Electron collisions with methylidyne (CH) radical using theR-matrix method

The R -matrix method is used to calculate the elastic and the excitation cross sections from the ground state X 2 Π to the four low-lying electronically excited states a 4 Σ - , A 2 Δ, B 2 Σ - and C 2 Σ + of methylidyne (CH) radical. Configuration interaction (CI) wavefunctions are used to represent the target states. In our CI model we keep the 1σ orbital doubly occupied and the remaining electrons are free to occupy the 2σ,3σ,4σ,1π,2π,3π and 1δ orbitals. This model gives an equilibrium bond length, R e , of X 2 Π state equal to 2.113 a 0 which is in excellent agreement with the experimental value of 2.116 a 0 and a CH equilibrium dipole moment of 1.53 D which is close to the experimental value of 1.46±0.06 D. Scattering calculations are performed in the static-exchange, static-exchange plus polarization, 5-state and 6-state models. Our best 6-state model also includes the D 2 Π state. The vertical excitation energies lie in the range 0.32-7.29 eV and agree within three per cent of the experimental values. We find a bound state of CH - of 3 Σ - symmetry with an electron affinity of 0.61 eV at R e . Below 1 eV there are shape resonances in 1 Σ + and 1 Δ symmetries. Both of these resonances have the configuration 3σ 2 1π 2 . Born correction is applied for dipole-allowed transitions to account for higher partial waves excluded in the R -matrix calculation. Cross sections are given for scattering energies up to 10 eV.