Charge exchange

The formation of an ion M+ can be accomplished through charge exchange between its neutral M and another ion X+, viz. 

Under favourable conditions, the internal energy E of M+ is given by [528, 541, 542] 

The experiments are best performed using tandem mass spectrometers. One mass spectrometer is used to mass-select the incident ion X+, which is subsequently retarded to the required velocity before collision with the gaseous neutral M. The ions M+ and its ionic fragmentation products are extracted from the collision chamber, either perpendicularly or in-line (longitudinally) with the incident beam and mass-analysed with the second mass spectrometer. The incident ion, X+, energies may be as low as 0.1 eV, which loosely corresponds to a translational temperature of something like 800 K. 

Breakdown diagrams can be constructed on the basis of charge exchange mass spectra [812, 813]. In the ideal case, these would be identical to those obtained by photoion—photoelectron coincidence, provided the sampling intervals, and t2 in eqn. (9), were the same in the two experiments. 

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There are two basic physical phenomena which govern atomic collisions in the keV range. First, repulsive interatomic interactions, described by the laws of classical mechanics, control the scattering and recoiling trajectories. Second, electronic transition probabilities, described by the laws of quantum mechanics, control the ion-surface charge exchange process. 

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. 

Charge-exchange reaction. Synonymous with charge-transfer reaction. 

Ion Scattering Spectrometry Low-Energy Ion Scattering Resonance Charge Exchange... 

For an ideally polarizable electrode, q has a unique value for a given set of conditions.1 For a nonpolarizable electrode, q does not have a unique value. It depends on the choice of the set of chemical potentials as independent variables1 and does not coincide with the physical charge residing at the interface. This can be easily understood if one considers that q measures the electric charge that must be supplied to the electrode as its surface area is increased by a unit at a constant potential." Clearly, with a nonpolarizable interface, only part of the charge exchanged between the phases remains localized at the interface to form the electrical double layer. 

Charge Exchange and Ion-Molecule Reactions Observed in Double Mass Spectrometers... 

Tn a double mass spectrometer several types of ion-molecule reactions - can be observed (a) charge exchange, A++B- A + B+, often followed by dissociation of B+ (b) transfer of part of A+ or B (e.g., proton transfer or hydride ion transfer) during the collisions (c) reactions at increased pressure in the collision chamber. 

In charge exchange collisions the cross-section depends upon the energetics of the reaction. To compute the energy defect, the initial and final states of the colliding particles must be specified. This can be done easily for the bombarded neutral molecule, which usually can be assumed to be in the ground state before the collision, but not for the incident ion which is often in one of its metastable states. 

Radius of mass spectrometer A is only 2.5 cm so that very low velocities of the ions (A) can be used. In the collision chamber, k, reaction products (ions B) are formed from the gas by charge exchange or ion-molecule reactions. All ions move in the same direction in the collision chamber and are accelerated by special electrostatic lenses hence, they all reach the slit r of mass spectrometer B (not shown) independently of their initial velocities in the collision chamber. The discrimination in mass spectrometer B can therefore be considered negligible... 

The selection rules obviously break down if the charge exchange takes place at very small distances between the colliding particles. 

If the recombination leaves the atom in a high atomic state, the recombination energy will be so low that charge exchange cannot take place. Such recombination energies are not included in Table II. 

Table II must be used with care in anomalous cases in which the transition probability for ionization of the molecule is very low in some energy ranges (e.g., acetylene, benzene, methylamine). In such cases higher RE s, not included in the table and normally of small importance, may be responsible for the charge exchange processes although with small cross-sections (cf. 9, 13).

If charge exchange occurs when the incident positive ion passes the neutral gas molecule with a certain velocity, transfer of translational energy will usually take [place. This transfer of translational energy... 

If no transfer of translational energy occurs, then the charge exchange process probably takes place when the distance between the ion and the molecule is large. This means, however, that the ion and the molecule can be considered as isolated from each other, and therefore, the recombination process of the ion and the ionization process of the molecule must obey the spectroscopic transition laws. On the other hand, if a large transfer of translational energy takes place, then the process probably takes place when the distance is small, and possibly then all selection rules break down. 

Maier (50) also investigated the charge exchange between Xe + and C2H4. These reactions were studied previously by Tal roze et al. (29, 30, 31, 32) in a perpendicular type apparatus in which the geometry seems to result in a smaller discrimination in the second mass spectrometer than in the Stockholm apparatus. Finally, the same reactions were observed in Stockholm by Szabo (19) during a detailed investiga-... 

If a charge exchange process, A + + B- A -f- B +, occurs when the distance between the two particles is large, we expect that no transfer of translational energy takes place in the reaction and that the same selection rules govern the ionization as in spectroscopic transitions. This means that if the molecule B is in a singlet state before the ionization, the ion B + will be formed in a doublet state after ionization of one electron without rearrangements of any other electrons, at least for small molecules. 

Processes of this type can be expected to predominate when using a perpendicular type apparatus in fact, it has been possible recently to observe the validity of selection rules when ionizing C02 and H20 by charge exchange (16, 17). 

Our finding that selection rules govern the ionization by means of charge exchange is interesting since other recent investigations seem to show that no selection rules are valid. 

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