Reactions Framework

The Reactions Framework surpasses previous interaction frameworks in both speed and flexibility in types of processes that can be added to simulations. See Reactions Text-Setup Introduction, as well as relevant sections in the VSim User Guide for more general information on the Reaction framework.

The Reaction framework collisions are available when particles in the Composer Basic Settings element is set to include particles and the collision framework is set to reactions. This will add a “Reactions” element to appear within the “Particle Dynamics” element.

When a collision process is added to a simulation, the user must specify each of the products and reactants from the drop down menus corresponding to each species in the chemical formula. Additionally, users must add a cross-sections to determine the reaction probability to determine how many particles to react within each cell for each timestep. Scroll to the bottom of this page for the reference on setting cross-sections.

Particle Particle Collisions

These collisions are for interactions between kinetically modeled particle species (for a process involving a background gas use the “Particle Fluid Collisions”). The following interactions are available by right-clicking the “Particle Particle Collisions” element and hovering the mouse pointer over the “Add CollisionType” menu.

• Charge Exchange A collision of the form \(A + B^+ \rightarrow A^+ + B\). This is the implementation of Charge Exchange in the visual setup.

  energyLoss

  The maximum energy lost during a single inelastic collision in eV. A choice of 0 (default) will give an elastic collision.

• Impact Ionization A collision of the form \(A + B \rightarrow A^+ + B + e\). This is the implementation of Impact Ionization in the visual setup. If one of the reactant species is an electron, the Electron Ionization productGenerator is used. Ionization energies are taken from the appropriate species blocks.

  Note

  If an electron is involved in an ionization process, the “product distribution” attribute won’t affect the simulation. The scattering distribution is determined automatically by a physical model.

• Elastic A collision of the form \(A + B \rightarrow A + B\). This is the implementation of Binary Elastic in the visual setup.

  energyLoss

  The maximum energy lost during a single inelastic collision in eV. A choice of 0 (default) will give an elastic collision.

• Dissociative Double Ionization A collision of the form \(AB + C \rightarrow A^+ + B^+ + C + 2e\). This is the implementation of Dissociative Ionization in the visual setup.

  dissociation energy

  The energy required to dissociate the molecule, in eV. This value also sets a threshold for whether or not the reaction will occur. So a pair of particles will need at least this much relative energy (i.e. energy in the center of momentum frame) in order to react. This can be set as a negative value to have the products gain kinetic energy.

• Dissociative Single Ionization A collision of the form \(AB + C \rightarrow A^+ + B + C + e\). This is the implementation of Dissociative Ionization in the visual setup.

  dissociation energy

  The energy required to dissociate the molecule, in eV. This value also sets a threshold for whether or not the reaction will occur. So a pair of particles will need at least this much relative energy (i.e. energy in the center of momentum frame) in order to react. This can be set as a negative value to have the products gain kinetic energy.

• Dissociative Recombination A collision of the form \(AB^+ + e \rightarrow A + B\). This is the implementation of Dissociative Recombination in the visual setup.

  dissociation energy

  The threshold energy for the reaction in eV. So, a pair of reactants will need at least this much relative energy (i.e. energy in center of momentum frame) in order to react. If the reaction occurs then this much energy is lost from the products to potential energy. If this is zero, then energy will not be lost. If negative, then the products will gain kinetic energy.

• General Binary Reaction A collision of the form \(A + B \rightarrow C + D\). This is the implementation of Binary Reaction in the visual setup.

  threshold energy

  The threshold energy for the reaction in eV. So, a pair of reactants will need at least this much relative energy (i.e. energy in center of momentum frame) in order to react. If the reaction occurs then this much energy is lost from the products to potential energy. If this is zero, then energy will not be lost. If negative, then the products will gain kinetic energy.

• Electron Impact Dissociation A collision of the form \(AB + e \rightarrow A + B + e\). This is the implementation of Electron Impact Dissociation in the visual setup.

  dissociation energy

  The energy required to dissociate the molecule in eV.

• Electron Attachment A collision of the form \(A + e \rightarrow A^-\). This is the implementation of Electron Attachment in the visual setup.

  binding energy

  The threshold energy for the reaction in eV. So, a pair of reactants will need at least this much relative energy (ie energy in center of momentum frame) in order to react. If the reaction occurs then this much energy is lost.

• Negative Ion Detachment A collision of the form \(A^- + B \rightarrow A + B + e\). The neutral reactant must be a kinetically modeled species. This is the implementation of Negative Ion Detachment in the visual setup.

  detachment energy

  The threshold energy for the reaction in eV. So, a pair of reactants will need at least this much relative energy (ie energy in center of momentum frame) in order to react. If the reaction occurs then this much energy is lost.

• Excitation A collision of the form \(A + B \rightarrow A^* + B\). This is the implementation of Binary Excitation in the visual setup.

  excitation energy

  The potential energy, in eV, gained by the excited species (a positive value for this attribute will result in an energy loss from the simulation). This is also a threshold energy. The reaction will not occur between reactants that have a center of mass energy less than this value.

• Inelastic Electron Scattering A collision of the form \(e + A \rightarrow e + A\). This is the implementation of Electron Scatter in the visual setup.

  scatter type

  • VahediSurendra Use the Vahedi-Surendra model to determine the scattering distribution. See Electron Scatter for more information.

  • isotropic Use an isotropic scattering distribution.

• Recombination A collision of the form \(C + B \rightarrow CB\). This is the implementation of Binary Recombination in the visual setup, for the recombination of particles of any charged state.

• General Reaction A collision that allows for the combination of any number of reactants and products. Reactants and products are added by right clicking the reaction and selecting a reactant or product to add. Note that while this is classified as a Particle Particle collisions, fluids are available as both reactants and products.

  threshold energy

  The reaction will not occur between reactants that have a center of mass energy less than this value.

Particle Fluid Collisions

These collisions are for interactions between kinetically modeled particle species and a background gas. The following interactions are available by right-clicking the “Particle Fluid Collisions” element and hovering the mouse pointer over the “Add CollisionType” menu.

• Elastic A collision of the form \(A + B \rightarrow A + B\). This is the implementation of Binary Elastic in the visual setup.

  energyLoss

  The user can choose either fully elastic which conserves kinetic energy, or partially elastic in which case the user

• Charge Exchange A collision of the form \(A + B^+ \rightarrow A^+ + B\). This is the implementation of Charge Exchange in the visual setup.

  energyLoss

  The maximum energy lost during a single inelastic collision in eV. A choice of 0 (default) will give an elastic collision.

• Impact Excitation A collision of the form \(A + B \rightarrow A^* + B\). This is the implementation of Binary Excitation in the visual setup.

  excitation energy

  The potential energy, in eV, gained by the excited species (a positive value for this attribute will result in an energy loss from the simulation). This is also a threshold energy. The reaction will not occur between reactants that have a center of mass energy less than this value.

• Impact Ionization A collision of the form \(A + B \rightarrow A^+ + B + e\). This is the implementation of Impact Ionization in the visual setup. If one of the reactant species is an electron, the Electron Ionization productGenerator is used. Ionization energies are taken from the appropriate species blocks.

  Note

  If an electron is involved in an ionization process, the “product distribution” attribute won’t affect the simulation. The scattering distribution is determined automatically by a physical model.

• Electron Attachment A collision of the form \(A + e \rightarrow A^-\). This is the implementation of Electron Attachment in the visual setup.

  binding energy

  The threshold energy for the reaction in eV. So, a pair of reactants will need at least this much relative energy (ie energy in center of momentum frame) in order to react. If the reaction occurs then this much energy is lost.

• Negative Ion Detachment A collision of the form \(A^- + B \rightarrow A + B + e\). The neutral reactant must be a kinetically modeled species. This is the implementation of Negative Ion Detachment in the visual setup.

  detachment energy

  The threshold energy for the reaction in eV. So, a pair of reactants will need at least this much relative energy (ie energy in center of momentum frame) in order to react. If the reaction occurs then this much energy is lost.

• Inelastic Scattering A collision of the form \(e + A \rightarrow e + A\). This is the implementation of Electron Scatter in the visual setup.

  scatter type

  • VahediSurendra Use the Vahedi-Surendra model to determine the scattering distrubution. See Electron Scatter for more information.

  • isotropic Use an isotropic scattering distribution.

• General Binary Reaction A collision of the form \(A + B \rightarrow C + D\). This is the implementation of Binary Reaction in the visual setup.

  threshold energy

  The threshold energy for the reaction in eV. So, a pair of reactants will need at least this much relative energy (i.e. energy in center of momentum frame) in order to react. If the reaction occurs then this much energy is lost from the products to potential energy. If this is zero, then energy will not be lost. If negative, then the products will gain kinetic energy.

• Dissociative Double Ionization A collision of the form \(AB + C \rightarrow A^+ + B^+ + C + 2e\). This is the implementation of Dissociative Ionization in the visual setup.

  dissociation energy

  The energy required to dissociate the molecule, in eV. This value also sets a threshold for whether or not the reaction will occur. So a pair of particles will need at least this much relative energy (i.e. energy in the center of momentum frame) in order to react. This can be set as a negative value to have the products gain kinetic energy.

• Dissociative Single Ionization

  A collision of the form \(AB + C \rightarrow A^+ + B + C + e\). This is the implementation of Dissociative Ionization in the visual setup.

  dissociation energy

  The energy required to dissociate the molecule, in eV. This value also sets a threshold for whether or not the reaction will occur. So a pair of particles will need at least this much relative energy (i.e. energy in the center of momentum frame) in order to react. This can be set as a negative value to have the products gain kinetic energy.

Three Body Reactions

• Three Body Recombination A collision of the form \(A^+ + e + B \rightarrow A + B\). This is the implementation of Three Body Recombination in the visual setup.

Field Ionization Process

The “Fluid Field” and “Particle Field” ionization processes both use Field Ionization productGenerator. The distinction is made in the visual setup for the benefit of the user.

• Particle Field Ionization This is the implementation of Field Ionization for ionization of kinetically modeled particles by electric fields in the visual setup.

  cross section type

  • DCADK Use this when the time step resolves the oscillations of Electric field. See Field Ionization DCADK for more information.

  • Average ADK Use this when the time step is much larger than the oscillations of Electric field. See Field Ionization Average ADK for more information.

• Fluid Field Ionization This is the implementation of Field Ionization for ionization of background gasses by electric fields in the visual setup.

  cross section type

  • DCADK Use this when the time step resolves the oscillations of Electric field. See Field Ionization DCADK for more information.

  • Average ADK Use this when the time step is much larger than the oscillations of Electric field. See Field Ionization Average ADK for more information.

Decay Process

• Decay This is the implementation of Decay in the visual setup

  lifetime

  The lifetime (in seconds) of the unstable species.

Emission Process

• Photon Emission This simulates the emission of an untracked photon from a particle. This is the implementation of Radiation Reaction

Cross-Sections

Interpolated from 2Column Data

Import cross sections from a data file with the independent variable (either velocity or energy) in the first column and the cross-section (dependent variable) in the second column. The imported file should NOT have any header.

cross section variable

The unit of the independent variable in the first column of the data file. Either velocity, or energy.

cross section data file

The name of the cross section data file. This file must be in the run directory.

Constant Cross Section

This can be used to set a user defined function for the cross section.

constant

A constant value (in m^2) for the cross section.

Power Law Cross Section

Use a cross section of the form \(Ax^E\). See Power Law Cross-Section for more information.

coefficient

The value for the constant A in the expression above.

exponent

The value for the constant E in the expression above.

cross section variable

The unit of the independent variable (\(x\) in the expression above). Either velocity, or energy.

Exponential Polynomial Cross Section

The cross-sections will take the functional from of the equation:

\[\sigma (y) = \frac{1}{y} \left( A \ln (y) + \frac{B \ln (y) + C*(y-"toZero")}{y + D} \right)\]

where

\[y = x/I\]

See Exponential Polynomial Cross-Section for more information.

Acoeff

The value for the fitting constant A in the formula above.

Bcoeff

The value for the fitting constant B in the formula above.

Ccoeff

The value for the fitting constant C in the formula above.

Dcoeff

The value for the fitting constant D in the formula above.

Icoeff

The value for the fitting constant I in the formula above. A scaling factor for the independent variable, x.

toZero

Sets the value for the toZero parameter in the formula above. Enter either “0” to have the cross-section go to zero at threshold, or “1” to have the cross-section go to a non-zero value.

cross section variable

Set the independent variable, x, in the equation above. Options are:

• velocity

• energy

Additional Collision Attributes

update periodicity

This sets how frequently the reaction is carried out. See the description of updatePeriod in RxnProcessSettings Block for more information. The update periodicity setting in the Visual Setup will set the updatePeriod when the .sdf is translated to a .in file. Options are:

every timestep

The reaction update will occur every timestep.

update period

not editable; defaulted to 1

custom

Set a custom update periodicity.

update period

Enter a constant to set the update periodicity.

product distribution

This will apply an anisotropy to the cross-section. See the section Anisotropy for more information. This attribute is only available for collisions with two reactant particles (or fluids) and two product particles (or fluids) except for the Inelastic Electron Scattering process which has the anisotropy set according to the Vahedi-Surendra model.

isotropic distribution

The default. Set no preference for forward or backward scattering.

anisotropic distribution

Add a preference for forward or backward scattering.

anisotropy

Set a value between -1 and 1 (inclusive). A value of -1 will give full backward scattering bias, and 1 will give full forward scattering bias.

diagnostics

Diagnostics can be enabled for any reaction. These will collect and report data about how often that reaction is utilized.

no diagnostics

The default. Nothing is collected.

diagnostics collected

If selected diagnostics are collected. An averaging period can be applied, specified in timesteps. This will provide a spatially resolved reaction frequency over the entire simulation domain