol/l could be explained solely in terms of a simple competition for free and geminate ions by the two solutes. However, it was found that the nitrous oxide and carbon dioxide systems could be rationalized on a similar model by taking account of possible secondary ion reactions and/or excited state formation as a result of ion recombination between Naph + and the negative ion of the competing scavenger. The secondary process occurring in the sulphur hexafluoride and chloracetic systems are more complex and an empirical fit of the data was not possible.

The Effect of Electron Scavengers on Photo-produced Electrons in Ethyleneglycol/Water Glass.

The Effect of Electron Scavengers on Photo-produced Electrons in Ethyleneglycol/Water Glass.

Trapped Electrons in Rigid Polar solution. A Study of Recombination Luminescence

AbstractThe afterglow (A) and thermoluminescence (TL) of tryptophan in ethyleneglycol/water‐glass irradiated with 250 nm UV‐light at 77°K have been studied. Both A and TL are attributed to recombination between tryptophan ions and trapped electrons produced by ionization of tryptophan by triplet absorption. The excitation energy of the molecule formed by the recombination is between 3.0 and 4.0 eV. The intensity I of A decays according to I = const t−1.09. The glow curve has two peaks: TL1 at 90°K and TL2 at 135°K. TL2 is associated with a glass transition of the solvent and occurs in the same temperature region as the optical absorption from the trapped electron disappears.The effect of electron scavengers on the thermoluminescence shows that the glow curve may be interpreted in terms of the distance be the trapped electron and the positive ion. A and TL1 is associated with electrons that are trapped fairly close to their mother ions whose recombination is guided by the Coulomb force. The electrons associated with TL2 are trapped at larger distances but probably so large as the mean trapping distance. As judged from scavenging experiments a lower limit of the mean trapping distance is 20 Å. On their way back to the mother ions the electrons do not behave like “thermal” but rather like solvated ones with respect to reactivity with H. The yield of trapped electrons increases and the electrons also seem to be trapped closer to their mother ions when the irradiation temperature increases. This may be caused by increased mobility of the molecular dipoles or OH‐groups and/or by the higher thermal energy of the electrons prior to trapping.

Ionic Processes in the Radiolysis of Nitrous Oxide. The Effect of Electron Scavenger and Rare Gas Setisitization

Abstract The effect of electron scavenger (SF6) on the radiolysis products of N2O was examined. The sensitization of N2O by various rare gases was also measured. In the presence of an electron scavenger we got 3.0 for the value of −ΔG(−N2O) in good agreement with the value of G(e−) for N2O. In the experiments of Xe or Kr containing small amounts of N2O, the G(−N2O) values were nearly equal to the respective G(e−) values for the rare gases, and the addition of SF6to the Xe-N2O system almost completely inhibited nitrogen formation. The results show that one electron decomposes just one molecule of N2O. This fact can not be accounted for by the hitherto assumed mechanism, and a new mechanism involving N2O− is proposed for ionic processes in the radiolysis of N2O. In the N2O-rare gas mixtures of various compositions, the sensitized yields of N2O decomposition per 100 eV energy absorbed by rare gases were divided into two groups, e.g., the yields from He, Ne mixtures were about twice as large as those from Ar, Kr and Xe mixtures. This result is discussed in terms of charge transfer and the Penning ionization.

The effect of electron scavengers on the γ-radiolysis of gaseous H2S

The effect of the electron scavengers SF6 and N2O on the γ-radiolysis of gaseous H2S has been studied. Neither scavenger lowered the radiation yield of H2, G(H2). With added N2O, nitrogen was produced, though not in the amount Ge, the ion pair yield in H2S. Possible explanations of the findings are discussed.

Radiolysis of Methylcyclohexane. II. The Effect of Cumene on Hydrogen Formation from γ-Irradiated Methylcyclohexane

Abstract The effect of cumene on the hydrogen yields from methylcyclohexane γ-irradiated at 100, 20, −72, −120 and −196°C has been investigated. Cumene did not compete with N2O for capturing electrons in methylcyclohexane and the yield of thermal H atoms in methylcyclohexane, determined by use of ethylene as an H atom scavenger, was smaller than the decrease in hydrogen yield in methylcyclohexane-cumene systems. Therefore, the decrease in hydrogen yield in methylcyclo-hexane-cumene systems can be ascribed mainly to positive charge transfer from methylcyclohexane to cumene. Further, the evidence of charge transfer from methylcyclohexane to cumene was found by the optical absorption method. The effect of cumene on the hydrogen yield from methylcyclohexane was similar in the temperature range from 100 to −196°C, especially at high cumene concentration. The effect of an electron scavenger at high concentrations was also examined with n-butyl chloride. In the present work, we have found that although the effect of a positive ion scavenger on the hydrogen yield from methylcyclohexane in the liquid state differs little from that in the glassy state, the effect of electron scavengers on the former differs considerably from that on the latter.

Effect of Electron Scavengers on the Positive Ion Reactions in the Radiolysis of Cyclopropane Solutions

Electron scavengers such as CCl4, SF6, N2O and CH3Br are found to increase the yield of the secondary positive ion reactions which occur in the radiolysis of cyclopropane solutions in cyclohexane. The observed increase can be understood in terms of an increase in the lifetime of the ion pairs which results when the electron initially formed is converted to a less mobile negative ion. The quantitative aspects can be treated in terms of the change in the mutual diffusion coefficient of the ion pair if one includes in the model the concentration dependence for ion scavenging found when only one solute is present. In the present study electron capture by CCl4 is found to result in a decrease in the diffusion coefficient of the ions by a factor of 17. From this and available experimental information on the diffusion coefficients of positive and massive negative ions the diffusion coefficient of the negative entity initially produced by the radiation in cyclohexane is estimated as 2.6 × 10−4 cm2sec−1. This diffusion coefficient is considerably higher than those for the massive negative ions so that there seems to be little question but that the electron is involved in a more or less free state. The absolute rate constant for the electron scavenging process is estimated to be of the order of magnitude of 1011M−1·sec−1 and that for the positive ion reactions of cyclopropane a factor of 30 smaller.

Effects of electron scavengers in the radiolysis of water vapor

Water vapor irradiated with X-rays in the presence of nitrous oxide gives mainly nitrogen and oxygen together with small amounts of hydrogen and, possibly, hydrogen peroxide. The yield of nitrogen, G(N2) = 3.0 ± 0.3, is constant over a wide range of conditions and G(O2) = 1.6 ± 0.3 after an induction period. The hydrogen yield is constant at low doses with G(H2) = 0.45 ± 0.1, but reaches a steady state at higher doses. The nitrogen yield is equated with the yield of scavengeable electrons in water vapor, the value g(e) = 3.0 ± 0.3 being in reasonable agreement with the theoretical value based on W(H2O) = 30 eV. Addition of other electron scavengers to water vapor containing nitrous oxide suppresses the nitrogen yield by competing efficiently for electrons. On the basis of their efficiency in suppressing the nitrogen yield, limiting values for the relative rates of reaction of N2O, SF6, CCl4, and HCl with electrons in water vapor are obtained and comparisons are made with their known electron attachment cross sections in the gas phase.

Studies on radiation‐induced ionic polymerization. Part VII. Effect of electron scavengers on polymerization of styrene

Studies on radiation‐induced ionic polymerization. Part VII. Effect of electron scavengers on polymerization of styrene

The Use of Nitrous Oxide in Elucidating Mechanisms of Radiosensitization in Chemical and Biological Systems

Radiosensitizing effects of electron scavengers in aqueous solutions of aromatic compounds and of certain alkylating agents and organic nitroxide free radicals in suspensions of erythrocytes and bacteria are discussed. In neutral aqueous solutions of aromatic compounds the radiosensitizers exert their effect through: 1) interference with back reactions involving e_aq and H, 2) interaction with solute radicals, and 3) conversion of e−sq, and H into oxidizing radicals.For biological systems (erythrocytes and bacteria) radiosensitizing mechanisms of certain alkylating agents (iodoacetic acid and N-ethylmaleimide) and organic nitroxide free radicals are discussed in light of their ability to interact with e_sq, OH and vital biological targets.It is shown that N2O is a useful tool in the elucidation of radiosensitizing mechanisms at the cellular and molecular level.

The Use of Nitrous Oxide in Elucidating Mechanisms of Radiosensitization in Chemical and Biological Systems

Radiosensitizing effects of electron scavengers in aqueous solutions of aromatic compounds and of certain alkylating agents and organic nitroxide free radicals in suspensions of erythrocytes and bacteria are discussed. In neutral aqueous solutions of aromatic compounds the radiosensitizers exert their effect through: 1) interference with back reactions involving e_aq and H, 2) interaction with solute radicals, and 3) conversion of e−sq, and H into oxidizing radicals.For biological systems (erythrocytes and bacteria) radiosensitizing mechanisms of certain alkylating agents (iodoacetic acid and N-ethylmaleimide) and organic nitroxide free radicals are discussed in light of their ability to interact with e_sq, OH and vital biological targets.It is shown that N2O is a useful tool in the elucidation of radiosensitizing mechanisms at the cellular and molecular level.

Effect of electron scavengers on the formation of hydrogen in the radiolysis of propane

Effect of electron scavengers on the formation of hydrogen in the radiolysis of propane