Free Ion Yield Research Articles

Estimation of the electron lifetime in tetramethylsilane

The variation of the collected charge with electric field in a diode-type ionization chamber is used to estimate the electron lifetime in tetramethylsilane (TMS). The primary ionizing particles are the 976 keV conversion electrons emitted from a 207Bi source. In the best purity sample of TMS we obtained an electron lifetime of 11 ± 2 μs which corresponds to an attenuation length at 10 kV/cm, equal to 11 cm. The free ion yield at zero electric field, G f(0), is determined to be 0.51 ± 0.05.

Electron thermalization distances and free ion yields in dense gases and liquids-effects of molecular structure

Radiolysis free ion yields were measured as a function of electric-field strength in vapor/liquid equilibrium fluids of ethene, propene, cyclopropane, propane, n-butane and i-butane at 0.15<or=d/d/sub c/<3(d/sub c/=critical fluid density). From these yields, secondary electron thermalization distances b/sub GP/ were estimated using the extended Onsager model. In the normal liquid at d/d/sub c/>1.5, the effects of molecular shape gave b/sub GP/d in methane>i-butane>ethane>propane>n-butane. The effect of pi -bonding gave n-butane>cyclopropane>ethene approximately propene; the permanent dipole of propene had negligible effect in the dense liquid. The conduction-band energy in supercritical fluids of n-butane, isobutane, n-pentane and isopentane was measured as a function of the fluid density. The conduction-band energy values in these fields show a density dependence that is related to their molecular structure. The correlation between the density dependence of conduction-band energy and of electron mobility is discussed in terms of the deformation potential theory.<<ETX>>

Comparison of photoinduced electron transfer reactions of aromatic carbonyl vs. cyano compounds with electron donors in condensed phase:the importance

Photoinduced electron transfer reactions in acetonitrile with bensopheneone, anthraquinone, 9-cyanoanthracene and 9,10-dicyanoanthracene as electron acceptors, and with 1,4-diasabicyclo[2,2,2]octane and N,N-dimethylaniline as electron donors have been studied with ns-laser flash photolysis and fluorescence quenching measurements. For these systems the resulting free ion yield depends on the spin state of the geminate ion pair: its separation is very efficient if formed in a triplet state (carbonyl compounds/donors), while it is very inefficient if formed in a singlet state (cyanoanthracenes/donors). In the triplet systems, geminate back electron transfer is limited by the rate of spin flip.

Theoretical analysis of free-ion yield in liquid argon under low-let irradiation

The relatively large probability (≈0.35) of escaping electron-ion recombination in liquid argon, irradiated at low-LET in the presence of small collecting fields, is not consistent with Onsager's geminate model. A diffusion-kinetic model allowing electrostatic interaction, with a cylindrical initial Gaussian distribution of standard deviation ≈ 3 μm can be made consistent with experimental results if a specific recombination rate ≈0.01 times the Debye value is used. Other possibilities are discussed.

Electron thermalization distances and free ion yields in liquid nitrogen from 77 K to near T c

The free ion yield in x-irradiated liquid nitrogen at zero applied field strength increases from G0fl=0.0034 ion pairs/100 eV at 77.1 K to 0.043 at 123.7 K, while the density-normalized thermalization distance of the secondary electrons increases from bEPd=4.4×10−6 to 5.6×10−6 kg/m2. The ratio of the GEfi values at 123.7 and 77.1 K decreases from 13 at E=0 to 3.1 at E=5.2 MV/m. Yields calculated from the extended Onsager model agree with the measured values only when the electron thermalization distance distribution is assumed to be exponential with an added power tail (YEP): F(y)=YE, 0&amp;lt;(y−y0)&amp;lt;3.5bEP; F(y)=YE+0.74b2EP/(y−y0)3, (y−y0)&amp;gt;3.5bEP; where YE=b−1EP exp[(y0−y)/bEP], y&amp;gt;y0. The electron thermalization process in liquid nitrogen appears to be truncated by electron capture to form an anion.

Electron mobilities, free ion yields, and electron thermalization distances in liquid long-chain hydrocarbons

At 296 K the electron mobility decreased in the liquids n-hexane&amp;gt;n-decane&amp;gt;n-dodecane &amp;gt;n-tetradecane in accord with the two state model of electron transport (e−il⇄e−qf); the percolation model was inadequate. The temperature dependences were determined for n-decane, n-dodecane, n-tetradecane to T&amp;gt;450 K. Free ion yields were also measured. At a given temperature Gofi was the same in all these liquids to within ±4%. The density-normalized thermalization ranges of secondary electrons, estimated from the extended Onsager model, were bGPd =(49±1)10−6 kg/m2 under all conditions.

Ionization and electron thermalization distances in isomeric pentanes: Effects of molecular shape and density

Free ion yields were measured in n-, iso-, and neo-pentane at densities d=1–600 kg/m3 along the vapor–liquid coexistence curve. The total yield of ionization Gtot is 4.3 ± 0.1 in the gases at d≤100 kg/m3. The free ion yield extrapolated to zero electric field strength G0fi decreases continuously with increasing density in n-pentane, whereas in iso- and neo-pentane it goes through a maximum at d≈230 and 350 kg/m3, respectively. The density-normalized thermalization distances bGPd, go through a maximum in neo-pentane and iso-pentane in the low-density liquids, while in n-pentane they remain relatively constant (∼4×10−6 kg/m2). The values of bGPd correlate with the density normalized mobilities nμ of hot electrons (at high field strengths) in both the gas and liquid phases of the isomeric pentanes. The effect of molecular shape (sphericity) on electron scattering is much greater in the liquid than in the gas phase, and much greater at thermal than at epithermal electron energies. The direction of the shape (sphericity) effect reverses on going from the gas to the liquid phase.

Electron thermalization distances and free-ion yields in dielectric fluids: Effect of electron scavengers.

Free-ion yields were measured in isobutane and in solutions containing sulfur hexafluoride, at densities from the liquid at 295 K to the supercritical fluid. A modified Onsager model was used to estimate the most probable thermalization distances b/sub GP/ (GP denotes a Gaussian power-law distribution). The density-normalized distance b/sub GP/d in isobutane increased from 6.3 x 10/sup -6/ kg/m/sup 2/ at 295 K to a maximum of 10.5 x 10/sup -6/ kg/m/sup 2/ at 395 K, and then decreased to 5.6 x 10/sup -6/ kg/m/sup 2/ at 408 K (T/sub c/). The maximum, which is evidence of conduction-band enhancement of b/sub GP/, was suppressed in solutions containing small amounts of SF/sub 6/. The SF/sub 6/ captured the electrons while they were still at epithermal energies, and terminated their flight away from their sibling ions. The values of b/sub GP/ and the free-ion yields were thereby decreased.

Effects of solvent structure on electron reactivity and radiolysis yields: 2-propanol/water mixed solvents

Reaction of solvated electrons with nitrobenzene, N, is nearly diffusion controlled in both pure solvents; kN ~ 1010 dm3/mol s. The value of kN is approximately proportional to the inverse viscosity η−1 in the pure solvents, and in the mixed solvents at different temperatures. However, on going from zero to 74 mol% water at the same temperature kN is independent of the 40% increase of η. Electron diffusion in the mixed solvents is not a simple function of fluidity.Reaction with the inefficient scavengers tryptophane (kS ~ 109 dm3/mol s) and phenol (kS ~ 107–108 dm3/mol s) correlates inversely with the electron optical absorption energy. The latter is related to the trap depth in the solvent; electrons in deeper traps have less tendency to react with molecules of low electron affinity.Addition of 3 mol% 2-PrOH to water at 296 K increases the value of Gεmax by 16%, although the value in pure 2-PrOH is three-fold smaller than that in pure water. The increase is attributed to an increase in the free ion yield, caused by an increase in the product of the electron thermalization range and the microscopic dielectric constant of the fluid between the ion and electron, averaged over the time that they exist as a correlated pair. Addition of a small amount of alcohol to water increases the orderliness of the water structure.

Electron thermalization ranges and free-ion yields in dielectric fluids: Effects of density and molecular shape

Secondary-electron thermalization ranges ${b}_{\mathrm{GP}}$ and free-ion yields ${G}_{\mathrm{fi}}^{E}$ have been measured in the coexistence vapor and liquid of spherelike (methane) and nonspherelike (ethane) molecules at densities $0.14\ensuremath{\lesssim}\frac{d}{{d}_{c}}\ensuremath{\lesssim}2.8$, where ${d}_{c}$ is the density of the critical fluid. The density-normalized range ${b}_{\mathrm{GP}}d$ at low $d$ is 5.5\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}6}$ kg/${\mathrm{m}}^{2}$ in both compounds. In the dense gas at $\frac{d}{{d}_{c}}\ensuremath{\gtrsim}0.3$, ${b}_{\mathrm{GP}}d$ increases slightly; the effective cross section for epithermal electron scattering is lessened. The large density fluctuations at these conditions, which cause quasilocalization of thermal electrons, appear to affect epithermal electrons in an opposite fashion. At $\frac{d}{{d}_{c}}&gt;1.0$, conduction-band formation lifts ${b}_{\mathrm{GP}}d$ in liquid methane to \ensuremath{\sim}4 times the value in liquid ethane. The density effect is in the same direction as, but less marked than, that for thermal electrons. At $\frac{d}{{d}_{c}}=2.5$, ${b}_{\mathrm{GP}}$ in methane is 1.2 times larger than at $\frac{d}{{d}_{c}}=1$, but the mobility $\ensuremath{\mu}$ of thermal electrons is 15 times larger than at $\frac{d}{{d}_{c}}=1$. In ethane at $\frac{d}{{d}_{c}}=2.5$, ${b}_{\mathrm{GP}}$ is 3.4 times and $\ensuremath{\mu}$ is 33 times smaller than at $\frac{d}{{d}_{c}}=1$. The degree of sphericity of the molecules greatly affects electron energy loss and localization interactions.

Solvent structure effects on electron thermalization ranges in water–alcohol mixed solvents; preferred EAmax values for e−s in alcohols

Addition of 2 mol% of an alcohol to water or of 1 mol% of water to an alcohol increases the value of Gfi εmax (e−s) above that in the pure solvent. The effect is attributed mainly to an increasing free ion yield Gfi. The increase of Gfi correlates with a "stiffening" of the solvent molecular rotational motions and a corresponding lower rate of energy loss by the epithermal electrons to dipole rotations. Thus the increase of Gfi is mainly attributed to a larger electron thermalization range bGP. The value of Gfi εmax passes through a maximum at 2 mol% of an alcohol in water; the maximum increases in the order methanol&lt; ethanol&lt; 1-propanol as additive. The solvent "stiffening" effect of these solutes increases in the same order.Some of the published values of EAmax and W1/2 for solvated electrons in alcohols appear to be too low. A set of preferred values is reported herein.

Free-ion yields, electron mobilities and electron-ion recombination rate constants in liquid and solid isooctane and several other nonpolar compounds

Free-ion yields ( G fi 0) in solid isooctane were determined by a new simple method which was a combination of the electric current measurement during irradiation of very low dose of 60Co γ-rays and the continuous measurement of thermostimulated current after the irradiation by applying d.c. high voltage to the solid isooctane. Electron mobilities (μ e) in the solid isooctane were determined by the free-ion yields and the measurement of peak electric current upon the X-ray pulse irradiation of a few nanoseconds width. In the liquid phase, however, G fi 0 and μ e were also measured by the usual charge clearing method and the decay curve analysis or peak current method, respectively. The effects of temperature from 26 to – 158°C and of phase change from liquid to solid on G fi 0, μ e and b (the most probable electron-thermalization length) are compared with those for neopentane, tetramethysilane, and cyclohexane. Some values of the G fi 0, μ e and b determined in liquid and solid isooctane are as follows: in liquid G fi 0=0.13 at -101°C, μ e =1.6 cm 2 V -1 s -1 at -99°C, b=103 A at -101°C; in solid, G fi 0=0.15 at -118°C, μ e =0 cm 2 V -1 s -1 at -118°C, b=104 A at -116°C. The values of apparent activation energy of μ e obtained by the Arrhenius plot are 0.049 eV in the liquid and 0.077 eV in the solid isooctane, respectively. As the results, it is concluded that in liquid and solid isooctane localized electrons play a main role in electron transport with somewhat larger contribution of quasi-free electrons than in liquid and solid cyclohexane and that the rather larger contribution of quasi-free electrons than in cyclohexane seems to relate to the variation in G fi 0 and b value with temperature and phase change. Electron-ion recombination reaction in some organic liquids and solids is also discussed.

Electron mobilities in liquid and solid cyclohexane

The electron mobility and the free ion yield in liquid and solid cyclohexane have been measured as a function of temperature using X-ray pulse radiolysis. The zero-field free ion yield decreases with decreasing temperature in both liquid and solid, and exhibits a discontinuous decrease on the phase change from liquid to solid. The electron mobility in liquid from room temperature to the freezing point decreases and approaches to that in solid cyclohexane which also decreases with decreasing temperature. The observed values of the electron mobility and the zero-field free ion yield just below the freezing point are 0.07 cm 2/V· s and 0.05, respectively.

Radiation induced conductivity in liquefied rare gases

Theory of electron thermalization, at various levels of sophistication, has been applied to calculate thermalization time and width of thermalization distance distribution with special reference to LAr. The different approaches give the thermalization time ∼ 1 ns. However the thermalization distance parameter (b-value), using cross-sections according to the Cohen-Lekner theory, calculates in the range 845–1142 nm which is a factor ∼ 3.6 – 4.8 larger than that inferred from free-ion yield measurements. The difference can be attributed to the effect of the coulombic field of the geminate (+ve) ion. The implication is that the mean free path for both momentum and energy transfer in the presence of the Coulombic field is the same and that it lies in the range 4 – 6 A.

Geminate recombination kinetics of solute radical ions: Singlet excited state formation in cyclohexane solutions of biphenyl

Transient absorption spectra of the solute anion, cation and triplet state and the solute fluorescence in the pulse radiolysis of 0.1 mole 1 -1 biphenyl in cyclohexane were observed on a nanosecond timescale longer than 1 ns after a 20 ps pulse. The formation of the solute excited singlet state is mainly due to the geminate ion recombination reaction even in the high concentrated solutions. The decay of the solute ions obeys the reciprocal square root dependence on time longer than 10 ns from the end of a 10 ps pulse. The slope of this reciprocal square root plots agrees with the literature value on a longer timescale obtained by microwave absorption. The yield of free ions obtained from the intercept of the slope agrees also with the literature values obtained by the field clearing method. Ratio of the formation rate of the solute excited triplet state to the decay rate of the solute anion changes in a time range between 5 and 20 ns. It is very well correlated with a theoretical calculation of spin correlation decay of the germinate ion pairs by Brocklehurst, although the formation of the solute triplet state was observed even on a timescale shorter than 5 ns from the end of a 20 ps pulse, where loss of spin correlation is negligibly small.

Geminate ion recombination in irradiated liquid CCl 4: As observed by means of microwave and optical absorption pulse radiolysis and compared with calculations based on a single ion pair model

The geminate recombination of the ions formed in liquid CCl 4 by high energy radiation has been studied by measuring the microwave absorption at 8.7 GHz (X-band) and 29 GHz (Ka-band) as well as the optical absorption as a result of irradiation with pulses of 3 MeV electrons of (sub)nanosecond duration. The observed kinetics has been analysed using a single ion pair model. The survival probability against recombination as a function of time, W, for pairs of oppositely charged ions has been calculated by numerical integration of the Smoluchowski equation. For quite different distributions of initial separations the survival probability at long times is adequately represented by W( τ)=(1+0.6 τ -0.6) W(∞), where W(∞) is the escape probability and τ= Dt/ r c 2 the generalized time with D the sum of the diffusion coefficients of the ions and r c the Onsager radius. This equation describes quite well the geminate ion recombination in pure CCl 4 at different temperatures and also in solutions of cyclohexane. Using this equation the sum of the mobilities of the positive and the negative ions in CCl 4 has been determined to be 4.1, 7.6 and 12.8×10 -4 cm 2 V -1 s -1 at -20, 20 and 60°C, respectively. Values of the free ion yield of 0.08, 0.095 and 0.095 (100eV) -1 have been found at the three temperatures, respectively. A comparison of the results observed at short times using the microwave absorption method with the kinetics calculated for different distributions of initial ion separations shows that good fits are obtained for an exponential distribution with an initial yield of G(0)=5 (100 eV) -1 as well as a Gaussian distribution with G(0)=2.3 (100 eV) -1. With the assignment of the optical absorption with a maximum at 480 nm to a product resulting from the geminately recombining ions, the growth kinetics observed on a subnanosecond time scale (risetime ∼ 200 ps) has been compared with the kinetics calculated for the two above mentioned distributions. It is found that the experimental results are in fair agreement with the kinetics expected for an exponential distribution of separations of ions with an initial yield 5 (100 eV) -1.

Liquid Ionization Chamber for Absorbed Dose Determinations in Photon and Electron Beams

The use of a liquid ionization chamber for measurements in electron and photon beams with energies above 1 MeV has been investigated. The liquid in the chamber is 2,2,4-trimethylpentane. Fundamental properties such as the reproducibility of the measured charge per absorbed dose, the temperature dependence, and the angular dependence have been experimentally analysed. The general recombination losses in pulsed beams are shown to be dependent on the transport time of the ions in the liquid. For a longer transport time than the time between two consecutive pulses the losses depend on the mean dose rate and for a shorter transport time the losses depend on the absorbed dose per pulse. The Jaffe method is used for determination of the initial recombination losses, and compared with results from other authors. The chamber is particularly useful for absorbed dose determination in electron beams since the relation between measured charge and absorbed dose to water is almost energy independent. In photon beams the relation is somewhat quality dependent particularly at qualities below 10 MV. The change of the free ion yield caused by the small change of mean LET with the quality of the photon beam is probably the reason. Procedures for the calibration of the liquid chamber and the absorbed dose determination in electron and photon beams are described.

Semiempirical formula for the electric field dependence of geminate ion recombination fluorescence

A semiempirical formula is derived which governs the electric field dependence of the fluorescence produced upon recombination of geminate ions. The formula is applied to some new data on the fluorescence which follows the ultraviolet irradiation of solutions of N,N,N′,N′-tetramethylparaphenylenediamine (TMPD) in tetramethylsilane and 2,2-dimethylbutane. From the two-parameter-least-squares lines which result from the application of the formula, we determine a number of derived quantities. Among these are Φip the quantum yield for photoionization of TMPD, and P(0,0) the probability of dissociation of the electron–ion pair produced by the photoionization. The values of P(0,0) are found to be nearly equal to the ratio Gfi/(Ggi+Gfi) where Gfi and Ggi are the free ion and geminate ion yields, respectively, obtained in 60Co γ irradiations of the pure solvents. The product of Φip and P(0,0) is Φe0, the free electron quantum yield. The values of Φe0 obtained from the fluorescence measurements are compared with those obtained from separate measurements of the photocurrent. Other quantities derived from the semiempirical formula include the first three moments of a function related to the distribution of electron–ion pair distances in the liquid.

Arguments against a spur model for muonium formation

The postulated spur model is scrutinized by reference to μ SR and MSR data in a variety of media. Specific arguments are given against the spur model when applied to: water and aqueous solutions of electrom scavengers; low dielectric media such as hydrocarbons and liquid argon-and particularly the contrast between the muonium yield and the radiolysis free-ion yield; CCl4 and its mixtures with hydrocarbons; and the absence of an effect of an external electric field. Comments are made regarding the timescale of muonium formation and on important comparisons with positronium formation. It is suggested that an “expanding track” model may account for the missing polarization which is observed in liquids but not in solids.

Decay kinetics of excess electrons in crystalline ice

The decay kinetics of highly mobile excess electrons formed on nanosecond pulse radiolysis of crystalline ice have been studied from -10 to -190/sup 0/C by using the microwave absorption method. The overall decay is controlled by four processes, two of which involve preexisting trapping sites present in the lattice and two others which involve reaction with products of irradiation formed in the pulse. The lattice trapping sites are tentatively assigned to a vacancy combined with a Bjerrum D defect (dressed vacancy) and a normal lattice vacancy. The former results in an electron decay rate (rate constant x trap concentration) of 1.1 x 10/sup 19/ exp(-0.48/kT) s/sup -1/, for kT in eV, above -80/sup 0/C and less than or equal to 10/sup 6/ s/sup -1/ below -100/sup 0/C. The latter is operative below -140/sup 0/C and presents a shallow trap, ..delta..H = -0.1 to -0.2 eV, for electrons. The main radiation product responsible for electron decay is the positive counterion. The electron-ion recombination rate constant is given by the Debye expression with a temperature-independent value of the effective dielectric constant epsilon/sub r/ of 2.1 +- 0.2. Based on an infinitely short pulse the first half-life of mobile electrons toward recombination ismore » approximately 500/R ns over the temperature range -120 to -50/sup 0/C, where R is the dose in the pulse in rads. The second product of irradiation which is thought to react with electrons is the OH radical with a rate constant of 3 x 10/sup 14//G(OH) dm/sup 3/ mol/sup -1/ (100 eV). The product of the electron mobility and the free-ion yield is found to increase from 3.0 x 10/sup -4/ to 4.2 x 10/sup -4/ m/sup 2/ V/sup -1/ s/sup -1/ (100 eV)-/sup 1/ over the temperature range -120 to -50/sup 0/C. Evidence for a second mobile species, thought to be the hole, with a mobility of ca. 1 x 10/sup -6/ m/sup 2/ V/sup -1/ s/sup -1/ and a lifetime of 50 to 100 ns is found above -40/sup 0/C.« less