Infrared Spectra and Structures of Large Water Clusters

Infrared spectra have been determined for aerosol ice samples with particles that vary in average diameter down to ∼2 nm. The aerosol spectra, obtained at 100 K, show that the crystalline core of the average particle decreases rapidly with decreasing particle size and vanishes near 4 nm (or 1000 molecules). Consequently, the combined FT-IR spectrum of the surface and subsurface regions has been observed directly for the first time and observed to be nearly invariant to ∼3 nm. Using a polarizable water potential, the structure and spectra of a 1000 molecule cluster has been simulated. The starting point was an approximately spherical cubic ice structure, which was subjected to relaxation by molecular dynamics. The resulting lower energy structure includes a disordered surface layer and an interior that clearly retains a degree of oxygen order. The simulated spectrum of the cluster is separable into components resembling the surface, subsurface, and core ice experimental spectra. The combined results support the description of ice nanoparticles as a central crystalline core surrounded by a subsurface region and a strongly disordered outer surface layer. The virtually crystalline subsurface, though strained by interaction with the surface, persists to a size of a few hundred H2O.