Electron-impact cross sections for dissociation processes of vibrationally excited CH radical

This paper presents a theoretical investigation of the cross sections for dissociative electron attachment and dissociative excitation processes in vibrationally excited CH radicals induced by electron impact. Resonant electron-CH collisions are analyzed using the ab-initio R-matrix method, while nuclear dynamics are explored within the Local Complex Potential framework. A comprehensive set of vibrationally resolved cross sections and rate coefficients is provided for both the ground and first excited electronic states of the CH molecule. These findings contribute to a better understanding of the kinetics of non-equilibrium systems containing CH molecules with applications in plasma technologies for CO2 reduction, combustion processes and various astrophysical contexts.

💡 Research Summary

This paper presents a comprehensive theoretical study of electron‑induced dissociation processes in vibrationally excited methylidyne (CH) radicals, focusing on two fundamental reaction pathways: dissociative electron attachment (DA) and dissociative excitation (DE). The authors employ an ab‑initio R‑matrix approach to characterize the resonant anionic states of the e + CH system, followed by a Local Complex Potential (LCP) treatment of nuclear dynamics to obtain vibrationally resolved cross sections and temperature‑dependent rate coefficients.

The R‑matrix calculations are performed with a detailed configuration‑interaction description of the target CH molecule (CAS‑CI active space including σ, π, and δ orbitals) and a spherical Bessel basis for the scattering electron. By partitioning space into an inner region (radius a) and an outer region, short‑range electron correlation and exchange are treated explicitly, while long‑range multipole interactions are handled analytically. Eigenphase sum analysis reveals three low‑lying resonances of symmetries 1Σ⁺, 3Π, and 5Σ⁻, with positions and widths (E_r, Γ_r) of approximately (0.31 eV, 0.22 eV), (1.52 eV, 0.81 eV), and (10.13 eV, 0.07 eV), respectively. These resonances are the doorway states that mediate DA and DE.

To describe the nuclear motion, the authors construct real potential energy curves (PECs) for the neutral CH X²Π ground state and the a⁴Σ⁻ excited state using high‑level MRCI calculations with an aug‑cc‑pVQZ basis set. The resonant anionic PECs obtained from the R‑matrix are smoothly merged with the MRCI curves, and the corresponding resonance widths Γ(R) are incorporated as the imaginary part of a complex potential. The Fourier Grid Hamiltonian (FGH) method is then used to solve the one‑dimensional Schrödinger equation for each electronic state, yielding vibrational eigenfunctions and eigenvalues for v = 0–5.

With the vibrational wavefunctions and the complex potentials, the authors solve the multichannel non‑adiabatic scattering equations to compute DA and DE cross sections as functions of incident electron energy (0–15 eV). The results show that DA cross sections increase dramatically with vibrational excitation, especially for v ≥ 3, and peak near the 1Σ⁺ resonance (≈0.3 eV). DE cross sections are dominated by the broad 3Π resonance, producing a pronounced maximum in the 1–3 eV range. The high‑energy 5Σ⁻ resonance contributes only at electron energies above ≈10 eV and is therefore less important under typical plasma conditions.

Rate coefficients are obtained by averaging the cross sections over a Maxwell‑Boltzmann electron energy distribution for temperatures ranging from 100 K to 10 000 K. At low temperatures (≤300 K) DA is the primary loss channel for CH, while at higher temperatures (≥3000 K) DE becomes dominant, reflecting the shift of electron population to energies where the 3Π resonance is active. This temperature dependence is crucial for kinetic modeling of non‑equilibrium plasmas, such as CO₂‑splitting discharges, hydrocarbon combustion plasmas, and interstellar molecular clouds.

The calculated electron affinity of CH⁻ (≈1.0 eV) and resonance positions are compared with available experimental data (e.g., Kasdan et al., 1.238 eV), showing agreement within 0.2–0.3 eV, which validates the computational protocol. However, the study neglects rotational contributions and assumes a thermal electron distribution; the authors acknowledge that non‑thermal electron energy functions, common in many laboratory and astrophysical plasmas, would require extensions of the present model.

In conclusion, the paper delivers the first set of vibrationally resolved DA and DE cross sections and corresponding rate coefficients for CH radicals, filling a critical data gap for plasma chemistry and astrochemical modeling. The combined R‑matrix/LCP methodology proves effective for diatomic radicals where resonant electron capture dominates low‑energy scattering. Future work is suggested to incorporate rotational‑vibrational coupling, multi‑electron channels (including ionization), and experimental validation to further refine the database for practical applications.