Apparent ion current

It was found that the apparent ion current I j decreases with an- increase in the film thickness d. A question arises as to whether the decrease in I i is an apparent phenomenon due merely to variation of potential on the probe surface or a real phenomenon based on the direct contact of polymer instead of metal with plasma. 

The effects of glow-discharge polymerized polystyrene films on the I-V characteristics of single probe were studied in nitrogen gas, where no polymerization occurs. It was found that the apparent ion current lai flowing into the single probe decreases inversely with the film thickness as was previously reported with the double probe. 

Figure 10. The apparent ion current I i as a function of R with a fixed capacitance of 0.625 fiF(a) and 0.005 fiF(b). The measurements were made with no apparent contamination, but with C—R circuit substituted in place of the contamination layer. (9) 100 Hz, (O) 0.5 Hz.

However, a maximum is reached at about 15 volts, and a decrease is observed at higher voltages. With increasing voltage, the kinetic energy of the primary ions is also increased. The increase in primary ion current apparently is counterbalanced and finally exceeded by the decrease in the cross-section of the ion molecule reaction. In addition, discrimination of CH5 + ions which are formed via a complex becomes more effective. At a potential of 20 volts between trap and chamber, most of the primary ions have kinetic energies around 20 e.v., and a secondary CH5+ ion... 

Similarly, the m/z = 60 ion current signal was converted into the partial current for methanol oxidation to formic acid in a four-electron reaction (dash-dotted line in Fig. 13.3c for calibration, see Section 13.2). The resulting partial current of methanol oxidation to formic acid does not exceed about 10% of the methanol oxidation current. Obviously, the sum of both partial currents of methanol oxidation to CO2 and formic acid also does not reach the measured faradaic current. Their difference is plotted in Fig. 13.3c as a dotted line, after the PtO formation/reduction currents and pseudoca-pacitive contributions, as evident in the base CV of a Pt/Vulcan electrode (dotted line in Fig. 13.1a), were subtracted as well. Apparently, a signihcant fraction of the faradaic current is used for the formation of another methanol oxidation product, other than CO2 and formic acid. Since formaldehyde formation has been shown in methanol oxidation at ambient temperatures as well, parallel to CO2 and formic acid formation [Ota et al., 1984 Iwasita and Vielstich, 1986 Korzeniewski and ChUders, 1998 ChUders et al., 1999], we attribute this current difference to the partial current of methanol oxidation to formaldehyde. (Note that direct detection of formaldehyde by DBMS is not possible under these conditions, owing to its low volatility and interference with methanol-related mass peaks, as discussed previously [Jusys et al., 2003]). Assuming that formaldehyde is the only other methanol oxidation product in addition to CO2 and formic acid, we can quantitatively determine the partial currents of all three major products during methanol oxidation, which are otherwise not accessible. Similarly, subtraction of the partial current for formaldehyde oxidation to CO2 from the measured faradaic current for formaldehyde oxidation yields an additional current, which corresponds to the partial oxidation of formaldehyde to formic acid. The characteristics of the different Ci oxidation reactions are presented in more detail in the following sections. 

At low pressures the measuring range is limited by two effects by the X-ray effect and by the ion desorption effect. These effects results in toss of the strict proportionality between the pressure and the ion current and produce a tow pressure threshold that apparently cannot be crossed (see Fig. 3.14). 

The interface between the polar phospholipid headgroups and the aqueous electrolyte solution provides a membrane surface which contains weakly selective cationic binding sites. (10) This generates a reservoir of cations available for conduction and is apparently much more important than bulk solution Ion content in the determination of permion and ion current density. (11)... 

Support for the general mechanism outlined in Scheme 7-14 is provided by gas-phase Fourier-transform mass-spectrometric studies of the anionic reaction products of several substrates with Oa - (produced by electron impact with Oa HO can be produced by electron impact with HaO). 0,51 in these experiments neutral products are not detected. Both Oa - and HO react rapidly with 1,2-diphenylhydrazine in the gas phase (P = 10 7 Torr) to give the anion radical of azobenzene (PhN -NPh m/e = 182) and the anion from deprotonation (PhN NHPh m/e = 183), respectively. When Oa - is ejected from the experiment, the peak at m/e = 182 disappears. In contrast to the exponential decay that is observed for the HO peak with time, the ion current for Oa - decays to a steady-state concentration. Apparently, the PhN -NPh product reacts with residual Oa (which cannot be ejected from the FTMS cell) to give Oa - and azobenzene in a process that is analogous to the (Oa l-induced auto-oxidation in aprotic solvents (Scheme 7-14). 

Similar trends in variation of the intensity of ion currents with sputtering depths were observed for Al-doped AILS (Fig. 45). In this case, the increase of La ion current is more pronoimeed than in the case of Sr-doped apatite. It apparently correlates with admixture of perovskite-like LaAIOs phase whieh can be located in the surface layer of apatite partieles. Indeed, in the densely-packed perovskite-like phase known for its high lattice stability, sputtering is expected to proeeed less easier than in the case of framework apatite structure. A higher bonding strength of eations in the surface layer revealed by SIMS and explained by preferential location of admixed phases in it is expected to be reflected in a lower grain boimdary eonductivity. 

Now we can, in part, answer our question about the parameters which influence the selectivity of an electrode. The ionic species which displaces the greatest number of charge carriers by entering a new phase determines the electrode potential, provided that no other processes are involved, such as diffusion of surface ions into the phase interior. In such a case the slowest partial reaction, as always in kinetics, controls the total reaction. Both cases are found with ion-selective electrodes. The table below shows some apparent exchange current density estimates for ion-selective electrodes [448]. 

This gas is electronegative and its molecules quickly absorb the free electrons in the arc path between the contacts to form negatively charged ions. This apparent trapping of the electrons results in a rapid build-up of dielectric strength after a current zero. The detailed. sequence of arc extinction may be summarized as follows. 

At a the net anodic reaction rate is zero (there is no metal dissolution) and a cathodic current equal to I" must be available from the external source to maintain the metal at this potential. It may also be apparent from Fig. 10.4 that, if the potential is maintained below E, the metal dissolution rate remains zero = 0), but a cathodic current greater than /"must be supplied more current is supplied without achieving a benefit in terms of metal loss. There will, however, be a higher interfacial hydroxyl ion concentration. 

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