Particle sources may change their behavior depending on their surface coverage. Examples, which are implemented in the DSMC algorithm are

• Sputtering sources with a given ion flux
• Thermal evaporation of coated material with high evaporation pressure

A sputtering source with a given ion flux is given by the command

add_surface_emission("material", ion_flux, ["rp_1", "rp_2", ...], [yield_1, yield_2, ...]);

Parameters are as in the following:

• material → Material phase in the surface layer. For different surface materials different sputtering processes can be defined.
• ion_flux → The effective ion flux is given in sccm.
• [“rp_1”, “rp_2”, …] → list of gaseous reaction products
• [yield_1, yield_2, …] → list of partial yield of the gaseous reaction products

The sputtering yield of a gasous reaction product is the product of the reaction probability and its partial yield. As an example, the surface chemistry and sputtering source for the reactive TiOx process is given:

# Specification of surface material phases and initial coverage:
add_material("Ti_metallic", 9e18);
add_material("TiO2",        9e18);
set_initial_coverage("TiO2", 0.9);
 
# Surface reactions:
add_surface_reaction("O2", 1, 1, 1.0, "Ti_metallic", "TiO2", [], []);
add_cover_deposition("Ti", 1, 1, "Ti_metallic", [], []);
 
# Sputtering source:
add_surface_emission("Ti_metallic", ion_flux_sccm, ["Ti"], [0.5]);
set_emission_sputter("Ti", 4.89, 1.5);
add_surface_emission("TiO2", ion_flux_sccm, ["Ti", "O2"], [0.05, 0.05]);
set_emission_sputter("Ti", 6.0, 1.5);
set_emission_sputter("O2", 6.0, 1.5);

In the above example, sputtering of metallic Ti and oxidized TiO₂ is handled separately. The sputtering yield of TiO₂ is by factor of 10 lower compared to metallic Ti. The energy distributions of the sputtered gaseous species are to be specified afterwards for each case.

Note: When sputtering the surface layer, it is being replaced by the material beneath the surface layer.

• If there is deposition on the sputtering surface, the replaced surface fractions are according to the stoichiometry of the material in the deposition layer.
• If there is no deposited material beneath the surface layer, the replacement material will be the material which is firstly defined in the deposition chemistry (i.e. Ti_metallic in the above example).

If surfaces are coated with materials with high vapour pressure, there is the possibility of thermal desorption of that material. For this purpose, a desorption source is implemented. It can be declared via the syntax

add_desorption("species", pressure, "material", amount);

The parameters are as follows:

• species: The species, which is to be emitted from the wall into the gas phase
• pressure: The vapor pressure in Pa. The vapor pressure generally depends on the wall temperature. For many materials, the vapor pressure curve as a function of temperature is available in literature¹⁾.
• material: The associated wall material.
• amount: The number of gas molecules needed to form a bond site of the wall material

If the surface material does not completely desorb but leaves behind some remainder, modified command is available:

add_desorption_r("species", pressure, "material", amount_gas_species, "remainder", amount_remainder);

Here, we have the following parameters:

• species: The species, which is to be emitted from the wall into the gas phase
• pressure: The vapor pressure in Pa.
• material: The associated wall material.
• amount_gas_species: The number of gas molecules needed to form a bond site of the wall material.
• remainder: The wall material which is left behind by the desorption process.
• amount_remainder: The number of wall sites of the remaining material created by the desorption process.

Example: If we have a metallic ZnAl mixed site and assume that only Zn can desorp and leave behind a half Al2 site, the command would look like follows:

add_desorption_r("Zn", vapor_pressure, "ZnAl", 1, "Al2", 0.5);