Low-energy electron scattering with polar moleculeNH3using theR-matrix method

Abstract

We report on the calculated elastic differential, elastic integral, momentum transfer, and excitation cross sections for the low-energy electron-${\mathrm{NH}}_{3}$ scattering using the $R$-matrix method. Elastic differential and momentum transfer cross sections are obtained by summing over rotationally elastic and rotationally inelastic cross sections for rotor states up to $J=4$. The excitation cross sections of the first four low-lying electronically excited states from the ground state of ${\mathrm{NH}}_{3}$, at its equilibrium geometry, are presented. These excited states have symmetries $a\phantom{\rule{0.2em}{0ex}}^{3}A_{1}$, $A\phantom{\rule{0.2em}{0ex}}^{1}A_{1}$, $b\phantom{\rule{0.2em}{0ex}}^{3}E$, and $B\phantom{\rule{0.2em}{0ex}}^{1}E$. The set of self-consistent-field molecular orbitals is obtained by optimizing these on the first excited state $^{3}A_{1}$. Configuration-interaction (CI) wave functions are used to represent the target states. In our CI model, we kept the core two electrons frozen in doubly occupied molecular orbital $1{a}_{1}$, and the remaining eight electrons moved freely among the five molecular orbitals $2{a}_{1}$, $3{a}_{1}$, $1e$, $4{a}_{1}$, $2e$. With this CI model, we obtain good agreement for the vertical spectrum of excited states with the experimental values. The Born approximation is employed to account for higher partial waves excluded in the $R$-matrix method to evaluate elastic cross sections. Cross sections are reported for the electron-impact energy range 0.025--$20\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$.