The Reaction Group node () allows input of a large number of reactions of electron impact reactions and heavy species reactions. More than one Reaction Group can be added to a component.

COMSOL Multiphysics automatically ensures that the Reaction Group selection is the same as the Heavy Species Model or Plasma Model selection.

This section is used to specify which type of reactions the current Reaction Group feature should be used for. The option is either Heavy species reactions (default) or Electron impact reactions. This selection should be made prior to entering any values in the Reaction Table since this selectable input is only enabled when the table is empty.

The functionality in the sections Import Reactions from File and the Reaction Table depends on the reaction group type selection as explained below.

This section allows to import reactions from a text file into the table in the Reaction Table section. The reaction data must be in a specific format, referred to as reaction group blocks. A reaction group block can be used either for heavy species reactions or for electron impact reactions. All reaction group blocks start with the keyword REACTIONGROUP. After that, the type of the block needs to be defined using the keywords HEAVYSPECIESREACTIONS or ELECTRONIMPACTREACTIONS for heavy species reactions or electron impact reactions, respectively. For electron impact reactions, there are two ways to supply the reaction rates data. Therefore, following the keyword of ELECTRONIMPACTREACTIONS, keywords of either CROSSSECTIONDATA or ARRHENIUSPARAMETERS need to be added, to define the reaction rates using cross sections data or Arrhenius parameters respectively. After supplying the reaction rates data in different ways, all the reaction group blocks need to be ended with the keyword END.

If All unlabeled is selected, all the data blocks with no customized labels will be read and their data will be combined.

For the HEAVYSPECIESREACTIONS block, the reactions are set explicitly below the keyword. The formula is defined on the left of “!” and the Arrhenius coefficients are defined on the right of “!” in sequence: Forward frequency factor, Forward temperature exponent, Forward activation energy, Reverse frequency factor, Reverse temperature exponent, and Reverse activation energy. If the formula is not reversible (defined with =>), only the forward coefficients (3 parameters) are needed. If the formula is reversible (defined with <=>), both forward and reversible coefficients (6 parameters) need to be provided. If a coefficient is not needed to define the reactions it should be set as “0”.

For the ELECTRONIMPACTREACTIONS with CROSSSECTIONDATA block, one or more files of electron impact cross sections data need to be provided. The files need to be in the same folder as the main plasma chemistry file. All the data within these files will populate the same table in a Reaction Group feature.

For the ELECTRONIMPACTREACTIONS with ARRHENIUSPARAMETERS block, the reactions are set explicitly below the keyword. Each line represents one electron impact reaction. The formula is defined on the left of “!”, and the collision types, the energy loss coefficients as well as the Arrhenius coefficients are defined on the right of “!”. The first field after “!” must define the Collision type that can be Elastic, Excitation, Ionization, or Attachment. Following the Collision type field, it is needed to given the Electron mass ratio if the collision type is Elastic, and to give the Energy loss if the collision type is Excitation or Ionization. The last three fields defined the Arrhenius coefficients.

It should be noted that multiple different types (or same type) of reaction group blocks can be put into the same text file, and all the content will be read. However, based on the selection of the Reaction Group Type in the previous section, only the related reactions will be put into the Reaction Table. For example, if the selection is Heavy species reactions, the reactions in all the HEAVYSPECIESREACTIONS blocks in the text file will be put into the same Reaction Table in the current Reaction Group feature, and the same rule also holds for Electron impact reactions.

If one wants to automatically put different reaction sets into different Reaction Group features and label them accordingly, it is recommended to use the Plasma Chemistry Import feature.

Based on the selection of Reaction Group Type, the Reaction Table has different appearance with different number of columns.

For a reaction table of type Heavy species reactions, the columns from left to right are for Reaction sequence number, Reactant, Product, Forward frequency factor, Forward temperature exponent, Forward activation energy, Reverse frequency factor, Reverse temperature exponent, Reverse activation energy.

For a Reaction Table of type Electron impact reactions, the columns from left to right are for Reaction sequence number, Reactant, Product, Collision type, Electron mass ratio, Energy loss, Specify reaction using, Forward frequency factor, Forward temperature exponent, Forward activation energy. The Collision type and the Specify reaction using columns both have selectable inputs, where the Collision type has options of Elastic, Excitation, Ionization, and Attachment, and the Specify reaction using has options of Cross-section data and Arrhenius parameters. The columns following these two columns will respond to the options selected in the cells of these two columns. The rules are as follows:

If the Collision type is

If Specify reaction using is set to

To manually add a reaction to the Reaction Table, click the Add button () and a new row will appear which represents a new reaction. Trivial default values will be provided to the reaction except the Reaction sequence number. The Reactant and Product of the reaction can then be entered, and COMSOL automatically update the list of species features and create new species features if it finds new species.

For Reaction Table with type of Electron impact reactions, the button () Plot Cross Section Data can be used to inspect imported cross-section data. In order to use it, a single cell or row need to be selected in the Reaction Table first, then click the button () and a new window will pop up where the Cross section (m2) is plotted with respect to the Electron Energy (eV). The cross-section data can only be imported using the Import Reactions from File functionality or using the Plasma Chemistry Import feature.