Pulsed conductance

Binders (TbC) 671 Bipolar pulse conductivity detector (LC) 588 Bonded phases (GC) 125 crosslinked 126 estersils 125 nonextractable 126 siloxane 125 Bonded phases (LC) 324 carbon loading 335 cleavage of ligands 336 eluotropic strength (LSC) 382 endcapping 326 hydrophobicity 364 metal impurities 369 models for surface 337 physical characteristics 333, 366... 

Electron attachment to solutes in nonpolar liquids has been studied by such techniques as pulse radiolysis, pulse conductivity, microwave absorption, and flash (laser) photolysis. A considerable amount of data is now available on how rates depend on temperature, pressure, and other factors. Although further work is needed, some recent experimental and theoretical studies have provided new insight into the mechanism of these reactions. To begin, we consider those reactions that show reversible attachment-detachment equilibria and therefore provide both free energy and volume change information. 

Figure 8.15 Bipolar pulse conductance measurement system.

Behrens G, Bothe E, Koltzenburg G, Schulte-Frohlinde D (1980) Formation and structure of 1,1-di-alkoxyalkene radical cations in aqueous solution. An in situ electron spin resonance and pulse conductivity study. J Chem Soc Perkin Trans 2 883-889... 

A number of experiments measuring the decay of the P state have revealed complicating factors in addition to the multi-exponential decay of the state described above. In measurements monitoring the decay of P using short (SOnlOOfs) laser pulses conducted by Vos and co-workers, oscillations were observed superimposed on the resulting kinetic traces (Vos et al., 1994a,b,c 1993,1991). These oscillations have been attributed to coherent nuclear motion associated with the P state (Vos and Martin, 1999 ... 

Beck G. (1983) A picosecond pulse-conductivity technique for the study of excess electron reactions. Radiat Phys Chem 21 7-11. 

At elevated temperatures, where the electron lifetime was much shorter than the pulse lengths of a few nanoseconds used, a second mobile species could be observed as a slowly decaying after-pulse conductivity component for large pulses. This was attributed to proton conduction with a proton mobility of 6.4 x 10 cm /Vs in H,0 ice and a somewhat lower value in D2O ice. ° In the case of the proton, the mobility was found to have an apreciable negative activation energy of 0.22 eV. The motion and trapping of protons was tentatively explained in terms of an equilibrium between free protons and a proton complexed with an orientational L-defect. °... 

The important influence of the surface on the electronic properties of Degussa P25 particles was illustrated by the dramatic effect of a layer of isopropanol on the decay kinetics of electrons. This resulted in an increase in the mobile electron lifetime from a few hundred nanoseconds to seconds. In addition, on repetitive pulsing the end of pulse conductivity increased to a value corresponding to a mobility of ca 1 cm /Vs. These effects were attributed to the retardation of surface recombination with holes which are removed from the surface as protons by reaction with the alcohol. 

Because of the fascination of synthetic organic chemists and molecular electronics device designers with ever-increasing charge mobilities, attention was focussed mainly on the magnitude of the end-of-pulse conductivity of PR-TRMC transients. Because of this, the after-pulse decay kinetics in discotic materials received only scant attention. The dramatic influence of the nature of the peripheral chains on the lifetime of the PR-TRMC conductivity transients, was in fact demonstrated early-on for octa-alkoxy phthalocyanine derivatives as mentioned previously in this section. This effect is illustrated with more recent data for some hexa-alkyl HBC derivatives in Fig. 7. 

K. J. Caserta, F. J. Holler, S. R. Crouch, and C. G. Enke. Computer controlled bipolar pulse conductivity system for applications in chemical rate determinations. Anal. Chem., 50,1534,1978. 

In effect, the measured current flow is always due to an instantaneous potential. Other detectors use a bipolar pulse conductance technique [4, 5). The technique consists of the sequential application of two, short (about 100 ps) voltage pulses to the cell. The pulses are of equal magnitude and duration and opposite polarity. At ejacdy the end of the second pulse, the cell current is measured and the cell resistance is determined by applying Ohm s law. Because an instantaneous cell current is measured in the bipolar pulse technique, capacitance does not affect the measurement and an accurate cell resistance measurement is made. 

Optical absorption spectrophotometry is probably the most commonly used technique [4,a]. Reaction cells are similar to those used in flash work. Photomultipliers cover the uv-visible range the initial photoelectric signal is amplified internally, by an amoimt controlled by selection of the number of dynodes. Nanosecond equipment is commercially available. Picosecond time-resolution has been achieved [l,h]. For the infrared and Raman region, semiconductor photodiodes cover the range 400-3000 nm the vibrational spectra yield structural information about transient species much more detailed and precise than that from electronic spectra. Resonance enhancement of Raman spectra increases their intensity by a factor of 10, and makes them attractive for detection and monitoring [4,b]. They can be recorded with time-resolution down to sub-nanoseconds. Fluorescence detection is sensitive, and fast with single-photon counting or a streak camera (Section 4.2.4.2), it has been used for times down to 30 ps after an electron pulse. Conductivity also provides a fast and sensitive technique [4,c,d,l,m], especially in hydrocarbon solutions, where... 

This reaction has recently been studied as a function of pressure and temperature in several solvents (Nishikawa et al., 1988). A pulse conductivity technique was used which allowed determination of both the... 

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