Electrons have so little mass that in less than a second they can tunnel through potential energy barriers that are several electron-volts high and several nanometers wide. Electron tunneling is a critical functional element in a broad spectrum of applications, ranging from semiconductor diodes to the photosynthetic and respiratory charge transport chains. Prior to the 1970s, chemists generally believed that reactants had to collide in order to effect a transformation. Experimental demonstrations that electrons can transfer between reactants separated by several nanometers led to a revision of the chemical reaction paradigm. Experimental investigations of electron exchange between redox partners separated by molecular bridges have elucidated many fundamental properties of these reactions, particularly the variation of rate constants with distance. Theoretical work has provided critical insights into the superexchange mechanism of electronic coupling between distant redox centers. Kinetics measurements have shown that electrons can tunnel about 2.5 nm through proteins on biologically relevant time scales. Longer-distance biological charge flow requires multiple electron tunneling steps through chains of redox cofactors. The range of phenomena that depends on long-range electron tunneling continues to expand, providing new challenges for both theory and experiment.

Long-Range Electron Tunneling

A review is given on the physical properties of electronic charge carriers (electrons, holes) in non-polar and polar liquids. The results are used to explain or discuss certain electric conduction phenomena, which occur when these liquids are subjected to a high electric field strength. Electronic conduction in ultrapure liquids is finding an increasing number of applications.

Electronic Conduction Processes in Dielectric Liquids

A new set-up, using a video camera, allowing easy and economical access to the velocity distribution of streamers is presented. Our study focuses on the behavior of negative streamers in a variety of liquids, either pure or containing ionic or non-ionic additives, in point-plane geometry. The propagation velocity distribution has always a more or less marked minimum, whatever the polarity and the shape of the streamer (bush-like or filamentary). Such a velocity distribution is similar to that of the field on a growing conducting sphere. There is generally a very close correlation between the shape and the velocity: the higher the velocity the more filamentary the streamer. This is still the case even when in the same liquid we have either subsonic or supersonic velocity. It is shown that the positive transient currents are also in favor of the model of the conducting sphere and not of a unique conducting filament even though there is evidence that the streamer is filamentary. Some of the theoretical models of streamer propagation already proposed are appraised, in particular that of Devins et al. which is thought to be the most accurate in describing experimental results. As concerns the initiation phase, the time lag until the appearance nce of the streamer dec, reases sharply when voltage is increased. This time lag also depends on space charge phenomena. Initiation voltages are always higher with positive polarity than for negative polarity for a series of tested simple or industrial liquids.

Prebreakdown Phenomena in Liquid Dielectrics

Through its Radiological and Chemical Physics Program, the Department of Energy (DOE) has been a primary source of funding for research in radiation physics and radiochemistry, supporting a wide range of explorations of the link between physical, chemical and biological events. This book is a series of articles by authors working within this field, most of whom have been central to the DOE-sponsored research. The opening papers focus on radiological physics; the second section covers radiation chemistry in a discussion that extends from the initial energy transfer to the production of intermediate chemical species and DNA damage. The third section explores the link between the physical and chemical events and the production of biological effects. Finally the book closes with a series of papers on molecular radiation biology.

Physical and Chemical Mechanisms in Molecular Radiation Biology

The electrical conduction of purified cyclohexane with point-plane electrode geometry was studied as a function of point radius ( approximately 0.1 to 10 mu m) and hydrostatic pressure (10/sup 4/ to 5*10/sup 6/ Pa). The parameter with the greatest influence on the observed phenomena is the radius r/sub p/ of the point. The results indicate a field emission or field ionization regime for sharp points only at r/sub p/ 0.5 mu m), the results depend greatly on polarity. For negative points, above a threshold electric field of 7+or-2 MV/cm (a field much lower than that required for field emission) a highly regular pulse regime is observed. This pulse regime shows all the characteristics of the Trichel regime observed in electronegative gases with the exception of no pressure dependence. This leads to the consideration of a mechanism of corona discharge occurring in the liquid phase. For positive points of r/sub p/>0.5 mu m the observed phenomena were poorly reproducible. Current bursts, randomly distributed, were also detected at fields at the point above 5 MV/cm but with a very low count rate. >

Electrical conduction of purified cyclohexane in a divergent electric field

Measurements of the photoconductivity of liquid n‐pentane, n‐hexane, cyclopentane, cyclohexane, neopentane, neohexane, and tetramethylsilane induced by far UV radiation (7⩽hν⩽10 eV) have been performed. We have studied the influence of the incident photon energy, of the applied electric field strength, and of the concentration of two electron scavengers (carbon tetrachloride and perfluoromethylcyclohexane). Our results show that in going from the gas phase to the liquid phase, the ionization energy threshold values of these low dielectric constant liquids are lowered by about 1.6 eV. The introduction of an electron scavenger reduces drastically the solvent photocurrent. Our results are consistent with the quenching, by the scavenger, of a solvent expanded excited state, precursor of the geminate ion pair, and not with an epithermal electron scavenging as in high energy photon irradiation.

Photoconductivity studies in some nonpolar liquids

A corrected curve of free ion yield versus electric field is given for n‐hexane as well as new curves for 2‐methylpentane, 2,2,4‐trimethylpentane, and carbon tetrachloride. Comparison with the Onsager theory of geminate ion recombination does not give reasonable agreement over the entire range of field strengths for any of the functions used.(AIP)

Dependence of free ion yield on electrical field strength in a dielectric liquid irradiated by 60Co γ rays

Electron mobilities and rate constants for electron-solute reactions in some non polar liquids

A comparison of the effects of perfluoromethylcyclohexane on the photoconductivity of tetramethylsilane and on photoionization quantum yields of n,n,n′,n′-tetramethyl-p-phenylenediamine dissolved in the same solvent

J. Casanovas

Papers

Photoconductivity studies in some nonpolar liquids

Dependence of free ion yield on electrical field strength in a dielectric liquid irradiated by 60Co γ rays

Electron mobilities and rate constants for electron-solute reactions in some non polar liquids

A comparison of the effects of perfluoromethylcyclohexane on the photoconductivity of tetramethylsilane and on photoionization quantum yields of n,n,n′,n′-tetramethyl-p-phenylenediamine dissolved in the same solvent

Photoconductivity studies in some nonpolar liquids