Charge exchange collision

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. 

E.E.Nikitin and AI.Reznikov, Theoretical total cross section and branching ratio for Kr ions produced in low-energy charge-exchange collisions of Kr with He, Chem.Phys.Lett. 149,212 (1988)... 

Wilmenius, P and B. Lindholm Dissociation of Methanol Molecule Ions Formed in Charge Exchange Collisions with Positive Ions. Ion-Molecnle Reactions of Methanol with very Slow Positiv Ions. Arkiv Phys. 21, 97 (1962). 

Not many results have been reported so far on atom-atom charge exchange collisions in the eV range (see Tables IV and VI), but those which have been obtained are extremely interesting. Total cross sections have been measured for Cs on O and Na, Li on I, whereas differential cross sections only have been reported for Na + I. 

H. von Koch, Dissociation of ethane molecule ions formed in charge exchange collisions with positive ions. Ion-molecule reactions of ethane, Arkiv Fysik 28, 559-574 (1965). 

Vi, accounts for velocity losses due to elastic and charge-exchange collisions of the rrii ion with mi neutral molecules, while the second term, -( f2 + 12)accounts for velocity losses due to collisions of ions with m2 neutrals. The last term accounts for velocity gains for mi ions resulting from charge-exchange collisions of 1712 ions with mi neutrals. 

In table I, only such metastable ionic states have been included for which the lifetime is at least 10 sec. If in applications the ions are produced in the gas itself, the time between the formation of the ion and the charge-exchange collision may be shorter. In such cases, other excited ionic states and other recombination processes must also be considered. 

The charge exchange collisions may result in highly excited states of the neutral species. Often the population of these excited states accumulates in metastable states with long lifetimes. This opens the possibility of performing laser spectroscopy on transitions between excited states. 

In the application of a dc potential, often the applied voltage and current (power - watts/cm ) to the surface are used as process parameters and control variables. However, it must be realized that the bombarding ions generally have not been accelerated to the full applied potential due to the position of their formation, charge exchange collisions, and physical collisions in the gas. The measured current consists of the incident ion flux (the ions may be multiply charged) and the loss of secondary electrons from the surface. The cathode power is a useful process parameter to maintain reproducibility only if parameters, such as gas composition, gas pressure, system geometry, etc., are kept constant. 

Cross-section The physical area in which an interaction can take place. Examples Cross-section for physical collision (sum of the radii of the particles) cross-section for electron-atom ionization cross-section for charge exchange collisions. 

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