### Abstract:

The equilibrium densities of hydrogen species have been determined by a
computational modeling. The thermodynamic equilibrium process is required to expose
plasma on a sample at a stable and controlled condition. Hydrogen species densities have been modeled based on the time-dependent continuity equation and a modified Arrhenius equation. These equations are used to integrate the density change over the time. This simulation is designed to find the equilibrium hydrogen species densities and reaction rates, both among the thermal hydrogen species and among the non-thermal hydrogen species, at a constant atmospheric pressure and low temperature. For the thermal hydrogen plasma, the equilibrium density of electron, H2, H, H2+, H+, and H- are obtained from the numerical calculation is 2.6×1010; 5.12×1024; 9.23×1022; 4.07×1021; 3.58×1022; and 5.12×1020 m-3, respectively. And, for the non-thermal hydrogen plasma,
the equilibrium density of electron, H2, H, H2+, and H+ are acquired from the numerical
simulation is 4.03×1023; 1.01 × 1025; 6.05 × 1024; 3.02 × 1024; and 3.36 × 1023 m-3,
respectively. From the reaction rate gained, it can be inferred that the dominant process
of electron in thermal hydrogen is the recombination, where its density decreases in
order to reach the equilibrium. Meanwhile, the dominant process for electron in non-
thermal hydrogen is the ionization, where its density increases in order to reach the
equilibrium. From this modeling, the relationship between equilibrium electron
densities in hydrogen plasmas and the required time to reach equilibrium was found.