TOTAL (ELASTIC AND INELASTIC) SCATTERING CROSS-SECTIONS FOR SEVERAL POSITRON-MOLECULE SYSTEMS AT 10-5000 EV - H2, H2O, NH3, CH4, N2, CO, C2H2, O2, SIH4, CO2, N2O, AND CF4

We report calculations on the total (elastic plus inelastic) positron-scattering cross sections from several diatomic and polyatomic molecules (H2, H2O, NH3, CH4, N2, CO, C2H2, O2, SiH4, CO2, N2O, and CF4) where experimental data are available. The impact energy (E) range is 10-5000 eV. A local spherical complex optical potential (SCOP) is calculated for each positron-molecule system from the target charge density [rho(r)], which in turn is determined from the corresponding molecular wave function at the Hartree-Fock level. The real part of the SCOP is composed of the repulsive static and attractive positron-correlation-polarization Potential of Jain [Phys. Rev. A 39, 2437 (1990)]. The imaginary component of the SCOP, the so-called absorption potential, is derived semiempirically as a function of p(r), E, and the mean excitation energy. The resulting complex optical potential is treated exactly in a variable-phase approach to yield complex phase shift and the total-cross-section quantities. In this intermediate- and high-energy region, the small contribution due to the nonspherical nature of the target is neglected. In addition, we fit the total-cross-section values to a simple analytic formula. For molecules possessing a permanent dipole or quadrupole moment, the present results are reliable only roughly above 50 eV.