Water is probably the most important molecule for the thermal properties of dense, interstellar gas, and as such plays a decisive role in establishing the rate at which molecular clouds collapse towards star formation. The synthesis of water is thought to be mainly due to dissociative recombination of ${\mathrm{H}}_3{\mathrm{O}}^{+}$ with electrons, although experiments have, as yet, provided no evidence that ${\mathrm{H}}_2\mathrm{O}$ is a product of the reaction. Here we present the first observation of water synthesis from the recombination of ${\mathrm{H}}_3{\mathrm{O}}^{+}$, with a branching ratio of 33%. This value is in good agreement with existing observations of the $\left[{\mathrm{H}}_2\mathrm{O}\right]/\left[{\mathrm{H}}_3{\mathrm{O}}^{+}\right]$ ratio in interstellar clouds.

• Received 9 July 1996

DOI:https://doi.org/10.1103/PhysRevLett.77.4891