Power-law Velocity Distribution of Light Radicals in Low-Temperature Plasmas.

Nonthermal velocity distributions of radical atoms in various low-temperature plasmas have been observed for decades. Although this nonthermality has been attributed to the generation of energetic radicals by dissociation, the distribution has been difficult to quantitatively understand because the theoretical form considering collisional relaxation remains missing. Here, based on the collision theory and experimental observations, we show that a power-law velocity distribution is robustly formed through the collisional energy cascade of energetic radicals. The dissociation-degree dependence of the power-law index is also found. We propose an analytical form to represent the distribution, which can be used for plasma-diagnostics.