Shurik Yatom methane decomposition in a short glow discharge at Princeton, NJ; H atoms peak at cathode; polymerization zone farther
Shurik Yatom methane decomposition in a short glow discharge at Princeton, NJ; H atoms peak at cathode; polymerization zone farther 1 Spatially-resolved methane decomposition in a short glow discharge: insights into suprathermal hydrogen and radical chemistry Shurik Yatom Princeton Plasma Physics Laboratory, Princeton University, Princeton, NJ 08543, United States of America Abstract 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 (TALIF), and optical emission spectroscopy (OES), 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 demonstrates how the structured energy landscape 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. ...