Calculations of Low Energy Electron-Impact Excitation Cross Sections of Diatomic Molecules by the Close-Coupling R-Matrix and Polarized-Orbital Methods.

Low energy electron-impact excitation cross sections of diatomic molecules are calculated by the close-coupling, R-matrix, and polarized-orbital methods. The standard close-coupling procedure is generalized to the excitation of the a('1)(PI)(,g) state of N(,2) in the energy range 12.5-50.0 eV. The target wave functions are calculated by the molecular self-consistent-field method using 10 s-type and 6 p-type Gaussian orbitals as basis functions. The calculation is ab initio and exact within the limits of two-state close-coupling. The calculated cross sections agree well with experiments above 30 eV but can be improved below 30 eV by including the induced dipole polarization of the target. Two improvements to the standard close coupling are presented. A merger of the close-coupling and R-matrix procedures yields an efficient technique for solving the scattering equations. Approximate close-coupling solutions are used as the basis for a R-matrix calculation within a sphere of radius a centered on the molecule. The R-matrix is then used as the boundary condition to integrate the scattering equations with the short-range exchange terms omitted in the region r > a. Cross sections for the excitation of the b('3)(SIGMA)(,u)('+) state of H(,2) calculated by this method are presented. Comparison with the standard close-coupling calculation on the same state shows agreement within 5% with a factor of five reduction in the computing time. The basis set consisted of only two or three basis functions per channel, which is a markedly smaller set than typically employed in R-matrix calculations. The second improvement is the inclusion of the induced dipole polarization of the target molecule. The polarized-orbital method is applied to the excitation of the B('1)(SIGMA)(,u)('+) state of H(,2). First-order adiabatic corrections to the molecular orbitals are generated, which yield the isotropic dipole polarizability of the ground state to within 8%. The ab initio long-range polarization potential constructed from these orbitals is included in the usual close-coupling procedure. The cross section is increased dramatically in the low energy peak region.