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

Abstract

Threshold electron-impact excitation of ammonia has been studied using both "trapped electron" 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 ∼0.1 eV. The total negative-ion cross section for N${\mathrm{H}}_3$ (5.74 × ${10}^{-18\mathrm{}}$ ${\mathrm{cm}}^2$) peaking at 5.65 eV is approximately 1.1 times larger than the N${\mathrm{D}}_3$ cross section (5.36 × ${10}^{-18\mathrm{}}$ ${\mathrm{cm}}^2$) 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 ∼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 ${\mathrm{Cl}}^{-}$/HCl in the mass-spectrometer study of the various ion-current peaks (e.g., ${\mathrm{H}}^{-}$, N${\mathrm{H}}_2^{-}$, N${\mathrm{H}}^{-}$, etc.) corresponded exactly to that obtained for the cross sections using the 19.31-eV ${\mathrm{He}}^{-}$ resonance in the total ionization measurements. Accurate measurements of the ${\mathrm{H}}^{-}$(${\mathrm{D}}^{-}$) ion kinetic energy as a function of incident electron energy yield a dissociation energy of 4.35 ± 0.15 eV for N${\mathrm{H}}_2$ - H (or N${\mathrm{D}}_2$ - D).