Ion-Molecule Reactions of NH3+ by Photoionization

Photoionization-efficiency curves were measured for NH₃⁺, and NH₄⁺, and H₃O⁺ ions in NH₃ gas and in gaseous mixtures of NH₃ and H₂O. The results are interpreted to determine how the relative reaction cross section depends on the vibrational energy of the NH₃⁺ ion for the reactions $${\mathrm{NH}}_3^{+}\mathrm{\hspace{0.17em}+\hspace{0.17em}}{\mathrm{NH}}_3\to {\mathrm{NH}}_3^{+}\mathrm{\hspace{0.17em}+\hspace{0.17em}}{\mathrm{NH}}_2,$$ $${\mathrm{NH}}_3^{+}\mathrm{\hspace{0.17em}+\hspace{0.17em}}{\mathrm{H}}_2\mathrm{O}\to {\mathrm{NH}}_4\mathrm{\hspace{0.17em}+\hspace{0.17em}}\mathrm{OH}.$$ The cross section for Reaction (1) decreases with increasing vibrational energy while that for Reaction (2) is nearly independent of vibrational energy. The results are discussed in terms of the formation of an intermediate complex and in terms of stripping models. The upper limit of the proton affinity was determined to be ≥ 8.54 eV for NH₃ and ≥ 7.0 eV for H₂O.