By means of the supersonic expansion technique in conjunction with a high-resolution mass spectrometer, two separate forms of doubly and triply charged water cluster have been observed. The doubly charged clusters have been identified as $\left({\mathrm{H}}_2\mathrm{O}\right)_n^{}{}_{}{}^{2+}$ and ${\left({\mathrm{H}}_2\mathrm{O}\right)}_n\mathrm{H}_2^{}{}_{}{}^{2+}$, and the triply charged as ${\left({\mathrm{H}}_2\mathrm{O}\right)}_n{\mathrm{H}}^{3+}$ and ${\left({\mathrm{H}}_2\mathrm{O}\right)}_n\mathrm{H}_3^{}{}_{}{}^{3+}$. Coulomb explosion imposes a minimum value of $n$ below which each form is unstable, and that value is different for each of the four types of multiply charged cluster.

• Received 21 December 1987

DOI:https://doi.org/10.1103/PhysRevLett.61.306