Electron-Impact Excitation and Negative-Ion Formation in NH3and ND3

Abstract

Threshold electron-impact excitation of ammonia has been studied using both trapped and S${\mathrm{F}}_{6}$ electron-scavenger techniques. Results from the two excitation studies show four well-defined peaks in the energy range from 6 to 12 eV. The scavenger spectrum exhibits a small peak below the first singlet state which is analogous to that previously found for ${\mathrm{H}}_{2}$O and is tentatively attributed to a triplet state. Also, an intense peak is observed at approximately 17 eV. Dissociative electron-capture cross sections for N${\mathrm{H}}_{3}$ and N${\mathrm{D}}_{3}$ have been measured with a total ionization chamber and the various ions contributing to these cross sections were recorded as functions of the electron energy with a TOF mass spectrometer using electron beams with resolution of \ensuremath{\sim}0.1 eV. The total negative-ion cross section for N${\mathrm{H}}_{3}$ (5.74 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}18}$ ${\mathrm{cm}}^{2}$) peaking at 5.65 eV is approximately 1.1 times larger than the N${\mathrm{D}}_{3}$ cross section (5.36 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}18}$ ${\mathrm{cm}}^{2}$) peaking at 5.86 eV which is analogous to previous results for water and heavy water. The total ionization cross section for N${\mathrm{D}}_{3}$ peaked at \ensuremath{\sim}0.21-eV higher energy than that for N${\mathrm{H}}_{3}$. This is ascribed to a difference in the zero-point energy for the two molecules. The electron energy scale determined from ${\mathrm{Cl}}^{\ensuremath{-}}$/HCl in the mass-spectrometer study of the various ion-current peaks (e.g., ${\mathrm{H}}^{\ensuremath{-}}$, N${\mathrm{H}}_{2}^{\ensuremath{-}}$, N${\mathrm{H}}^{\ensuremath{-}}$, etc.) corresponded exactly to that obtained for the cross sections using the 19.31-eV ${\mathrm{He}}^{\ensuremath{-}}$ resonance in the total ionization measurements. Accurate measurements of the ${\mathrm{H}}^{\ensuremath{-}}$(${\mathrm{D}}^{\ensuremath{-}}$) ion kinetic energy as a function of incident electron energy yield a dissociation energy of 4.35 \ifmmode\pm\else\textpm\fi{} 0.15 eV for N${\mathrm{H}}_{2}$ - H (or N${\mathrm{D}}_{2}$ - D).