Rotational excitation of CH4and H2O by slow electron impact

Abstract

Cross sections for rotationally elastic and inelastic scattering of slow electrons by spherical (CH4) and asymmetric top (H2O) molecules are calculated in the adiabatic-nuclei-rotation (ANR) approximation. The elastic scattering amplitude is derived in the molecule-fixed frame and the corresponding K matrix is obtained by solving a set of coupled differential equations; in which the total interaction potential includes an ab initio molecular core potential from the ground state near Hartree-Fock wavefunctions, an exchange term in the free-electron-gas-exchange (FEGE) model plus the orthogonalisation procedure and the distortion of the target through a recently proposed nonparametric polarisation potential of Jain and Thompson (1982). Symmetry properties and the selection rules for the asymmetric rotor in general are discussed. In order to avoid convergence problems for the molecule with a permanent dipole moment, an alternative two-stage hybrid S-matrix theory of Collins and Norcross (1978) along with the closure formula of Crawford and Dalgarno (1971) is adopted to obtain converged cross sections (in which proper use of the unitarised Born approximation is made). Results are compared with recent available measurement and calculations; there is a qualitative agreement for CH4 with recent calculations performed in a different model. In the case of H2O, some difficulties are discussed in comparing directly the authors' data with recent experimental cross sections.