Vacuum Ultraviolet Photochemistry. III. Primary Processes in the Vacuum Ultraviolet Photolysis of Water and Ammonia

Abstract

Water and ammonia have each been photolyzed in the absence and presence of C₂D₄ which served to scavenge H atoms. Wavelengths used were: for ammonia, 1849 and 1236 A; for water, 1236 A. Under conditions where H atoms are efficiently scavenged by C₂D₄, the production of H₂ signifies a primary photochemical process giving molecular H₂ directly. It is found that at 1849 A, ammonia decomposes almost entirely to H+NH₂. At 1236 A, two primary processes are observed. $$\begin{array}{c}(\mathrm{a}){\mathrm{NH}}_3\to {\mathrm{H}}_2+\mathrm{NH},\\ (\mathrm{b}){\mathrm{NH}}_3\to \mathrm{H}+{\mathrm{NH}}_2.\end{array}$$ Process (a) is about ⅙ as probable as process (b). At 1236 A, the photolysis of water proceeds via two primary processes. $$\begin{array}{c}(\mathrm{c}){\mathrm{H}}_2\mathrm{O}\to \mathrm{H}+\mathrm{OH},\\ (\mathrm{d}){\mathrm{H}}_2\mathrm{O}\to {\mathrm{H}}_2+\mathrm{O}.\end{array}$$ The probability of process (c) being three times that of process (d). It is suggested that primary process (d) constitutes a reasonable photochemical mechanism for hydrogen formation in the earth's upper atmosphere.