Electron-impact study of formaldehyde using the R-matrix method

Abstract

The R-matrix method, involving eight target states in the close-coupling formalism, is used to calculate elastic integral, differential and momentum transfer cross sections for electron impact on the formaldehyde molecule. We have also obtained the excitation cross sections for the seven lowest-lying electronically excited states which have symmetries 1 3A1, 1 1,3A2, 1 1,3B1 and 1 1,3B2. Their vertical excitation energies from the equilibrium geometry of the ground state X 1A1 lie in the range 3.58–9.99 eV. Configuration interaction (CI) wavefunctions are used to represent the target states. In our CI model, we keep the six core electrons frozen in doubly occupied molecular orbitals 1a1, 2a1 and 3a1. The complete active space consists of ten valence electrons that are allowed to move freely among the eight molecular orbitals: 4a1, 5a1, 6a1, 1b1, 2b1, 1b2, 2b2 and 3b2. In this CI model, we obtain good agreement of the dipole moment of the ground state with the experimental value, and a good representation of the vertical excitation spectrum of the excited states included in our calculation. We have also investigated the electron impact rotationally elastic and rotationally excitation transitions for the ground state for this asymmetric top molecule and obtained the rotationally resolved differential and integral cross sections for energies up to 20 eV. Our calculations do not detect any bound H2CO− states at the equilibrium geometry of the H2CO molecule. We find a shape resonance of the 2B1 symmetry with its resonance position at 1.32 eV and a corresponding resonance width of 0.546 eV at the equilibrium geometry of the molecule. This resonance provides a pathway for dissociative electron attachment when the CO bond is stretched beyond 3a0. Born correction is applied for the elastic and the dipole allowed transitions to account for higher partial waves excluded in the R-matrix calculation. We also compare R-matrix differential, partial, momentum transfer and excitation cross sections with the other work.