Water clusters (H$_{2}$O)$_{n}, n=$9-20 in external electric fields: ``Exotic'' O-H stretches as precursors of breakdown

Neutral, deformable clusters of water (H$_{2}$O)$_{n}$[$n =$ 9-20] subject to the highest tolerable ``threshold'' external electric fields on the verge of their breakdown exhibit some ``exotic'' O-H stretching modes in their infrared spectra. These normal vibrations, in which only \textit{one or two} O-H stretchings contribute, emerge in the frequency range of 2327-2965 cm$^{-1}$, markedly below the O-H stretching frequencies that commonly occur in the 3200-3600 cm$^{-1}$ region. These exotic modes essentially characterize the high degree of instability in these clusters immediately preceding their breakdown. On the other hand, a few clusters among those endowed with a C$_{n}$ axis of symmetry remain ``robust,'' in that, they exhibit no major structural changes right up to their electric-field thresholds, wherein dominant stretches in the form of \textit{collective} modes appear only beyond 3200 cm$^{-1}$.