Primary and Secondary Ions

At a finite gas pressure and in the condensed phase, secondary ions are generated by reactions of primary ions with neutral gas molecules. 

Further references Meisels and Ethridge (1972) Miyajima et al.(1974) Miller and Boring (1974) Jones (1973) Bichsel (1974) Takahashi et al. (1975) Jesse and Platzman, (1962) and Jesse (1964). W values reported are for a-radiation except where indicated in the parenthesis. In general, W values obtained with /3-radiation is smaller. bI determined spectroscopically. 

Secondary ions formed in an excitation transfer process—namely, A + B— A + B+ + e—require that E(A) I(B), where E(A) is the excitation 

TABLE 4.2 Appearance Potentials of Some Parent and Fragment Ions 

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One of the benefits of catalytic cracking is that the primary and secondary ions tend to rearrange to form a tertiary ion (a carbon with three other carbon bonds attached). As will be discussed later, the increased stability of tertiary ions accounts for the high degree of branching associated with cat cracking. 

To integrate one must know ts, which of course is a function of tp, and the form of this function depends upon the mechanism assumed for Reaction P. At this point we restrict Reaction P to a hydrogen transfer reaction in which the transferred species may be either a proton, hydrogen atom, or hydride ion and for which the masses of the primary ion, the molecule, the secondary ion, and the neutral fragment are identical and large compared with the transferred hydrogen. Three situations must be considered where the type of collision is defined by the relationship between uP and vQ, the velocities of the primary and secondary ions ... 

Table III. Reactivities of Primary and Secondary Ions with Acetylene...

The method is based on the principle of chromatographic separation of components of a mixture on a GC column, followed by their identification from their mass spectra. The compounds are separated on a suitable GC column, following which, the components eluted from the column are subjected to electron-impact or chemical ionization. The fragmented and molecular ions are identified from their characteristic mass spectra. Thus, the substances present in the sample are determined from their characteristic primary and secondary ions and also from their retention times. 

In 1949 Herzog and Viehbock reported a novel ion source for mass spec-trography (Fig. 4.2) [9]. This source provided separate accelerating fields for the primary and secondary ions and thus became the first modem instrument designed specifically for SIMS. The design included acceleration of the positive secondary ions from an equipotential surface through an electric field acting as an electron-optic lens. 

The role of "silent" species, in particular, hydrogen atoms, "dark" states, proton adducts, and carbonium ions. How do these species form and interact with each other, excited states, and primary and secondary ions Are there chemically-significant short-lived intermediates, such as carbenes and biradicals or some other unrecognized species ... 

Alternatively, halogenated hydrocarbons may be analyzed by GC/MS using a purge and trap technique (EPA Methods 624, 524, and 8240) or a thermal desorption method. GC/MS analysis is a confirmatory test in which the primary and secondary ions characterizing the compounds can be identified and quantified. 

Perhaps the most widely used method of identifying the precursor of a given secondary ion employs the matching of appearance potentials of the primary and secondary ions. Obviously, a secondary ion must have the same appearance potential as its precursor. This provides a simple and direct method of identifying the ions taking part in a certain reaction. The method suffers, however, from some very serious limitations. Thus,... 

Because of the high reactivity of primary and secondary ions under these conditions, the alkylation reaction is complicated by hydride transfer and related competing reactions. However, in this mechanism it is implicit that an energetic primary cation will react directly with methane or ethane. This opens the door to new chemistry through activation of these traditionally passive molecules. 

As stated earlier, the magnetic field at which each ion passes through resonance (when the magnetic field is swept) is directly proportional to mass. Since the drift velocity is inversely proportional to B, then Tj and for the respective primary and secondary ions will differ, and are related as follows ... 

In this case even though i and j are the same ion, the intensities of i and j due to each primary are not necessarily equivalent and will not cancel. As discussed in Section 3.9 the primary- and secondary-ion mass factors in Eq. (64) will also depend on whether the ion density in the cell is sufficiently large for space charge to become an important consideration. 

If the primary ion is very reactive, its intensity should be approximately correct. However, if the secondary ion is not too reactive, it can make a number of nonreactive collisions, and still satisfy the requirement of not reacting used in developing Eq. (58). Since collisions of any type will interrupt power absorption, the observed intensity would then be less than the true intensity for the secondary ion. Hence, calculation of a rate constant using Eqs. (55) and (58) and the observed ICR intensities for the primary and secondary ions would yield a rate constant which is too small in this instance. 

Consider a reaction sequence similar to those described by reactions (H) and (I), but where a number of primary and secondary ions are present. If a primary ion reacts to give a secondary the secondary ion current Si(t) is related to by Eq. (68) ... 

Fig. 11. Typical double-resonance spectra for reactions under conditions of low conversion to product. The extent of conversion is determined by the relative intensities of primary and secondary ions in the mass spectrum. Curves 1 -4 are typical of exothermic reactions. Curves 5 and 6 are typical of endothermic reactions. The difference between endothermic and exothermic reactions in a double-resonance experiment is that no product is present off resonance in the endothermic case. (Curves 5 and 6.)...

The potential across the probe orifice may vary in such a way that the probe functions as an electrostatic lens. The falsification of the sample is produced by the lens deflecting the ions that diffuse to the lens region away from the orifice. Under such conditions, nearly all the observed ions are formed in the lens. The collection of ions almost exclusively from a region near the walls produces a short reaction time, with the result that the variation of primary and secondary ions with pressure resembles that obtained using mass spectrometer ion sources as reactors. Figure 4 gives the variation in concentration of the ions in reactions (21) and (22) below with pressure. 

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