Gordon R. Freeman

Electron mobilities in gaseous, critical, and liquid xenon: Density, electric field, and temperature effects: Quasilocalization

The mobility μ of electrons in xenon varies with the density n : μαnx. For thermal electrons in the gas, x=−1.0 when n<4×1020 molecule/cm3, independent of temperature. This corresponds to pressures ≲20 atm at 300 K and is characteristic of simple scattering by a polarization field. At higher gas densities x becomes more negative, then passes through a minimum, and μ has a positive temperature coefficient at constant n. At these densities thermal electrons form quasilocalized states in, or suffer enhanced scattering by, microscopic regions of relatively high density. The localization effect maximizes in the critical fluid, at n=5×1021 molecule/cm3 and T=290 K, and appears to be negligible again at n?6.8×1021. In the liquid phase x?+9 up to n=1.2×1022 molecule/cm3, then changes to x?−8 at higher densities. The positive value of x is due to the formation of a conduction band, and the negative value to scattering by the repulsive cores of the closely spaced atoms. In the gas and the low density liquid (n<1.2×...

Dependence of Radiation‐Induced Conductance of Liquid Hydrocarbons on Molecular Structure

The radiolytic ion mobilities u, ion‐neutralization rate constant k, steady‐state radiation‐induced conductance κ, and free‐ion yield Gfi have been measured for a number of saturated hydrocarbon liquids of different molecular shape. The values of G>fi in the following liquids at 20° were; cyclohexane, 0.11; n‐hexane, 0.11; n‐pentane, 0.12; 2,2‐dimethylbutane, 0.40; 2,2‐dimethylpropane, 0.81. The unusually high values for the last two compounds were partly associated with a current “overshoot” that occurred at the beginning of current‐vs‐time oscillograms that were obtained for the evaluation of k / u. The overshoot was not present in oscillograms obtained from the first three liquids. The causes of the overshoot and the high Gfi values are not known, but their magnitudes correlate with the closeness to spherical shape of the molecules. The facility of electron trapping and of electron energy loss in the subvibrational‐excitation energy region are probably dependent on molecular shape and on liquid structu...

Electron ranges in liquid alkenes, dienes, and alkynes: Range distribution function in hydrocarbons

Free ion yields have been measured as functions of temperature T and electric field strength E in hydrocarbon liquids. A range distribution of the secondary electrons in hydrocarbons, suitable for use in the one‐ion‐pair‐per‐spur model, is F(y)=(4y2/π1/2bGP3)exp(−y2/bGP2), y 2.4 bGP; where F(y)dy is the fraction of thermalized electron‐ion pairs that have initial separation distances between y and y+dy, and bGP is the dispersion parameter. This equation is suitable for all of the liquids that have been studied thus far, at all values of T and E that have been used. The value of bGPd for a given liquid is independent of T, where d is the liquid density. The density‐normalized ranges in a family of hydrocarbons with the same number of carbon atoms and analogous carbon skeletons decrease in the order alkane>alkene‐1>alkyne‐1. Two terminal double bonds are more effective than one terminal triple bond in reducing the electron range. Conjugation of the do...

Molecular Structure Effects on the Free‐Ion Yields and Reaction Kinetics in the Radiolysis of the Methyl‐Substituted Propanes and Liquid Argon: Electron and Ion Mobilities

The free‐ion yields in liquid alkanes increase with increasing molecular sphericity and increasing temperature, in agreement with earlier observations. The Arrhenius temperature coefficient of free‐ion formation is 0.8 kcal/mole in propane and 0.7 kcal/mole in 2‐methylpropane. The free‐ion yield in purified argon at 87°K appears to be Gfi=2.0. Addition of oxygen to liquid argon decreased the free‐ion yield, mainly because the efficiencies of electron energy loss processes with the diatomic oxygen molecules were much greater than those with monatomic argon molecules; the oxygen served to decrease the ion—electron separation distance more by de‐energizing the electron than by scavenging it before it reached the end of its normal track. In pure liquid oxygen at 87°K, Gfi=0.013. The ion—electron neutralization rate constant equals 8 × 10−5 cm3/ion · sec in pure argon at 87°K and 3.1 × 10−4 cm3/ion · sec in liquid methane at 120°K. The mobility of electrons in liquid methane is 300 cm2/V · sec and decreases in...

Electron mobilities and ranges in methyl substituted pentanes through the liquid and critical regions

Density normalized penetration ranges bGPd of secondary electrons and mobilities μe of thermal electrons vary in similar manners as the temperature of the solvent hydrocarbon is increased toward the critical. These quantities pass through maxima in 3,3‐dimethylpentane, 2,2,4‐trimethylpentane, and 2,2,4,4‐tetramethylpentane, but not in 3‐methylpentane. In 2,3‐dimethylbutane μe has a small maximum just below the critical point while bGPd reaches a short plateau. As the molecules in a series become more spherelike, the maxima grow and the temperature at which they occur moves farther below the critical. Changes of the range and mobility are caused by changing scattering properties of the fluids. The decrease of μe on the high temperature side of the maximum is attributed to a decrease of mobility in the conduction band μ0h rather than to an increase in electron trap depth that might have been caused by the greater density fluctuations that occur as the critical region is approached. The results support the e...

Effect of Electric Field Strength on the Gamma‐Radiation‐Induced Conductance of Liquid Alkanes

The γ‐radiation‐induced conductance of liquid alkanes has been studied as a function of electric field strength, irradiation dose rate, and temperature over wide ranges. The conductance at fields below about 102 V/cm seems to be governed by electrode processes, but is not understood. The conductance at fields between about 102 and 103 V/cm was explained earlier [G. R. Freeman, J. Chem. Phys. 39, 978 (1963)]. A theory developed by Onsager has been extended to explain the results at fields greater than 103 V/cm. It appears that the number of electrons that escape their parent ions or spurs increases markedly with increasing field strength at high fields, but is nearly independent of temperature.

I. Kinetics of Reactions of Electrons during Radiolysis of Liquid Methanol. II. Reaction of Electrons with Liquid Alcohols and with Water

The gamma radiolysis of liquid methanol has been studied over the temperature range from its melting point ( − 98°) to its critical temperature (240°). The value of G(H2) varies from 4.95 at −98° to 5.45 at 25° and 8.0 at 240°. Sulfuric acid and nitrous oxide were added as electron scavengers at −97°, 25°, and 150°. The nitrogen and hydrogen yields were calculated as functions of the nitrous oxide concentration at each of the three temperatures. The calculations were based on a proposed mechanism. Homogeneous kinetics were used for the reactions of the free ions and nonhomogeneous kinetics were used for the reactions in spurs. At −97°, G(total ionization) = 4.6 and the value is assumed to be independent of temperature. The yield of free ions is G(esolv−)fi = 2.0 ± 0.2, independent of temperature from −97° − 150°. The reaction esolv−→CH3Osolv−) + H has a rate constant of 4.6 × 105 sec− 1 at 25° and an activation energy equal to that of dielectric relaxation (3.7 kcal/mole); the entropy of activation of the...

Densities against temperature of 17 organic liquids and of solid 2,2-dimethylpropane

Abstract Densities against temperature of liquids under their equilibrium vapor pressures are reported for three fluoromethanes, three aromatic hydrocarbons, four olefins, bicyclo[2,2,2]octane. 2-methyltetrahydrofuran, and {0.786(1,4- c -C 6 H 8 ) + 0.214C 6 H 6 }. The volume change on melting 2,2-dimethylpropane at the triple-point temperature is 9.6 per cent. A density parameter for n -pentane by Das and coworkers ( J. Chem. Eng. Data 1977 , 22, 3) has been corrected.

Ion and electron mobilities in cryogenic liquids: Argon, nitrogen, methane, and ethane

Ion mobilities at low fields have been measured over the entire liquid range in argon, nitrogen, and methane. Gas‐phase values for argon and nitrogen are also given. Comparison of the ratio of electron to ion mobilities confirmed that the electron remains nearly free in argon and methane liquids but is an anion in liquid nitrogen. At T≲116 K, the product of viscosity and either cation or anion mobility is constant in liquid nitrogen under its vapor pressure. At T>118 K, an abrupt destabilization of the anion in the low density liquid occurs; the breakdown strength of nitrogen should sharply decrease. The utility of breakdown studies in liquid ethane and propane in bridging studies of cryogenic inorganic liquids to studies of normal liquid hydrocarbons is discussed.

Effects of density and temperature on mobilities of electrons in vapors and liquids of pentane isomers: Molecular structure effects

The mobility of electrons in low density gaseous n‐pentane is independent of applied electric field strength up to E/n=2 Td. The mobility in isopentane increases at E/n≳0.4 Td, and in neopentane at ≳0.2 Td. The ratio of the threshold drift velocity for electron heating to the velocity of sound, vd(threshold)/co, increases as the molecules become less spherelike. The value of the ratio in low density xenon is ∼0.7, in neopentane ∼5, isopentane ∼15, and n‐pentane ≳100. The high field mobilities in neopentane and isopentane vapors seem to approach the field independent value in n‐pentane. Representing the velocity dependence of the scattering cross section as σv=e/mbjv1−j, the value of 1−j increases with increasing sphericity of the molecules. Values of 1−j and σav=〈v〉/〈v/σv〉 for thermal electrons in the dilute gases at 300 K are respectively: n‐pentane, 1.1, 1.3×10−15 cm2; isopentane, 1.5, 1.8×10−15 cm2; neopentane, 3.2, 4.0×10−15 cm2. The variation of (1−j) and σav with sphericity for the isomeric pentanes...