Atom-ion reactions

LIF has been used to study state-selected ion-atom and ion-molecule collisions in gas cells. Ar reactions with N2 and CO were investigated by Leone and colleagnes in the 1980s [13, 14] and that group has... 

Figure Bl.7.12. A schematic diagram of a typical selected-ion flow (SIFT) apparatus. (Smith D and Adams N G 1988 The selected ion flow tube (SIFT) studies of ion-neutral reactions Advances in Atomic and Molecular Physics vol 24, ed D Bates and B Bederson p 4. Copyright Academic Press, Inc. Reproduced with pennission.)...

McDaniel E W and McDowell M R C (eds) 1972 Case Studies in Atomic Coiiision Physics (Amsterdam North-Holland) vol 2 McDaniel E W, i rmak V, Dalgarno A, Ferguson E E and Friedman L (eds) 1970 ion-Moiecuie Reactions (New York Wiley) Bates D R (ed) 1962 Afom/c and Moiecuiar Processes (New York Academic)... 

Charge-exchange (charge transfer) ionization. Occurs when an ion/atom or ion/molecule reaction takes place in which the chaise on the ion is transferred to the neutral species without any dissociation of either. 

Ionizing collision. An ion/neutral reaction in which an electron or electrons are stripped from the ion and/or the neutral species in the collision. Generally, this term describes collisions of fast-moving ions or atoms with a neutral species in which the neutral species is ionized. Care should be taken to emphasize if charge stripping of the ion has taken place. 

Another reaction mechanism, which is conveniently mentioned under this heading, is due to Hill [479] who suggested that ions (atoms or molecules) frorh the product may move through the dislocation network of the reactant and activate potential nuclei, particularly in the vicinity of the reaction interface. Thus a reaction zone, within which potential nucleusforming sites are activated, is developed in front of an advancing interface. With appropriate assumptions, this reaction model provides an alternative explanation of the exponential rate law, eqn. (8), which in Sect. 3.2 was discussed with reference to chain reactions. 

Reactions of D with D20 and of 0 with 02, N20, and N02 have been studied with a magnetic sector mass spectrometer. Competition between electron transfer and ion-atom interchange has been observed in the production of 02 by reaction of 0 with 02, an endothermic reaction. The negative ion of the reacting neutral molecule is formed in 02, N2Of and N02 but not in D20. Rate constants have been estimated as a function of repeller potential. 

Reaction 6 might conceivably proceed through either an ion-atom interchange mechanism (Reaction 7) or a charge transfer mechanism (Reaction 8),... 

Realizing that the last four reactions of the ion-atom interchange mechanism listed each have only one-half the statistical probability of occurring as do the first four and assuming no isotope effect on the rate constants, we can write the following set of rate equations ... 

Pulsed source techniques have been used to study thermal energy ion-molecule reactions. For most of the proton and H atom transfer reactions studied k thermal) /k 10.5 volts /cm.) is approximately unity in apparent agreement with predictions from the simple ion-induced dipole model. However, the rate constants calculated on this basis are considerably higher than the experimental rate constants indicating reaction channels other than the atom transfer process. Thus, in some cases at least, the relationship of k thermal) to k 10.5 volts/cm.) may be determined by the variation of the relative importance of the atom transfer process with ion energy rather than by the interaction potential between the ion and the neutral. For most of the condensation ion-molecule reactions studied k thermal) is considerably greater than k 10.5 volts/cm.). 

Reactions involving a transfer of a proton or a hydrogen atom are an extremely common type of ion-molecule reaction and are particularly suited for study by the pulsed source technique. The secondary ion will usually occur at an m/e ratio where it is not obscured by abundant primary ions, and the product and reactant ions frequently will differ only slightly in mass, thus minimizing discrimination effects. 

Table II. Proton and H Atom Transfer Ion-Molecule Reactions...

Thus we think of the chemical ionization of paraffins as involving a randomly located electrophilic attack of the reactant ion on the paraffin molecule, which is then followed by an essentially localized reaction. The reactions can involve either the C-H electrons or the C-C electrons. In the former case an H- ion is abstracted (Reactions 6 and 7, for example), and in the latter a kind of alkyl ion displacement (Reactions 8 and 9) occurs. However, the H abstraction reaction produces an ion oi m/e = MW — 1 regardless of the carbon atom from which the abstraction occurs, but the alkyl ion displacement reaction will give fragment alkyl ions of different m /e values. Thus the much larger intensity of the MW — 1 alkyl ion is explained. From the relative intensities of the MW — 1 ion (about 32%) and the sum of the intensities of the smaller fragment ions (about 68%), we must conclude that the attacking ion effects C-C bond fission about twice as often as C-H fission. 

Acetylene Ion. No evidence for the contribution of ion-molecule reactions originating with acetylene ion to product formation has been obtained to date. By analogy with the two preceding sections, we may assume that the third-order complex should dissociate at pressures below about 50 torr. Unfortunately, the nature of the dissociation products would make this process almost unrecognizable. The additional formation of hydrogen and hydrogen atoms would be hidden in the sizable excess of the production of these species in other primary acts while the methyl radical formation would probably be minor compared with that resulting from ethylene ion reactions. The fate of the acetylene ion remains an unanswered question in ethylene radiolysis. 

The rate constants for unimolecular dissociation of the intermediate ions suggested earlier indicate that all ions containing seven or more carbon atoms arise from reactions of the dissociation products of Steps 9, 13, and 17 when pressures are of the order of a few torr and of Step 20 and its analogues at pressures in excess of a few hundred torr. The product ions are generally quite complex, and the simple exothermicity rule given earlier will not apply. Thus, we may well expect that there will be inefficient ion-molecule reactions in the sequences originating with these ions as well. 

The positive charge should reside on a complex entity, and there is no ready means for assessing the products of the neutralization process. Although we know that neutralization must yield 3.8 intermediates/100 e.v., there is no chemical evidence for their contribution to the product distribution. This cannot be interpreted by neutralization yielding predominantly hydrogen atoms, ethyl radicals, or methyl radicals. One can quantitatively account for these intermediates on the basis of the distribution of primary species and second- and third-order ion-molecule reactions (36). 

The conclusions on the occurrence of ion-molecule reaction in the radiolysis of ethylene are not seriously affected by the uncertainties in the neutralization mechanism. It must be assumed that neutralization results in the complex species which constitute the ionic polymer, — i.e., the fraction of the ethylene disappearance which cannot be accounted for by the lower molecular weight products containing up to six carbon atoms. 

Argon ion-molecule reaction. . Atom transfer, elastic collisions with. 

Laser desorption FTMS is fundamentally different from SIMS because the desorption and ionization steps are separate. With FTMS, neutral atoms and molecules desorbed by the laser are ionized by the electron beam after they have moved about 3 cm away from the surface. As a result, complications Introduced into SIMS spectra by gas-phase reactions above the surface are minimized because neutral-neutral reactions are typically two-orders of magnitude slower than ion-molecule reactions. We believe, therefore, that laser desorption FTMS spectra are representative of the species actually present on the surface. 

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