This work presents the spatially resolved mapping of methane decomposition chemistry in a short direct-current glow discharge, revealing segregated zones for dissociation and polymerization in an Ar-CH4 mixture at 400 mTorr. Laser-induced fluorescence (LIF), two-photon absorption LIF, and optical emission spectroscopy, are used to map the absolute number densities of atomic hydrogen (H), methylidyne (CH), and dicarbon (C2) radicals. The results reveal a segregated chemical environment driven by the non-local electron kinetics. The primary dissociation of methane is confined to the cathode sheath (<4 mm), where the H atom density peaks at a value (∼1.8 × 1021 m−3). Analysis of the Hα line profile identifies two distinct suprathermal H atom populations with peak kinetic energies of ∼110 eV and ∼17.5 eV, attributed to ion-surface reflection and electron-impact dissociation, respectively. The densities of secondary radicals CH and C2 peak further from the cathode at the sheath-negative glow boundary (y ≈4–6 mm), identifying this region as the primary zone for polymerization, with the peak C2 density (∼5.6 × 1017 m−3) higher than that of CH (∼1.5 × 1015 m−3) by a factor of ∼370. The discharge operates under strong non-equilibrium conditions, confirmed by the disparity between the low bulk gas temperature (∼570 K) and the high vibrational temperature (3300–4700 K) of the emitting CH(A) radicals, the latter serving as a signature of a specific high-energy dissociative excitation pathway. This work shows how the energetic structure of a short glow discharge spatially separates methane dissociation from subsequent polymerization. Kinetic estimates suggest that suprathermal H atoms may play a role in bulk plasma chemistry via abstraction reactions, with predicted CH4 activation rates exceeding those of the thermal H population.
