Γ-irradiated 3-methylpentane Research Articles

Photoconductivity induced by excitation of geminate trapped electrons in hydrocarbons glasses

Experimental data on photoconductivity in γ-irradiated 3-methylpentane glass are considered for the case when most radiation produced trapped electron forms geminate pairs with their parent positive ions. It is assumed that mobile electrons generated by photoexcitation of trapped electrons have an excess energy and there is a thermalized mobile precursor of stable trapped electrons which is governed by Smoluchowski's equation. The model allows to describe experimental data on photocurrent kinetics. It predicts that for germinate trapped electrons, apart from “free” or not germinate electrons, photocurrent dependes on the thermalization ranges of photoliberated electrons even if nonthermalized electrons are not affected by electric field.

Low energy electron localization in hydrocarbon glasses. Method of photoassisted ion pair separation

Experimental results on increase of the probability of geminate ion pair separation into free ions caused by photoexcitation of trapped electrons in γ-irradiated 3-methylpentane glasses at 77 K are discussed. The increase is associated with injection of “hot” electrons in the medium. After thermalization the motion of the electrons is suggested to be governed by Smoluchowski's equation until stable trapped electron states occur. The dependence of the mean-square thermalization length on the electron energy in the interval 0.1–1.5 eV and the mean-square length (45 Å) of thermalized precursor of stable trapped electrons are estimated. It is concluded that electron localization precedes to total thermalization and is effective at electron energies <0.1 eV.

The isothermal luminescence of γ-irradiated 3-methylpentane glass at 77 K

The isothermal luminescence (ITL) at 77 K of pure 3-methylpentane was investigated. The influence of dose on ITL intensity, spectral distribution and decay was examined. The effect of pre-irradiation dose on ITL was found. The results were explained assuming the recombination of e - t -cation (in the low dose range) and anion-cation (in the high dose range, ⪞ 10 23 eV dm -3). The recombining anions and cations are formed as a result of charge transfer from primary ions to the radicals as well as to stable radiolysis products.

A photoconductivity study of trapped electrons in γ-irradiated hydrocarbon glasses at 77 K

Photoconductivity spectra of trapped electrons in γ-irradiated 3-methylpentane, 2-methylpentane, 3-methylheptane, and 3-methylpentane–methylcyclohexane (1:9 volume ratio) glasses at 77 K were measured. From the comparison of photoconductivity and optical absorption spectra in these systems we have obtained relative values of quantum efficiencies for a trapped electron in the ground state to be excited to the conduction band by photo-induced transition. For electrons stably trapped, we have measured the ranges of trap depths from the shallowest to the deepest in these systems.

Isothermal luminescence of γ-irradiated 3-methylpentane containing two solutes at 77 K

A 3-methylpentane glass containing a pair of solutes was γ-irradiated at 77 K and the isothermal luminescence was measured. Two pairs of solutes were examined: naphthalene-9,10-diphenylanthracene and toluene-naphthalene. For constant concentration of left-hand solutes ( S1) the concentration of right-hand solutes ( S2) was changed. When the S2 concentration increases, the luminescence intensities measured at the emission bands of S2 increase. The results can be explained in terms of a tunnelling charge transfer model and a competitive charge capture scheme.

The mechanism of isothermal luminescence of γ-irradiated hydrocarbon glasses in the presence of naphthalene at 77 K

The isothermal luminescence of γ-irradiated 3-methylpentane and methylcyclohexane glasses in the presence of naphthalene (10 −4−10 −1 mol dm −3) was investigated at 77 K. The isothermal luminescence is explained by the tunnelling mechanism involving electrons and cations in the range of low solute concentrations ( ca. ≤7 × 10 −3 mol dm −3) and solute anions and cations in the range of high solute concentrations (up to 10 −1 mol dm −3). Evidence is presented that the β-coefficient in Bagdasar'yan equation is related to the distribution of recombining ion pairs.

Photostimulated reactions of isolated and paired radicals in solid hydrocarbons

Ultraviolet radiation absorbed by radicals trapped in γ-irradiated 3-methylpentane (3MP), 3-ethylpentane (3EP), and 3-methylheptane (3MHp) glasses and in polycrystalline n-hexane at 77 K causes a major improvement in resolution of the ESR spectra without change in the radical concentration, indicating that the energy of the absorbed photons changes the nature, the configuration, or the environment of the radicals. The new spectral shape slowly reverts to the old in the dark at a rate which is dependent on the matrix and the temperature. Ultraviolet irradiation also accelerates the decay of paired radicals and other radicals which are removed by intraspur processes, but not of those which react by random encounter after diffusion. This effect is attributed to localized heating of the matrix in the immediate vicinity of a radical which has absorbed a photon. Secondary 3-ethylpentyl radicals in glassy 3EP undergo spontaneous conversion to an isomer or change to a new rotamer configuration in the dark and the process is reversed by exposure to uv light. The ΔMs=2 ESR spectrum of radical pairs in γ-irradiated 3MHp has been observed.

Heats of reaction of trapped intermediates in γ-irradiated organic glasses and relaxation processes in unirradiated glasses measured by low temperature differential thermal analysis

Differential thermal analysis (DTA) has been used to determine the following: (1) the heat of photobleaching of trapped electrons in γ-irradiated 3-methylpentane (3MP) glass as a function of γ dose; (2) the heats of combination of radicals and of ions in γ-irradiated 3MP and methyltetrahydrofuran (MTHF) glasses; (3) the glass transition temperatures and the rates of enthalpy loss during 77 K annealing of 3MP-h14, 3MP-d14, 3-ethylpentane, and MTHF; (4) the effect of cooling rate and annealing time on crystallization of n-hexane in 3MP cooled to 77 K. The ΔH of neutralization of photobleached electrons in γ-irradiated 3MP at 72 K is ∼−150 kcal mole−1 at low dose, implying an upper limit of 80 kcal mole−1 for the solvation energy of the combining charges, and decreases to ∼80 kcal mole−1 for a dose of 1.2×1020 eV g−1 implying an increasing ratio of reaction with radicals to reaction with cations as the dose is increased. When γ-irradiated MTHF is warmed from 77 K, a large fraction of the trapped electrons appear to react with radicals to form carbanions, which then react with cations, the total ΔH being ∼80 kcal mole−1. The ΔH of ion pair solvation is estimated to be −120 to −150 kcal mole−1. The decrease in the enthalpy of the matrix due to relaxation during annealing in 3MP at 77 K parallels the decrease in the decay rate of trapped electrons for similar samples. Seemingly anomolous effects of sample size and shape on trapped electron decay rates in 3MP glass as a function of time of annealing at 77 K can now be rationalized in terms of the restrictions on viscous flow in different types of sample tubes.

Electrical conductivity as a source of information on charge carriers in low molecular weight organic solids

By determination of the temperature thresholds for current flow in hydrocarbon glasses γ-irradiated at 77 K in an electrical conductivity cell, different populations of trapped charge carriers have been distinguished and information has been obtained on their orientation and migration under the influence of the applied field. Characterization has been assisted by the use of charge scavengers, photobleaching and blocking electrodes. The recent data suggest that the warmup conductivity current peak at 85 K from γ-irradiated 3-methylpentane, previously assigned to electron movement, results from the migration of C 6H 13 - carbanions and monomeric cations, while that at 102 K is due to carbanions and dimeric cations. 3-Ethylpentane and methylcyclohexane give qualitatively similar curves. In all cases the fraction of the total trapped charge which reaches the electrodes is small. The warmup conductivity curves of unirradiated hydrocarbon glasses and of methyltetrahydrofuran show current peaks attributable to orientation of their molecular dipoles. These can be avoided if the electrical field is applied before cooling.

Behavior of Positive Hole in a γ-Irradiated 3-Methylpentane Glass at 77°K

Abstract In order to elucidate the behavior of a positive hole in irradiated alkane glasses, the effect of solutes having ionization potential lower than that of a glass matrix molecule upon the decay of a trapped electron was examined in a 3-methylpentane glass at 77°K by means of electron spin resonance. The decay was found to be explained in terms of the superposition of two exponentially decaying processes, as in the absence of a solute. Although the fast process was not affected by a solute, the slow process was further slowed by the addition of a solute. The effect of a solute is described by the empirical relation, k⁄k0=1⁄(1+fN), where k and k0 are the rate constants of the slow decay in the presence and in the absence of a solute, respectively. N is the mole fraction of a solute. The observed values of the slowing-down efficiency, f, increase in parallel with the decreasing values of ionization potential of solutes: 2-methyltetrahydrofuran, diethylether, methylvinylether, 2-methylpentene-1 and n-butylvinylether. The results are interpreted in terms of trapping of a positive hole by a solute molecule and positive hole tunneling from a hole trap to a trapped electron.

Recombination Luminescence in γ-Irradiated 3-Methylpentane Glass at 77°K

Thermoluminescence from γ-irradiated 3-methylpentane at 77°K has been studied. Two kinds of emission spectra are observed in the visible region according to temperature. The one has its maximum intensity at 425 nm and is observed in the temperature range of 77°∼83°K (Spectrum I), the other its maximum at 530 nm, appearing at 83°∼90°K (Spectrum II). The effects brought upon the integrated luminescence intensity by differences in radiation dose, by IR and UV bleaching, by the addition of electron or hole scavengers, and by standing in dark were examined, and the results are compared with those obtained from optical absorption in the IR and UV regions under various experimental conditions. The results suggest that the spectrum I is caused by the recombination of positive holes with electrons released by the warming, while the spectrum II would appear attributable the recombination of positive holes with some anionic species produced by the reaction of electrons with radicals.

Recombination Luminescence in γ-Irradiated 3-Methylpentane Glass at 77°K

Thermoluminescence from γ-irradiated 3-methylpentane at 77°K has been studied. Two kinds of emission spectra are observed in the visible region according to temperature. The one has its maximum intensity at 425 nm and is observed in the temperature range of 77°∼83°K (Spectrum I), the other its maximum at 530 nm, appearing at 83°∼90°K (Spectrum II). The effects brought upon the integrated luminescence intensity by differences in radiation dose, by IR and UV bleaching, by the addition of electron or hole scavengers, and by standing in dark were examined, and the results are compared with those obtained from optical absorption in the IR and UV regions under various experimental conditions. The results suggest that the spectrum I is caused by the recombination of positive holes with electrons released by the warming, while the spectrum II would appear attributable the recombination of positive holes with some anionic species produced by the reaction of electrons with radicals.

Fluorescence and Phosphorescence Produced by Electron–Ion Recombination in 3-Methylpentane at 77°K

Spontaneous luminescence and photoinduced luminescence from γ-irradiated 3-methylpentane (3MP) at 77°K have been investigated. In addition to the previously reported band at 415 nm, a second band is observed in the emission spectrum at 230 nm and is assigned to 3 MP fluorescence. The presence of the latter band confirms the existence of 3MP+ ion centers in neat 3MP. Measurements of the decay of recombination luminescence induced by a brief pulse of light indicate lifetimes of ≤ 2 msec for the 230-nm band and 4.4 sec for the 415-nm band. The decay of the fluorescence sets an upper limit on the rate of the electron–positive hole recombination in 3MP and establishes that the long decay time of the 415-nm band is intrinsic to the emission process itself. This phosphorescence is assigned to emission from the lowest triplet state of 3 MP

Coincidence of times for thermal decay of trapped electrons in γ-irradiated 3-methylpentane at 77°K by electron spin resonance and optical observations

Agreement has been obtained between ESR and optical measurements on the decay of trapped electrons in identical samples of γ-irradiated 3-methylpentane at 77°K.

Viscosity-Dependent Ion Recombination Luminescence in Organic Liquids and Solids from Electron and Gamma Irradiation

Recombination luminescence of triphenylamine (TPA), both fluorescence and phosphorescence, has been used to investigate ionic processes in (a) γ-irradiated 3-methylpentane (3MP) at 77°K, (b) thermoluminescence (TL) in γ-irradiated 3MP, and (c) in 1-MeV electron-pulse-irradiated paraffin oil and squalane from 215°–298°K. The shape of the time-dependent decay of luminescent intensity, I(t), in (a) was independent of a 500-fold change in irradiation dose, providing evidence for substantially complete correlation of geminate charge pairs. Photoionization of TPA under otherwise similar conditions led to the same I(t), which provides evidence that the distribution, P(R), of charge separation produced by γ irradiation is determined by very low-energy electrons. TL curves in part (b) showed two well-defined maxima of intensity I′ attributed to recombination of TPA+− e− at 83°K and I″ attributed to TPA+− TPA− at ∼95°K. The ratio I″ / I′ increased with the concentration of TPA, I′ decreased with ir bleaching of solvent-trapped e− or addition of CO2 while both I′ and I″ decreased with addition of methyltetrahydrofuran, a hole trap. In part (c) intensity I was measured from 10−6–10−3 sec as appropriate at various temperatures and solvent viscosities which ranged from 1– ∼ 103 P. Excepting Cerenkov radiation, the luminescence spectrum coincides with TPA fluorescence, the phosphorescence intensity being negligible since its half-life is ∼ 2 sec. Linear plots of lnI vs t can be resolved into two exponential decays, with t1/2 of the slow component proportional to (viscosity)1/2, and attributed to recombination of TPA+ with TPA−. The faster component of I is attributed to recombination of TPA+ with e−, but t1/2 was too short for reliable measurement over most of the range. The dependence t1/2 ∝ η1/2 demonstrates the inapplicability of Stokes' law (which requires t1/2 ∝ R3) to recombination of isolated ion pairs, the high field reducing the randomness of relative ion displacements R. If the probability per ion jump mean free path λ against the central field at R is P = exp(− e2λ / εR2kT), the net displacement for dN jumps is dR = [1 − exp(− e2λ / εR2kT)]λdN. This leads to t1/2 ∝ Rm with 1 ≤ m ≤ 3. This relation combined with dn± / dt = (dn± / dR) (dR / dt) gives the ion pair distribution dn± / dR. From this expression and an assumed dependence Rn ∝ N for free charge one obtains the free-charge distribution function.

ESR Evidence on Trapped Electrons and ``CO2−'' in γ-Irradiated 3-Methylpentane

The singlet ESR signal of γ-irradiated 3-methylpentane (3-MP) glass [K. Tsuji, H. Yoshida, and K. Hayashi, J. Chem. Phys. 46, 810 (1967)], which is attributable to physically trapped electrons, and the singlet and doublet resulting from reaction of electrons with CO2 [P. M. Johnson and A. C. Albrecht, J. Chem. Phys. 44, 1845 (1966)] in 3-MP glass have been examined further. Immediately after production by a 1019-eV g−1, 5-min γ dose, the electron line decays with an apparent half-life of less than 20 min. It is readily removed by illumination at 13 500 Å. ESR saturation is significant at 10−3 mW and prevents detection of the signal at powers above ∼1 mW. The ESR g value and the radiation G value are estimated to be ∼2.002 and 0.5±0.2, respectively. When 3-MP or 2-methyl-pentene-1 (2-MP-1) containing CO2 is γ irradiated at 77°K, a singlet (g=∼2.0007; linewidth=∼3.1 G) and a flanking doublet with a splitting of ∼8 G are produced. The singlet, but not the doublet, shows saturation effects starting at ∼0.05 mW. It may be removed by illumination of the sample at wavelengths of 4800 Å and below, producing new species, but is unaffected at 5500 Å and above. The initial ratio of the singlet to the doublet, the growth on standing at 77°K, and the rate of bleaching with ultraviolet light are sensitive to the amount of CO2 present. The observed radiation G value in samples containing 1 mole % CO2 and 99 mole % 3-MP, with a vapor—solid ratio of 8, is ∼0.3. The spectra are the same as those produced (Johnson and Albrecht) by photoionization of tetramethylparaphenylenediamine (TMPD) in 3-MP containing CO2 at 77°K.

Electrical Conductivity of γ-Irradiated 3-MP Glass and of Photoionized 3-MP—TMPD during Warming from 4° and 77°K

The electrical conductivity of γ-irradiated 3-methylpentane (3-MP) glass has been studied as a function of γ dose, temperature, applied voltage, post γ illumination, and the nature and concentration of additives. Three distinct populations of charge carriers are found in samples which have been γ irradiated at 77°K. The first, attributed to the combined mobility of weakly trapped electrons and positive charges, decays almost completely in 30 min in the dark at 77°K. It is reduced by 0.01 mole % of the electron scavenger CCl4, but is not affected by 1 mole % of the positive hole trap 2-methylpentene−1 (2-MP−1). The second, attributed to trapped electrons, is revealed by current flow between 83° and 88°K during warmup. It is eliminated by infrared illumination at 77°K before warm up (which causes a burst of current), or by alkyl halides, and is increased by 2-MP−1. The 85°K peak is absent on warmup of γ-irradiated 3-MP containing biphenyl, but appears if electrons are liberated from the biphenylide ion by illumination with 4100-Å light prior to warmup. The third population of charge carriers becomes mobile above about 92°K, giving maximum current at about 102°K. It is not affected by alkyl halide additives, and must be ascribed to migration of positive and negative ions. Additives capable of forming larger ions of lower mobility lower this current peak and produce new peaks. The areas of the peaks at 85° and 102°K are proportional to the applied voltage over the range tested (4×102 to 8×104 V cm−1). They increase linearly with dose below 1×1018 eV g−1 but more slowly at higher doses. Blocking electrodes reduce these peaks. Between about 88° and 92°K the warmup current of γ-irradiated 3-MP becomes negative, suggesting randomization by thermal agitation of radiation-produced charges which have been oriented by the applied electric field at 77°K. The warmup current spectrum of 3-MP following γ irradiation at 4°K shows a peak at about 60°K in addition to the peaks observed for warmup from 77°K. When tetramethyl-p-phenylenediamine (TMPD) in 3-MP at 77°K is photoionized, the warmup current shows a peak centered at 108°K, which differs from the similar peak produced by γ irradiation in that it is reduced by the presence of alkyl halides. The effect is attributed to complexing of TMPD by the alkyl halide with resultant trapping of the photoionized electrons in close proximity to the positive ion.

The Detection of Trapped Electrons in γ-Irradiated 3-Methylpentane Glasses by Electron Spin Resonance

The Detection of Trapped Electrons in γ-Irradiated 3-Methylpentane Glasses by Electron Spin Resonance

ESR Spectra of Trapped Electrons in γ-Irradiated 3-Methylpentane

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Positive Hole Migration and Trapping in γ-Irradiated 3-Methylpentane at -196°

Positive Hole Migration and Trapping in γ-Irradiated 3-Methylpentane at -196°