Cross Sections and Rate Coefficients

Ion Processes

The needs for fundamental data on ion processes can be subdivided by energy range as indicated in Table 6.1. The division in energy range is based on two premises. First, the ion distribution in the bulk plasma for the majority of plasma processing reactors can be well described by a Maxwellian or moderately drifting Maxwellian. The temperatures are usually less than 0.1 eV. Transport coefficients and cross sections for processes in this range of energies are usually measured using a swarm or drift tube technique, and characterized by a "random" temperature. Ion energies greater than 0.1 eV are usually found only in the presheath or sheath regions of the plasma. Cross sections for collisional processes in the higher energy range are measured by beam techniques, and usually characterized by a directed energy.

TABLE 6.1 Categorization of Data Needs for Ion Processes

Process

Thermal

Superthermal

 

(e < 0.1 eV)

(0.1 < e < 1 keV)

Momentum transfer

Ion-molecule and charge exchange

Ion-ion neutralization

 

Electron-ion recombination

 

Ion-neutral and neutral-neutral excitation

 

Momentum Transfer

Momentum transfer collisions are elastic and/or inelastic processes resulting in a change in momentum and in which the identity of the ion does not change.

Beam-measured cross sections should be resolved in energy and angle. Swarm-measured cross sections or rate coefficients should be temperature dependent.

The availability of ion swarm parameters (usually mobilities, diffusion coefficients, and characteristic energies vs. E/N) is good for ions of interest in atmospheric or combustion applications, and poor for ions of interest to plasma processing. These data are typically scattered in the literature. A compendium of ion transport coefficients dating to 1984 is available from Ellis et al.1

Ion-Molecule and Charge Exchange Reactions

Collisions resulting in transfer of positive or negative charge from the incident ion are ion-molecule or charge exchange reactions.

If the identity of the ion does not change, this process is known as a symmetric charge exchange and is sometimes difficult to distinguish from a momentum transfer collision. Beam-measured cross sections should be resolved in energy and angle. Swarm-measured cross sections or rate coefficients should be temperature dependent. Distribution of products from the target molecule should also be identified.

The availability of data for thermal ion-molecule and charge exchange reactions for molecules of atmospheric or combustion interest is relatively good. Many compendia are available, such as those of Sieck and Lias,2 Albritton,3 and Ikezoe et al.4



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