Structure of solid water clusters formed in a free jet expansion

Abstract

Clusters are produced in a free jet expansion of water vapor. Keeping constant the nozzle diameter (d=0.4 mm) and temperature (T=430 K), an increase in inlet vapor pressure from 1 to 5 bar produces an increase in mean cluster size from several tens to several thousands of molecules per cluster. An electron diffraction analysis provides information about the cluster structure and dynamics. A direct observation of diffraction patterns shows that the largest clusters exhibit mainly a crystalline structure, namely, diamond cubic which is the metastable phase of bulk solid water, whereas the smallest ones are amorphous. In order to elucidate the local order in the latter, a comparison is made between the experimental curves and the diffraction functions calculated for various noncrystalline models. The best agreement is obtained with a model which presents distorted rings of three to six H2O molecules, constructed by cooling a liquid water droplet through a molecular dynamics calculation. In particular, this means that the regular dodecahedral structure often referred to in mass spectroscopy is not a realistic model for neutral clusters.

Vibrational spectroscopy of hydrated electron clusters(H2O)15–50− via infrared multiple photon dissociation

“…We attribute this observation to the finite size of the cluster examined here. In neutral water clusters, the onset of crystallinity occurs somewhere in the size range from 200 to 1000 water molecules 29 , i.e., at much larger cluster sizes than probed here. Smaller water clusters correspond to quasi spherical nanoparticles with a crystal interior and a disordered "reconstructed" surface 30 , while even smaller clusters contain only "surface" water molecules.…”

Ab initio investigation of water clusters (H2O)n (n = 2–34)

“…In comparison, Xantheas and coworkers [60] reported the structural alternation between interior (n ϭ 17, 19, 21) and all-surface (n ϭ 18, 20) global minima in the n ϭ 17-21 size regime from electronic structure calculations. Moreover, the low-energy structures of all (H 2 O) n (n ϭ 4, 8,12,16,20,28,32) clusters have the cyclic tetramers as building units with opposite hydrogen bond orientation, and so it is not surprising they have similar total dipole moments.…”

“…4. The results for water clusters (H 2 O) n (n ϭ 4, 8,12,16,20,28,32) show that the low-energy structures of these clusters present a certain trend. That is, these low-energy structures all have the cyclic tetramers as building units with opposite hydrogen bond orientation.…”

Cluster cross sections from pickup measurements: Are the established methods consistent?

“…3 ) several other methods have been developed, e.g., electron diffraction, [29][30][31][32] cluster beam scattering by a buffer gas, 33 pickup of sodium atoms and photoionization, 34 and helium atom diffraction on clusters. 35 Here we are concerned with two methods for the mean cluster size determination using the pickup process: (1) The first method consists in measuring the variations of the average beam velocity with the pickup pressure.…”