Differential and integral cross sections for the valence-shell excitations in D2O studied by fast electron impact

Abstract

The experimental generalized oscillator strengths (GOSs) for the valence-shell excitations of heavy water in 6.50--12.17 eV are determined by fast electron scattering with an incident electron energy of 1500 eV and an energy resolution of 80 meV. The experimental data of the $\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{A}{\phantom{\rule{0.16em}{0ex}}}^{1}{B}_{1}\ensuremath{\leftarrow}\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{X}{\phantom{\rule{0.16em}{0ex}}}^{1}{A}_{1}$ transition between ${\mathrm{H}}_{2}\mathrm{O}$ and ${\mathrm{D}}_{2}\mathrm{O}$ show significant discrepancies around the minimum, and further theoretical calculations with nuclear effects accounted are recommended. The comparisons of the present GOSs of $\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{A}{\phantom{\rule{0.16em}{0ex}}}^{1}{B}_{1}\ensuremath{\leftarrow}\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{X}{\phantom{\rule{0.16em}{0ex}}}^{1}{A}_{1}$ and $\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{B}{\phantom{\rule{0.16em}{0ex}}}^{1}{A}_{1}\ensuremath{\leftarrow}\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{X}{\phantom{\rule{0.16em}{0ex}}}^{1}{A}_{1}$ transitions in ${\mathrm{D}}_{2}\mathrm{O}$ with theoretical calculations within the first Born approximation and the second Born approximation indicate that the higher Born terms become prominent at large momentum transfers. In particular, the GOSs of the dipole-forbidden transition ${}^{1}{A}_{2}\ensuremath{\leftarrow}\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{X}{\phantom{\rule{0.16em}{0ex}}}^{1}{A}_{1}$ are extracted, benefiting from the present high-energy electron scattering, where most of electronic dipole-forbidden transitions are not involved. By extrapolating the GOSs to the zero momentum transfer, the optical oscillator strengths for the corresponding dipole-allowed transitions are determined, which provide an independent cross-check for previous optical measurements. Moreover, the integral Born cross sections are scaled to an accurate level with the aid of the binding energy--excitation energy--scaling method. The relevant experimental data in this work can supplement the molecular database with electron collision data involving heavy water.