Spectroscopic snapshot for neutral water nonamer (H 2 O) 9 : Adding a H 2 O onto a hydrogen bond-unbroken edge of (H 2 O) 8 .

Structural characterization of neutral water clusters is crucial to understanding the structures and properties of water, but it has been proven to be a challenging experimental target due to the difficulty in size selection. Here, we report the size-specific infrared spectra of confinement-free neutral water nonamer (H 2 O) 9 based on threshold photoionization, using a tunable vacuum ultraviolet free-electron laser. Distinct OH stretch vibrational fundamentals in the 3200-3350 cm -1 region are observed, providing unique spectral signatures for the formation of an unprecedented (H 2 O) 9 structure evolved by adding a ninth water molecule onto a hydrogen bond-unbroken edge of the (H 2 O) 8 octamer with D 2d symmetry. This nonamer structure coexists with the five previously identified structures that can be viewed as derived by inserting a ninth water molecule into a hydrogen bond-broken edge of the D 2d /S 4 octamer. These findings provide key microscopic information for systematic understanding of the formation and growth mechanism of dynamical hydrogen-bonding networks that are responsible for the structure and properties of condensed-phase water.