Mass Spectrometric Study of Barium Oxide Vapor

Ions formed by electron bombardment of the vapor in thermodynamic equilibrium with BaO solid have been studied mass spectrometrically. The BaO solid was heated in a Knudsen cell and a sample of the effusing vapor entered the ion source of a mass spectrometer. At cell temperatures between 1500°K and 1800°K, ion currents of Ba⁺, BaO⁺, Ba₂O⁺, Ba₂O₂⁺, and Ba₂O₃⁺ were observed. The variations of ion intensities with temperature were measured and appearance potential curves were obtained. Possible mechanisms of ion formation were considered, and evidence for the existence of gaseous molecular species of BaO, Ba₂O, Ba₂O₂, and Ba₂O₃ was obtained. The major molecular species, effusing from the Knudsen cell, was BaO. A thermodynamic treatment of ion currents yields the following heats of reaction $$\begin{array}{c}\mathrm{BaO}\mathrm{(s)=}\mathrm{BaO}{\mathrm{(g);\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\Delta H}}_0^{\circ }\text{=104±4\hspace{0.17em}}\mathrm{kcal}/\mathrm{mole}\\ \mathrm{BaO}\mathrm{(g)=}\mathrm{Ba}\mathrm{(g)+}\mathrm{O}{\mathrm{(g);\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\Delta H}}_0^{\circ }\text{=130±5\hspace{0.17em}}\mathrm{kcal}/\mathrm{mole}\\ {\mathrm{Ba}}_2{\mathrm{O}}_2\text{(g)=2}\mathrm{BaO}{\mathrm{(g);\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\Delta H}}_{1700}^{\circ }\text{=89±11\hspace{0.17em}}\mathrm{kcal}\\ {\mathrm{Ba}}_2\mathrm{O}\mathrm{(g)=}\mathrm{Ba}\mathrm{(g)+}\mathrm{BaO}{\mathrm{(g);\hspace{0.17em}\hspace{0.17em}\Delta H}}_{1700}^{\circ }\text{=93±17\hspace{0.17em}}\mathrm{kcal}.\end{array}$$