Low-energy electron scattering with H2O and NH3 molecules.

Abstract

Calculated total, differential, and momentum-transfer cross sections are reported for the vibrationally elastic scattering of electrons from ${\mathrm{H}}_{2}$O and ${\mathrm{NH}}_{3}$ molecules in the range of energy 0.5--20 eV. The total cross sections are approximated by a sum of the rotationally elastic ones and the rotational transition from ground to first excited state (00\ensuremath{\rightarrow}10). The first-order Born approximation with the rotating-molecule model is used in the calculation of the first rotational excitation cross section. The experimental value of the dipole moments of ${\mathrm{H}}_{2}$O and ${\mathrm{NH}}_{3}$ are used in this step of the calculation. Spherically approximate molecular wave functions are applied to calculate the spherical part of the interactions of the incident electron with the molecule and the rotationally elastic-scattering cross section, in which the static and exchange interactions are calculated exactly within the accuracy of the molecular wave function and a parameter-free correlation-polarization potential is used to account for the polarization effect. The agreement of the differential cross sections with the experimental data is good at small angles and reasonably good at intermediate and large angles. Good agreement with measured values is obtained for the momentum-transfer cross sections. Total cross sections are reported with theoretical results on the e-${\mathrm{NH}}_{3}$ system below 10 eV. The possible sources of the discrepancies between the current theoretical and experimental results are analyzed for the total cross sections at very low energies by a quantitative consideration of the contributions of very strong forward scattering at near-zero angles to the total cross sections.