Ionization by charge transfer

The single most unique characteristic of the SIMS technique is its sensitivity. It can be as good as one part per billion (ppb). For example, if silicon is sputtered at a rate of 10 A/sec over an area of 100-pm x 100-/zm, then 10"u cm3/sec of material is removed. Given the density of silicon, this reduces to approximately 5 x 1011 atoms/sec. If 1% of these atoms are ionized (by charge transfer with the surface) and 10% of those ionized are collected in the mass spectrometer, then the measured ion intensity will be 5 x 10s ions/sec. If we assume we can distinguish 5 ions/sec, then a detection sensitivity of 1 part in 108 is achievable. This sensitivity is many orders of magnitude better than other techniques. 

Two general mechanisms are usually advanced to explain ionization of molecules in flames direct ionization by thermoionization, photoionization, or chemiionization and indirect ionization by charge transfer with other ions. The assessment of both mechanisms requires knowledge of the ionization potential of molecules. In the following discussion, computations developed in the Appendix are used to estimate approximate ionization potentials of polynuclear aromatic hydrocarbons. 

Ionization by Charge Transfer. Many ion-molecule reactions between small species (< 50 amu) occur in flames (24), As mentioned above, the only primary ion in fuel-lean flames (non-sooting) is CHO", formed by chemi-ionization (13, 24, 25). 

In our third example (52), dissociative chemisorption of Li2, B2, C2, 02, N2, F2, CO, NO and ethylene on (100)W and Ni surfaces was examined. The metal surfaces are represented by means of nine-atom clusters, arranged as in Fig. 35. Experimental geometry was used for the adsorbates. The standard EHT method was used, i.e. with charge-independent atomic ionization potentials. Charge transfer between adsorbate and surface was explored... 

Similar interface to that used for ESI. In APCI, a corona discharge is used to ionize the analyte in the atmospheric pressure region. Ions are formed by charge transfer from the solvent as the solution passes through a heated nebulizer into the APCI source... 

Figure 3.9a may also represent the interaction of a nonbonded ( lone-pair ) orbital with an adjacent polar n or a bond [67]. If a polar n bond, one can explain stabilization of a carbanionic center by an electron-withdrawing substituent (C=0), or the special properties of the amide group. If a polar a bond, we have the origin of the anomeric effect. The interaction is accompanied by charge transfer from to A, an increase in the ionization potential, and a decreased Lewis basicity and acidity. These consequences of the two-electron, two-orbital interaction are discussed in greater detail in subsequent chapters. 

Therefore, one might expect similar long-range charge transfer effects in the radiolysis of solid n-hexane. In irradiated hexane possible electron donors to the hexane ion radical are the hexenes, dimers, and hexyl radicals, all of which have lower ionization potentials than n-hexane. Additives of lower ionization potential than n-hexane would also be expected to act as electron donors. Another approach is to use a material that has a higher ionization potential than n-hexane to form a mixture in which the n-hexane fraction is sma l and to study the effects produced by charge transfer to the n-hexane. 

Because of its low ionization potential, NO+ is a relatively easily formed ion and can be produced by charge transfer to NO. Thus, this ion is an important one, both in the upper atmosphere and in ballistic missile reentry wakes. 

Figure 36. Electron spectrum for transfer ionization system He+-Ba at various collision energies.110 Indicated a tentative identification of some autoionizing states of Ba+ formed by charge transfer to He+.

Scheme 35 Formation of the arene-olefin (Ar-Ol) exciplex and its deactivation by charge-transfer effects induced by changes of olefin ionization potential (IP) and solvent polarity according to Leismann et al. (From Ref. 182.)...

We have demonstrated that Nj may form a salt with Nj [5] although side-reactions like the formation of covalently bound N10 or loss of N2 from the anion may pose a problem. We found that the kinetic stability of Nj is decreased when charge is diminished by charge transfer from the anion. Since the vertical ionization potential of Nj (5.6 eV [5,11]) is lower than the vertical electron affinity of N5, we also explore derivatives of Nj that may have a larger ionization potential and/or greater kinetic stability. 

The [Ruv(N40)(0)]2+ complex is shown to oxidize a variety of organic substrates such as alcohols, alkenes, THF, and saturated hydrocarbons, which follows a second-order kinetics with rate = MRu(V)][substrate] (142). The oxidation reaction is accompanied by a concomitant reduction of [Ruv(N40)(0)]2+ to [RuIII(N40)(0H2)]2+. The mechanism of C—H bond oxidation by this Ru(V) complex has also been investigated. The C—H bond kinetic isotope effects for the oxidation of cyclohexane, tetrahydrofuran, propan-2-ol, and benzyl alcohol are 5.3 0.6, 6.0 0.7, 5.3 0.5, and 5.9 0.5, respectively. A mechanism involving a linear [Ru=0"H"-R] transition state has been suggested for the oxidation of C—H bonds. Since a linear free-energy relationship between log(rate constant) and the ionization potential of alcohols is observed, facilitation by charge transfer from the C—H bond to the Ru=0 moiety is suggested for the oxidation. 

Thus, the negative APPI mode will produce molecular ions from the analytes by charge exchange or by electron capture if they have a sufficient electron affinity. Analytes that have high gas-phase acidity will be mainly ionized by proton transfer to yield deprotonated molecular ions. 

In ionizing solvents, atoms and free radicals arc capable of being oxidized or reduced by charge transfer, and their modes of reaction can become very complex. One of the systems in which redox reactions involving atoms and radicals have been extensively studied is that between Fe" "" and TT2O2. When equimolar, acidic solutions (<0.01 M) of Fe " and H2O2 are mixed together, there is a nearly quantitative reaction... 

Whatever the case may be, the excellent electron-donating properties of phenothiazine are well-established. The ionization potentials are low, as indicated indirectly by charge-transfer spectra and directly by photoelectric work in the solid state, firom which the value 4.36 ev was derived for the ionization potential. ... 

Sensitization by charge transfer requires that the LUMO of the diacetylene monomer be below the excited singlet level of the dye. The latter is at Is — Eg where I, and Eji are ionization potential and energy of the first singlet state of the dye, respectively. On the basis of polarographic data I, — E j —2.7. .. —3.3 eV is estimated for cyanine dyes The lowest unoccupied orbital of a previously neutral diacetylene... 

A most careful study to probe experimentally the effect of internal energy on the structure and reactivity of ionized cyclopropane was recently published by Lias and Buckley They generated by charge transfer to cyclopropane from ... 

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