Gas-phase diagnostic studies of H₂ and CH₄ inductively coupled plasmas

Determination of molecular temperatures within low-temperature plasmas is critical to understanding the reactions that drive the chemistry of these systems and the mechanisms involved in plasma-surface interactions. Optical emission spectroscopy was employed to investigate gas-phase processes in H₂ and CH₄ inductively coupled plasma systems. Specifically, rotational temperatures (T_R) have been determined for H₂ $d3Πu→a3Σg+$ and CH $A2Δ→X2Π$ under a variety of plasma parameter conditions. In 100% H₂ plasmas, T_R(H₂) values are ∼500–550 K, whereas generally higher T_R(H₂) values (∼500–700 K) are reported for 100% CH₄ plasmas. Disparities in the rotational temperature values and trends of H₂ $(d3Πu)$ between H₂ and CH₄ plasmas highlight the differences in H₂ excitation pathways occurring in each of these two plasma systems as T_R can be affected by the mechanism for molecule formation and excitation within plasma systems. As such, mixed gas CH₄/H₂ plasma systems were also explored to gain further insight into these mechanistic details. These results emphasize the connections between fundamental plasma properties and plasma parameters, a key component to understanding and optimizing plasma conditions for the future development of a host of plasma technologies.