Water, Formation and Photodissociation

Synonyms

H₂O

Definition

Water vapor can be formed in gas phase and on the ice mantles of dust grains (see “Water in the Universe”). Once formed, the molecule is mainly destroyed by photodissociation through the effect of UV photons. In gas phase the molecule can be formed in low-temperature clouds through the chain of reactions:

$$ {\mathrm{H}}^{+}+\mathrm{O}\to \mathrm{H}+{\mathrm{O}}^{+}{\mathrm{H}}_2+{\mathrm{O}}^{+}\to \mathrm{O}{\mathrm{H}}^{+}+\mathrm{H}{\mathrm{H}}_3^{+}+\mathrm{O}\to \mathrm{O}{\mathrm{H}}^{+}+{\mathrm{H}}_2/{\mathrm{H}}_2{\mathrm{O}}^{+}+\mathrm{H}\mathrm{O}{\mathrm{H}}^{+}+{\mathrm{H}}_2\to {\mathrm{H}}_2{\mathrm{O}}^{+}+\mathrm{H}{\mathrm{H}}_2{\mathrm{O}}^{+}+{\mathrm{H}}_2\to {\mathrm{H}}_3{\mathrm{O}}^{+}+\mathrm{H}{\mathrm{H}}_3{\mathrm{O}}^{+}+{\mathrm{e}}^{\hbox{--}}\to {\mathrm{H}}_2\mathrm{O}+\mathrm{H}/\mathrm{O}\mathrm{H}+{\mathrm{H}}_2 $$

In this set of reactions, H₂ is formed on the surface of dust grains, and H₃ ⁺is the product of the reaction...