22 research outputs found
A size resolved investigation of large water clusters
Size selected water clusters are generated by photoionizing sodium doped clusters close to the ionization threshold. This procedure is free of fragmentation. Upon infrared excitation, size- and isomer-specific OH-stretch spectra are obtained over a large range of cluster sizes. In one application of this method the infrared spectra of single water cluster sizes are investigated. A comparison with calculations, based on structures optimized by genetic algorithms, has been made to tentatively derive cluster structures which reproduce the experimental spectra. We identified a single all-surface structure for n = 25 and mixtures with one or two interior molecules for n = 24 and 32. In another application the sizes are determined at which the crystallization sets in. Surprisingly, this process strongly depends on the cluster temperature. The crystallization starts at sizes below n = 200 at higher temperatures and the onset is shifted to sizes above n = 400 at lower temperatures
Infrared detection of (H2O)(20) isomers of exceptional stability: A drop-like and a face-sharing pentagonal prism cluster
Water clusters with internally solvated water molecules are widespread models that mimic the local environment of the condensed phase. The appearance of stable (H2O)(n) cluster isomers having a fully coordinated interior molecule has been theoretically predicted to occur around the n = 20 size range. However, our current knowledge about the size regime in which those structures become energetically more stable has remained hypothetical from simulations in lieu of the absence of precisely size-resolved experimental measurements. Here we report size and isomer selective infrared (IR) spectra of (H2O) 20 clusters tagged with a sodium atom by employing IR excitation-modulated photoionization spectroscopy. The observed absorption patterns in the OH stretching region are consistent with the theoretically predicted spectra of two structurally distinct isomers of exceptional stability: a drop-like cluster with a fully coordinated (interior) water molecule and an edge-sharing pentagonal prism cluster in which all atoms are on the surface. The drop-like structure is the first experimentally detected water cluster exhibiting the local connectivity found in liquid water
Size-resolved infrared spectroscopic study of structural transitions in sodium-doped (H2O)n clusters containing 10-100 water molecules
In water clusters containing 10â100 water molecules the structural transition takes place between âall surfaceâ structures without internally solvated water molecules to amorphous water clusters with a three dimensionally structured interior. This structural evolution is explored with rigorous size selection by IR excitation modulated photoionization spectroscopy of sodium-doped (H2O)n clusters. The emergence of fully coordinated interior water molecules is observed by an increased relative absorption from 3200 to 3400 cmâ1 in agreement with theoretical predictions and earlier experimental studies. The analysis has also shown that the intermediate-sized water clusters (n = 40â65) do not smoothly link the structures in the largest and smallest analyzed size regions (n = 15â35 and n = 100â150) in line with previous reports suggesting the appearance of exceptionally stable water cluster isomers at n = 51, 53, 55, and 57. In the size range from n = 49 to n = 55 a reduced ion yield, a plateau in the total IR signal gain and signatures in the distribution of free OH stretch oscillator absorption have been observed. Recently reported putative global minima structures for n = 51 and n = 54 point to the presence of periplanar interior rings in odd-numbered clusters in this size range, which may affect cluster (surface) stability and the shape of the free OH stretch absorption peak. Potential links between pure and sodium-doped water cluster structures and the signatures of solvated electrons in photoelectron spectra of anionic water clusters are discussed
The temperature and size distribution of large water clusters from a non-equilibrium model
A hybrid Lagrangian-Eulerian approach is used to examine the properties of water clusters formed in neon-water vapor mixtures expanding through microscale conical nozzles. Experimental size distributions were reliably determined by the sodium doping technique in a molecular beam machine. The comparison of computed size distributions and experimental data shows satisfactory agreement, especially for (H2O) n clusters with n larger than 50. Thus validated simulations provide size selected cluster temperature profiles in and outside the nozzle. This information is used for an in-depth analysis of the crystallization and water cluster aggregation dynamics of recently reported supersonic jet expansion experiments
Determination of ppb concentrations of transition metals by radioisotope-excited energy-dispersive X-ray spectrometry
A new method for quantitative determiantion of copper by photoactivation analysis using a betatron
Investigation of a method for the determination of lead, cadmium, mercury, arsenic and tellurium in food by means of X-ray fluorescence analysis
Nucleation of Mixed Nitric AcidâWater Ice Nanoparticles in Molecular Beams that Starts with a HNO<sub>3</sub> Molecule
Mixed (HNO<sub>3</sub>)<sub><i>m</i></sub>(H<sub>2</sub>O)<sub><i>n</i></sub> clusters generated in supersonic
expansion of nitric acid vapor are investigated in two different experiments,
(1) time-of-flight mass spectrometry after electron ionization and
(2) Na doping and photoionization. This combination of complementary
methods reveals that only clusters containing at least one acid molecule
are generated, that is, the acid molecule serves as the nucleation
center in the expansion. The experiments also suggest that at least
four water molecules are needed for HNO<sub>3</sub> acidic dissociation.
The clusters are undoubtedly generated, as proved by electron ionization;
however, they are not detected by the Na doping due to a fast charge-transfer
reaction between the Na atom and HNO<sub>3</sub>. This points to limitations
of the Na doping recently advocated as a general method for atmospheric
aerosol detection. On the other hand, the combination of the two methods
introduces a tool for detecting molecules with sizable electron affinity
in clusters