Collision cross-sections charge transfer

Fig. 8. Relative velocity dependence of integral cross sections calculated for Na + O collisions for the indicated exit channels. The solid curve is the charge transfer cross section calculated using a multichannel Landau-Zener formalism (see text). The dashed curve is the two-state Landau-Zener cross section. Charge transfer calculations by van den Bos are indicated by triangles. Full circles and squares are the respective excitation channels as determined using the multichannel Landau-Zener model.

Despite the fact that Bohr s stopping power theory is useful for heavy charged particles such as fission fragments, Rutherford s collision cross section on which it is based is not accurate unless both the incident particle velocity and that of the ejected electron are much greater than that of the atomic electrons. The quantum mechanical theory of Bethe, with energy and momentum transfers as kinematic variables, is based on the first Born approximation and certain other approximations [1,2]. This theory also requires high incident velocity. At relatively moderate velocities certain modifications, shell corrections, can be made to extend the validity of the approximation. Other corrections for relativistic effects and polarization screening (density effects) are easily made. Nevertheless, the Bethe-Born approximation... 

The only reactions of SiCl4 with He+, Ne", and Ar+ are dissociative charge-transfer processes (Fisher and Armentrout, 1991b). The example of the Ar+ system (statistical distribution of spin-orbit states) is shown in Fig. 12. The total reactivity in all three systems is fairly high, with total cross sections that are comparable to the collision cross section at all energies. All SiCl+ ( = 0 to 4)... 

Fig. 22. Excitation function for the total charge-transfer cross section for the reactants Ar + CH4. Open circles refer to data obtained by the longitudinal tandem/pulsed ejection technique illustrated in Fig. 3. Solid circles refer to data obtained by the single-source impulse technique discussed in Section 3.4.4c. The relative excitation function of Koski is also shown and is normalized to Masson s absolute excitation function at 10 eV. Shown as a dashed line is the close-collision cross section predicted from the Langevin theory.

If the Langevin cross section or any other close-collision cross section is to be used to set an upper bound on the cross section of a charge-transfer reaction, it must be shown, by velocity analysis of the products, or from their angular distribution, that the trajectories responsible did indeed cross the centrifugal barrier. 

In Figure 2, we show the total differential cross-section for product molecules in the vibrational ground state (no charge transfer) of the hydrogen molecule in collision with 30-eV protons in the laboratory frame. The experimental results that are in arbitrary units have been normalized to the END... 

During slow collisions, the main contribution to the charge transfer cross-section is made by the impact parameters which exceed the size of a neutral atom. In this case, the potential barrier for tunneling is mainly formed by the electric field of the multiply charged ion in the vicinity of the neutral atom (Fig. 8). This field is equal to F = zjR2. The probability of... 

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