Negative-ion Cycles

Thermochemical information about neutral species can also be obtained from measurements of ions. Indeed, accurate bond dissociation energies for neutral molecules have been obtained from gas-phase ion chemistry techniques. In this section, we will summarize both the negative-ion and hydride-affinity cycles involving silicon hydrides (RsSiH) which are connected to electron affinity (EA) and ionization potential (IP) of silyl radicals, respectively [22-24]. 

Thermodynamic properties related to RsSiH can be obtained from negative-ion gas-phase studies. The following thermochemical cycle (cf. Scheme 2.1) ... 

To obtain an estimate of the combined enthalpies of hydration of such cations and anions, the thermodynamic cycle shown in Figure 2.6 is helpful. In Figure 2.6 it is assumed that the element X is a halogen and forms a uni-negative ion. 

A plasma potential that is positive with respect to electrode potentials is primarily a consequence of the greater mobility of electrons compared with positive ions. When there are many more negative ions than electrons in the plasma (e.g., in highly electronegative gases), plasma potentials are below electrode potentials, at least during part of the rf cycle (19b). 

The thermochemistry of silicon-containing anions has very recently been compiled in an excellent review by Damrauer and Hankin4, as well as in an earlier work by Damrauer3. In these reviews the authors give a detailed introduction into the experimental techniques as well as the cycles used to obtain thermodynamic data from negative-ion gas-phase chemistry. We will therefore confine ourselves here to the discussion of a few exemplary cases, and for a more detailed overview the reader is referred to the above-mentioned publications and the literature cited therein. 

The electron affinities of elements (Chap, 7) that form negative ions may be calculated by considering the formations of compounds containing such negative ions. The formation of such a compound from the elements (the heat of such a reaction being directly measurable) may be broken down into a series of simpler steps. The treatment is again called a Born-Haber cycle and is analogous to the treatment of the conversion of an alkali metal to its hydrated ion (discussed in Chap. 6). Consider the formation of sodium chloride from the elements ... 

Llneberger, W.C. and Engelking, P.C., "Negative Ion Energetics," Conference on High Temperature Sciences Related to Open Cycle, Coal-Fired MHD Systems Argonne National Laboratory, April 1977. 

Negative-ion gas-phase photoelectron spectroscopy has also recently been nsed to evalnate the reorganization energies of tetrahedrally coordinated Fe+ anions that serve as models for the active sites of mononuclear iron sulfur proteins such as rubredoxin that cycles between high spin Fe+ and high spin... 

The electron affinities Ea of the main group atoms are the most precisely measured values. Recall that the Ea is the difference in energy between the most stable state of the neutral and a specific state of a negative ion. It was once believed that only one bound anion state of atoms and molecules could exist. However, multiple bound states for atomic and molecular anions have been observed. This makes it necessary to assign the experimental values to the proper state. The random uncertainties of some atomic Ea determined from photodetachment thresholds occur in parts per million. These are confirmed by photoelectron spectroscopy, surface ionization, ion pair formation, and the Born Haber cycle. Atomic electron affinities illustrate the procedure for evaluating experimental Ea. 

First, let s note that experimentally the EA stands as a wild quantity being mostly indirect measured, for instance trough the Haber-Bom cycle and only recently improved since the advent of the laser photo-detachment experiments with negative ions (Hotop Lineberger, 1985) and by the electron transmission spectroscopy (Jordan Burrow, 1987). 

Figure 3.7. Opraation of a quadrupole in (A) the Tz-plane and B) the yz-plane. In the xz-plane, the positive rods act as a high-pass fito for positive ions. The ions convrage toward the z-axis (at point a) dne to the action of the positive dc potential and are defocused due to the action of the negative half-cycle of the rf potential. Similar actions make the negative pair of rods low-pass filto- in the yz-plane.

Fig. 7.3 ToF-SIMS depth profiles (negative ions) of Si nanowire electrodes, cycled in a Li metal cell with 1 M LiPF s in 1 1 EC DMC. Reproduced with permission from [63] copyright 2(X)6,...

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