Masaru Nishikawa

Pressure effects on electron reactions and mobility in nonpolar liquids

Abstract High pressure studies have elucidated the mechanisms of both electron reactions and electron transport in nonpolar liquids and provided information about the partial molar volumes of ions and electrons. The very large volume changes associated with electron attachment reactions have been explained as due to electrostriction by the ions, calculated with a continuum model, but modified to include the formation of a glassy shell of solvent molecules around the ion. The mobilities of electrons in cases where the electron is trapped can now be understood by comparing the trap cavity volume with the volume of electrostriction of the solvent around the cavity. In cases where the electron is quasi-free the compressibility dependent potential fluctuations are shown to be important. The isothermal compressibility is concluded to be the single most important parameter determining the behavior of excess electrons in liquids.

Density inhomogeneities and electron mobility in supercritical xenon

The low-field mobility of electrons in supercritical Xe has been measured isothermally as a function of density above the critical temperature (289.7 K). At 293 K the mobility varies from a high of 890 cm2/V s at 9.2×1021 atoms/cm3 to a minimum value of 4.6 cm2/V s at a density of 3.5×1021 atoms/cm3, which is just below the critical density. The density dependence of the mobility is reasonably well predicted by the deformation potential model if the adiabatic compressibility is used to characterize the electron–medium interactions. Approximate agreement indicates that electrons are quasifree in supercritical xenon.

Pressure tuning of electron attachment to benzoquinones in nonpolar fluids: continuous adjustment of free energy changes.

Changing pressure from 1 to 2500 bar continuously tunes free energy changes for electron attachment to molecules in nonpolar liquids by nearly 0.3 eV. Rate constants for electron attachment to substituted benzoquinones were determined over an extended free energy range of nearly 1 eV by a combination of solute, pressure, temperature, and use of solvents with differing energies of the quasifree electron, V0: tetramethylsilane (TMS) and 2,2,4-trimethylpentane (TMP). The rates of attachment to both benzoquinone (BQ) and 2,5-dichlorobenzoquinone in TMS increase as the pressure increases to 2500 bar, while in TMP the rates are higher but change little with pressure; the rate of attachment to fluoranil in TMS is similarly high at 1 bar but decreases with increasing pressure. Together the observed rate constants can be qualitatively interpreted to yield a rate vs free energy relation having both normal and Marcus inverted region behavior. Because the electron attachment reactions yield excited states, quantitative interpretation of the free energy dependence requires knowledge of the excited state energies. The electron enters the second lowest π* orbital to form a π*-π* excited state, which quickly relaxes to the lower n-π* excited state. The rate of attachment to this excited state is low when the free energy of reaction, ΔG°, is positive and increases as ΔG° decreases until near -0.2 eV, after which the rate decreases. While excited state energies are uncertain, reasonable estimates are obtained from absorption, excitation, and fluorescence spectra of the product radical anions measured here. The results are modeled using Marcus theory with inclusion of a high frequency molecular vibration.

STUDIES OF E- + CO2 CO2- EQUILIBRIA IN HEXAMETHYLDISILOXANE AND BIS(TRIMETHYLSILYL) METHANE

Abstract Electrons react reversibly with CO 2 in bis(trimethylsilyl)methane and hexamethyldisiloxane. This reaction is studied as a function of both temperature and pressure. The ground state energies of electrons are deduced and are quite low consistent with the observed high electron mobility. A linear dependence of the log of the attachment rate on the free energy of reaction is observed. The dependence of the equilibrium constant on pressure indicates the electrostriction volume of CO 2 − to be ≈−200 cm 3 /mol in these liquids.

Electron transport in {ital o}- and {ital m}-xylene under high pressure

The electron drift mobility (μ) was measured by a time‐of‐flight method in pure liquid o‐ and m‐xylene under high pressures up to 300 MPa, and in the temperature ranges from 15 to 120 °C and 0 to 100 °C, respectively. In both liquids μ increases in the lower pressure region at lower temperatures. At higher pressures μ decreases gradually with pressure at all temperatures studied. The pressure dependence of μ was interpreted in terms of a two‐state model and a hopping model. When μ increases with pressure this interpretation leads to a positive volume change upon introduction of electrons into the liquid, showing electrons reside in cavities of radius 0.31 to 0.32 nm, whereas in the high pressure region electron attachment to xylene molecules occurs, accompanied by hopping of electrons between molecules.

ELECTRON TRANSPORT IN O- AND M-XYLENE UNDER HIGH PRESSURE

The electron drift mobility (μ) was measured by a time‐of‐flight method in pure liquid o‐ and m‐xylene under high pressures up to 300 MPa, and in the temperature ranges from 15 to 120 °C and 0 to 100 °C, respectively. In both liquids μ increases in the lower pressure region at lower temperatures. At higher pressures μ decreases gradually with pressure at all temperatures studied. The pressure dependence of μ was interpreted in terms of a two‐state model and a hopping model. When μ increases with pressure this interpretation leads to a positive volume change upon introduction of electrons into the liquid, showing electrons reside in cavities of radius 0.31 to 0.32 nm, whereas in the high pressure region electron attachment to xylene molecules occurs, accompanied by hopping of electrons between molecules.

Electron mobility in liquid n‐hexane/2,2‐dimethylbutane mixtures under high pressure

The effect of pressure on the electron mobility in mixtures of n‐hexane and 2,2‐dimethylbutane was studied over the whole concentration range. The variation of mobility with pressure is discussed in terms of a two‐state model, and provides information on the volume changes occurring on localization. The observed volume changes are interpreted as the difference between the cavity volume and the electrostriction volume of the localized electron.

Mobility of excess electrons in hexamethyldisiloxane and bis(trimethylsilyl)methane

Abstract The mobility of excess electrons in bis(trimethylsilyl)methane is 63 cm 2 /V s, and in hexamethyldisiloxane 22 cm 2 /V s. For these and related silicon-containing compounds, the mobility is greater than for those alkanes which have comparable free-ion yields.

Mobility of electrons in supercritical krypton: Role of density fluctuations

Excess electrons were generated in supercritical krypton by means of pulsed x-ray irradiation, and the electron transport phenomena were studied. Electron signals immediately after a 30 ps pulse showed a distinctive feature characteristic of the presence of the Ramsauer-Townsend minimum in the momentum transfer cross section. The dependence of the drift velocity v(D) on field strength was found to be concave upward in the low field region and then to go through a maximum with increasing field strength, which is also typical of the presence of a minimum in the scattering cross section at an intermediate field strength. A minimum in the electron mobility was observed at about one-half the critical density. The acoustical phonon scattering model, which successfully explained the mobility change in this density region in supercritical xenon, was again found to account for the mobility in supercritical krypton.

Behavior of excess electrons in supercritical fluids-electron attachment

The behavior of excess electrons in supercritical ethane was investigated by measuring mobility and reaction rates. Mobilities were measured by means of a time-of-flight method at 306-320 K as a function of pressure. Mobility values decreased at all temperatures with increasing pressure, but showed a small minimum or a shoulder at the pressure where the compressibility /spl chi//sub /spl Upsi//, has a peak. Electron attachment to CO/sub 2/, NO, pyrimidine and C/sub 2/F/sub 4/ over the same temperature range was studied as a function of pressure. Both attachment rate constants k/sub a/ for NO and C/sub 2/F/sub 4/, and equilibrium constants K(=k/sub a//k/sub d/) for CO/sub 2/ and pyrimidine increased sharply at pressures of /spl chi//sub /spl Upsi// peaks. Activation volumes V/sub a/* and reaction volumes /spl Delta/V/sub r/ are very large and negative in the critical region. The volume change is mainly due to electrostriction around ions formed. The results are compared to volume changes predicted by a compressible continuum model.