Pulse radiolysis was used to study the rates of several electron transfer reactions in supercritical Xe over a range of pressures. The rates of electron attachment to oxygen and 4,4′-bipyridine were also studied; the latter reaction is sufficiently fast to compete with electron–ion recombination, which is very fast in supercritical Xe. Electron transfer from 4,4′-bipyridine anion to benzoquinone is diffusion controlled. A peak in the rate is observed in the pressure region of maximum compressibility as expected for diffusion control. The rates of electron transfer from 4,4′-bipyridine anion and from hexafluorobenzene anion to O2 are an order of magnitude slower than expected for diffusion. The energetics of these reactions is discussed.

Rate constants for electron attachment to C60 were measured by pulse radiolysis in several nonpolar solvents. The rates for this highly exoergic reaction are fast, comparable to the rates found for the most efficient acceptors, such as SF6. The linear dependence of the rate on electron mobility for liquids in which the mobility is low, indicates the reaction is diffusion limited, but the rate drops below the diffusion rate for liquids in which the mobility is high.

The mobility of one of two types of fast moving negative charge carriers was measured by a time-of-flight method over a wide range of the solvent density p in gaseous, supercritical, and liquid carbon dioxide. Charge carriers are formed by laser photoinjection into neat CO2. At low densities below 2 mol/L, the mobility decreases inversely proportional to rho(6.3). The mobility becomes minimum near the critical density and increases at higher densities. The location of the mobility minimum approximately coincides with that of the isothermal compressibility maximum. The mobility increase at higher densities is understood as the result of the onset of the hopping electron transport or of the formation of the conduction band. Both of the hopping and the equilibrium (the conduction band) models are consistent with the mobility behavior observed.

Excess electrons can be introduced into liquids by absorption of high energy radiation, by photoionization, or by photoinjection from metal surfaces. The electron's chemical and physical properties can then be measured, but this requires that the electrons remain free. That is, the liquid must be sufficiently free of electron attaching impurities for these studies. The drift mobility as well as other transport properties of the electron are discussed here as well as electron reactions, free-ion yields and energy levels, Ionization processes typically produce electrons with excess kinetic energy. In liquids during thermalization, where this excess energy is lost to bath molecules, the electrons travel some distance from their geminate positive ions. In general the electrons at this point are still within the coulombic field of their geminate ions and a large fraction of the electrons recombine. However, some electrons escape recombination and the yield that escapes to become free electrons and ions is termed G{sub fi}. Reported values of G{sub fi} for molecular liquids range from 0.05 to 1.1 per 100 eV of energy absorbed. The reasons for this 20-fold range of yields are discussed here.

The results of an initial study of the pulse radiolysis of supercritical xenon are reported. In pure xenon, transients are formed that absorb broadly throughout the visible. These transients are assigned to excimer species, Xe-2*, on the basis of lifetime and kinetic data. The formation of excimers by electron-ion recombination was time-resolved by pulse-probe measurements. The excimers can be quenched by adding small amounts of ethane, which then facilitates detection of other transients by absorption spectroscopy. The added ethane also accelerates the thermalization of electrons and allows measurements of fast reaction rates of thermal electrons. Electron attachment to hexafluorobenzene occurs near the maximum rate at high pressures in xenon-ethane mixtures. The C6F6- anion formed absorbs with a maximum at 500 nm and disappears by second-order kinetics. The mobility of this anion, as measured by conductivity, indicates sizable clusters of solvent around the ion at all pressures, which are of maximum size near critical density. The rate of electron transfer from C6F6- to benzoquinone exceeds 1 x 10(11) m(-1) s(-1) at most pressures. The rate maximizes near 62 bar at 21.4 degreesC. A maximum at this pressure is predicted by diffusion. The maximum is related to the increase in cluster size around the anion, which occurs at this pressure.

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.

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.

Free ion yields were measured for tetramethylgermane (TMG) in both the liquid and vapor phase and for Kr gas exposed to X-rays: The X-ray energy was varied across the K-edges of Ge and Kr, respectively. In Kr the relative W value increases slightly at the K-edge, which is at 14.3 keV. In liquid TMG the observed ion yield drops at the Ge K-edge (11.1 keV) and shows two minima separated by 10 eV. This ion-yield spectrum is a mirror image of the absorption spectrum, as represented by the gas-phase ion-yield spectrum. The observation of such an inverted spectrum in liquids is shown to be due in large part to inefficiency of collection of charges. This is a consequence of the large Ge cross sections above the edge which concentrates the region of irradiation near the entrance window, increasing the local dose rate and enhancing recombination. The yield of excited states in mixtures of TMG and toluene drops at the Ge K-edge by the amount expected considering the large X-ray fluorescence yield.

Pyrazine and methylpyrazine attach electrons reversibly in supercritical (SC) ethane in the interesting region near critical densities. Very rapid shifts in the attachment equilibrium occur over very narrow pressure ranges. Reaction volumes for electron attachment to these solutes range from -1.0 to -45 L/mol, depending on conditions. The agreement of the values of DeltaV(r) with the calculated values from the compressible continuum model is excellent, indicating that the model predicts the electrostriction volume by ions in SC ethane quite accurately, and also suggesting that the partial molar volumes of neutral pyrazine and methylpyrazine are small by comparison to that of the ions.

The reaction of excess electrons with pyrimidine and styrene was Studied in supercritical ethane, The equilibrium constant for attachment was large when the Solute was pyrimidine and small for styrene, although their electron affinities are comparable. At pressures above 100 bar, the rate constant for electron attachment to pyrimidine (k(a)) was time resolved using the short pulse of the laser-electron accelerator facility (LEAF). The rate constant, k(a), is large and nearly independent of pressure and temperature; thus, the volume of activation is close to zero. At lower pressures, the equilibrium constants for this reaction were derived from the changes in the mobility of excess electrons. The free energy is a function of the polarization energy, which was evaluated with a compressible continuum model. The small equilibrium constant in the case of styrene is attributed to a smaller polarization energy. Values of Delta V-r, obtained from changes of Delta G(r) with pressure, range from -9.0 to -0.4 L/mol. The observed volume changes are compared to electrostriction volumes calculated by the model. Electron attachment occurs with a large decrease in entropy associated with clustering of ethane molecules around the ion formed. Observed values of Delta S-r are comparable to expected values calculated from Delta S-r = (alpha/chi(T))Delta V-r.

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.

Thermal electrons attach reversibly to CO2 in supercritical ethane. The equilibrium constants range from 40 to 2 x 10(4) m(-1) for pressures from 50 to 220 bar at 33 and 37 degrees C. The attachment rate increases and the detachment rate decreases as the pressure increases. The lifetime of CO2- is between 20 and 100 ns in the high-pressure range. The reaction volume changes more than an order of magnitude, from -20.0 to -0.5 L/mol over the pressure and temperature ranges studied. The activation volume is approximately 50% of the reaction volume. Electrostriction volumes of the CO2- ion, calculated by a compressible continuum model, account for the main part of the observed reaction volumes. The calculation shows that the high-density region around each CO2- extends to 1 nm.

A photocathode immersed in an insulating liquid might inject electrons. The analysis of the wavelength versus photocurrent curve enables one to evaluate the energy of the injected, quasi-free electron in the liquid, i.e. the bottom of the conduction level V0. The pressure dependencies of V0 observed with several liquid hydrocarbons indicate certain discrepancies between experimental data and theoretical expectations. Thermodynamic analysis shows that, contrary to theoretical descriptions, the injection proceeds via a polytrope which runs between the isothermal–isochoric and isoentropic–isobaric curves. This being attributed to local perturbations of temperature and pressure by the injection process, an estimate of the time scale of injection can be given, finding characteristic times to be in the order of 0.01–0.1 ps.

The yields of free ions formed following absorption of 1.6–3.5 keV X-rays were determined for several nonpolar liquids using a conductivity technique. The yields are much less for X-rays than for gamma rays; this effect is largest for branched hydrocarbons. A minimum in yield is observed around 2 keV. The dependence of Gfio on X-ray energy is in good agreement with computer simulations. For tetramethylsilane a sharp dip in ion yield is observed at the Si ls → σ∗ resonance, indicating that the free electron yield is even less at this energy.

Electrons react reversibly with CO2 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 CO2− to be ≈−200 cm3/mol in these liquids.

The mobility of excess electrons in bis(trimethylsilyl)methane is 63 cm2/V s, and in hexamethyldisiloxane 22 cm2/V s. For these and related silicon-containing compounds, the mobility is greater than for those alkanes which have comparable free-ion yields.

The yields of excited singlet states formed in some liquid hydrocarbons are reported for exposure to synchrotron X-rays of energies between 5 and 14 keV. The yields are derived from measurements of ultraviolet fluorescence emission of the hydrocarbons. Results reported are relative to the yield of excited singlets in liquid benzene. The yields decrease with decreasing X-ray energy, and at the lowest energies studied G(S-1*) = 1.5/100 eV for cis- and trans-decalin. Results are compared to computer simulations.