Electric deflection studies of rhodium clusters

The static electric dipole polarizabilities of rhodium clusters Rhn, n=5-28, have been measured via a molecular beam deflection method. Uniform high-field beam deflections, indicative of induced polarization, were observed for all Rhn except Rh7 and Rh10 which by contrast exhibited beam broadening and anomalously high effective polarizabilities. Analysis of the beam deflection profile of Rh7 indicates that it possesses a permanent dipole moment of 0.24+/-0.02 D. Unlike the other clusters in the n=5-28 size range, the polarizability of Rh10 is observed to decrease with increasing source temperature. We attribute this temperature dependence to paraelectric behavior, suggesting that Rh10 is a fluxional molecule possessing a dipole moment that spatially fluctuates, uncorrelated with overall rotation.     

Electric deflection of rotating molecules

We provide a theory of the deflection of polar and nonpolar rotating molecules by inhomogeneous static electric field. Rainbowlike features in the angular distribution of the scattered molecules are analyzed in detail. Furthermore, we demonstrate that one may efficiently control the deflection process with the help of short and strong femtosecond laser pulses. In particular, the deflection process may be turned off by a proper excitation, and the angular dispersion of the deflected molecules can be substantially reduced. We study the problem both classically and quantum mechanically, taking into account the effects of strong deflecting field on the molecular rotations. In both treatments we arrive at the same conclusions. The suggested control scheme paves the way for many applications involving molecular focusing, guiding, and trapping by inhomogeneous fields.     

Magnetic deflection of neutral metal clusters in a beam

A new apparatus for measuring the magnetic properties of metal clusters has been constructed. The technique involves the conventional Stern-Gerlach deflection scheme together with modern pulsed laser vaporization source technology and time of flight mass spectrometry. High field seeking monodirectional deflections have been measured for cobalt clusters containing between 40 and 400 atoms. The measured magnetic moments per atom are found to be lower than the known values for the bulk. Special attention has been given to velocity measurements of the metal clusters and the carrier gas atoms in the beam. The residence time of the particles in the source cavity has been measured.     

Theoretical studies on hydroquinone-benzene clusters

High-level ab initio calculations were carried out to evaluate the interaction between the hydroquinone and benzene molecules. The intermolecular interaction energy was calculated using the M?ller-Plesset second-order perturbation theory at the complete basis set limit and also at the coupled cluster theory with single, double, and perturbatively triple excitations. The calculated binding energy is larger than the benzene dimer interaction energy. The T-shaped cluster (T-a) and the parallel conformation (P-a) are calculated to be nearly isoenergetic. Owing to the large energy gain in the attraction by electron correlation, the dispersion interaction is important for the attraction.     

Beam broadening of polar molecules and clusters in deflection experiments

A beam of rotating dipolar particles (molecules or clusters) will broaden when passed through an electric or magnetic field gradient region. This broadening, which is a common experimental observable, can be expressed in terms of the variance of the distribution of the resulting polarization orientation (the direction cosine). Here, the broadening for symmetric-top and linear rotors is discussed. These two types of rotors have qualitatively different low-field orientation distribution functions, but behave similarly in a strong field. While analytical expressions for the polarization variance can be derived from first-order perturbation theory, for experimental guidance it is important to identify the applicability and limitations of these expressions, and the general dependence of the broadening on the experimental parameters. For this purpose, the analytical results are compared with the full diagonalization of the rotational Stark-effect matrices. Conveniently for experimental estimations, it is found that for symmetric tops, the dependence of the broadening parameter on the rotational constant, the axial ratio, and the field strength remains similar to the analytical expression even outside of the perturbative regime. Also, it is observed that the shape envelope, the centroid, and the width of the orientation distribution function for a symmetric top are quite insensitive to the value of its rotational constant (except at low rotational temperatures).     

Electric dipole polarizabilities of copper clusters

The static electric dipole polarizabilities of Cu(9)-Cu(61) have been measured via a molecular beam deflection method. The clusters display per-atom polarizabilities that decrease monotonically with size, from approximately 16 A(3) per atom Cu(9-10) to approximately 5 A(3) (Cu(45-61)). Absent are any discernible discontinuities or odd-even alternations due to electronic shell filling or electron pairing effects. For the smallest clusters, the experimental polarizabilities are approximately 3 times larger than those predicted classically for conducting ellipsoids, and approach the classical values only for clusters containing more than approximately 45 atoms.     

Cyclic voltammetry studies on pure lead

A potentiometric study of complex formation between gallium and L-serine, L-tyrosine, L-methionine shows the existence of the complexes Ga(L₀)³⁺ and Ga(L_?)²⁺. In the case of L-cysteine, the complex Ga(L_2?)⁺ exists also. L₀, L_?, L_2? designate respectively the zwitterion and the mono- and divalent anions of amino acid. The stability constants of the complexes have been determined.     

Reaction studies of carbon clusters

Reaction studies of carbon clustersC _n in the rangen=8–37, produced by laser vaporisation in a supersonic nozzle, have been investigated using time-of-flight mass spectrometry. Clear differences in reaction products formed on hydrogenation are detected which show that even and odd clusters behave quite differently and furthermore thatat least two different types of even cluster appear to exist. The reactivity patterns for clusters C _n withn=16, 18 and 22 are in a different class from those withn=20, 24, 26 ..., a behaviour consistent with the existence of closed cage fullerene structures for even clusters with 20 or more carbon atoms (other thann=22).     

A molecular beam electric deflection study of clusters: acid and salt molecules in microscopic aqueous and ammonia clusters

Results of a molecular beam electric deflection study of the polarity of gas phase clusters comprised of HNO₃H₂O, HNO₃H₂ONH₃, and NH₄HSNH₃ are reported. No refocusing was observed for any species containing more than two molecular subunits. These results are contrasted with previous studies made in our laboratory which showed focusing for higher polymers of acetic acid; possible reasons are given for observed defocusing in the present systems where ion-pair formation is expected to occur.     

Size effect on the structural and electronic properties of lead telluride clusters

Theoretical computations of (PbTe)_n (n = 21–45) clusters based on density functional theory have demonstrated that at cluster size of (PbTe)₂₂ there is a transition from the strong preference of fivefold coordination to sixfold coordination of lead and tellurium atoms. (PbTe)₂₄ cluster is the smallest tetragonal structure in which its central atoms have bulk‐like coordination. This quantum dot (QD) contains a single‐unit cell of lead telluride crystal, thus can be considered as an “infant crystal.” (PbTe)₃₂ cluster is a perfectly cubic cluster for which its inner (PbTe)₄ core enjoys bulk‐like coordination. This (PbTe)₄ core unit of (PbTe)₃₂ cubic cluster has exactly the same environment as a primitive cell of lead telluride crystal. The (PbTe)_8n, (n ≥ 3) clusters are the magic number species with bulk‐like structure such that (n = 3–5) the nanoblocks considered here (PbTe)₂₄, (PbTe)₃₂, and (PbTe)₄₀ clusters exhibiting bulk‐like structure that can be replicated to obtain the bulk crystal. The calculated dimensions of this special clusters provided a rubric for understanding the pattern of aggregation, that is, the creation of defined nanoblocks [(PbTe)_8n, (n ≥ 6)], when they were accumulated on an appropriate surface. It is evident that the QDs (PbTe)_8n, (n = 3–5) clusters show high stability compared to their neighboring clusters. This can also be seen from the second‐order energy difference, binding, and fragmentation energy graphs. © 2014 Wiley Periodicals, Inc.     

Structural evolution and stabilities of neutral and anionic clusters of lead sulfide: joint anion photoelectron and computational studies

The geometric and electronic structures of both neutral and negatively charged lead sulfide clusters, (PbS)(n)/(PbS)(n)(-) (n = 2-10) were investigated in a combined anion photoelectron spectroscopy and computational study. Photoelectron spectra provided vertical detachment energies (VDEs) for the cluster anions and estimates of electron affinities (EA) for their neutral cluster counterparts, revealing a pattern of alternating EA and VDE values in which even n clusters exhibited lower EA and VDE values than odd n clusters up until n = 8. Computations found neutral lead sulfide clusters with even n to be thermodynamically more stable than their immediate (odd n) neighbors, with a consistent pattern also being found in their HOMO-LUMO gaps. Analysis of neutral cluster dissociation energies found the Pb(4)S(4) cube to be the preferred product of the queried fragmentation processes, consistent with our finding that the lead sulfide tetramer exhibits enhanced stability; it is a magic number species. Beyond n = 10, computational studies showed that neutral (PbS)(n) clusters in the size range, n = 11-15, prefer two-dimensional stacking of face-sharing lead sulfide cubical units, where lead and sulfur atoms possess a maximum of five-fold coordination. The preference for six-fold coordination, which is observed in the bulk, was not observed at these cluster sizes. Taken together, the results show a preference for the formation of slightly distorted, fused cuboids among small lead sulfide clusters.     

Multiphoton ionization studies of xenon clusters

Non-resonant multiphoton ionization of xenon clusters has revealed the same magic numbers as found in the case of electron impact ionization. Large dissociation rates are found for the trimer through pentamer ion, measured on a time scale of ≈ 10⁻⁷ s after ionization.     

Electric Birefringence studies on hexaethyleneglycol bodecyl ether

Transient electri birefringence has been used to reveal variable micelle morphology in the L₁ micellar dispersion region of hexaethyleneglycol dodecyl ether. The results are consistent with the occurrence of rod micelles in the solution adjacent to the hexagonal phase, and their gradual disappearance as the hexagonal phase melts.     

Thermal studies on lead(II) salts

The thermal decomposition of synthetic lead(II) hydroxide chloride has been studied between 490 and 520 K over a range of partial pressures of water vapour. The reaction rate is controlled by diffusion of water vapour and Arrhenius parameters have been calculated. The activation energy varies between 92 and 145 kJ mol^?1 and the frequency factor between 10⁹ and 10¹⁵ sec^?1. Water vapour pressure has a profound effect on the reaction rate and a double minimum in the rate is observed. Enthalpimetric data are also presented for the decomposition.     

Core edge energy loss studies of Pd clusters on graphite

In order to verify that the d-band of Pd clusters moves towards higher binding energies when the cluster size decreases, we have carried out energy loss measurements of the M_4,5 core edge of Pd clusters with different diameters evaporated on polycristalline graphite. Generally the shifts have been attributed in the past both to initial and or final state effects. In the last case one could not observe variations with our technique which probes the partial unfilled density of states. Our experimental results show an enhancement of the absorption and a narrowing of the lineshape with increasing cluster size. We attribute the observed effects to a genuine change in the electronic structure which occurs in the initial state of the cluster. Comparative studies of this effect have been performed following the evolution of the M_4,5 VV Auger levels and the collective excitations in the range 0–30 eV far from the elastic peak.     

Molecular beam multiphoton-ionization studies on ammonia—water binary clusters

Two-photon ionization mass spectra are obtained for NH₃H₂O binary clusters both with a nozzle beam and an ArF excimer laser. The detected major ions are H⁺(NH₃)_n(H₂O)_m(1 <m + n < 9). The results suggest that ammonia molecules constitute an inner shell which is surrounted by water molecules.     

Photoionization studies of free sodium ammonia clusters

Free sodium ammonia clusters Na(NH₃) _n up ton=45 were generated in a pickup source by injecting a beam of neutral sodium atoms into the expansion zone of a piezo driven pulsed nozzle. The clusters thus formed are studied by one-photon ionisation in the region of 266 nm to 520 nm, time-of-flight mass spectrometry as well as photoelectron spectrometry. Ionisation thresholds for clusters up ton=18 and dissociation energies for the neutral Na(NH₃) _n up ton=6 are reported.     

Ab initio studies of small AlmFen clusters

The equilibrium geometrical structures of small Al_mFe_n clusters have been determined through ab initio calculation of the cluster total energy at the UB3LYP/Lanl2dz level. For dimers of iron and aluminum, the dissociation energies, the equilibrium atomic distances, and the vibrational frequencies were calculated. The agreement between calculations and experiments is reasonable. The ground stable geometrical structures of Fe₄, FeAl₃, Fe₃Al and Fe₂Al₂ clusters favor three-dimension configurations, but Al₄ tetramers are planar structures. The Al-rich tetramers are more stable than the other two composition tetramers. This is different from that of bulk alloys.     

Computational studies of nonstoichiometric sodium auride clusters

The molecular structures of low-lying isomers of anionic and neutral sodium auride clusters have been studied computationally at the second-order M?ller-Plesset perturbation theory level using quadruple-ζ basis sets augmented with a double set of polarization functions. The first vertical detachment energies were calculated at the M?ller-Plesset level as the energy difference between the cluster anion and the corresponding neutral cluster. The photodetachment energies of higher-lying ionization channels were calculated by adding electronic excitation energies of the neutral clusters to the first vertical detachment energy. The excitation energies were calculated at the linear response approximate coupled-cluster singles and doubles level using the anionic cluster structures. The obtained ionization energies for NaAu(-), NaAu(2)(-), NaAu(3)(-), NaAu(4)(-), Na(2)Au(2)(-), Na(2)Au(3)(-), Na(3)Au(3)(-), and Na(2)Au(4)(-) were compared to values deduced from experimental photoelectron spectra. Comparison of the calculated photoelectron spectra for a few energetically low-lying isomers shows that the energetically lowest cluster structures obtained in the calculations do not always correspond to the clusters produced experimentally. Spin-component-scaled second-order M?ller-Plesset perturbation theory calculations shift the order of the isomers such that the observed clusters more often correspond to the energetically lowest structure, whereas the spin-component-scaled approach does not improve the photodetachment energies of the sodium aurides. The potential energy surface of the sodium aurides is very soft, with several low-lying isomers requiring an accurate electron correlation treatment. The calculations show that merely the energetic criterion is not a reliable means to identify the structures of the observed sodium auride clusters; other experimental information is needed to ensure a correct assignment of the cluster structures. The cluster structures of nonstoichiometric anionic sodium aurides have been determined by comparing calculated ionization energies for low-lying structures of the anionic clusters with experimental data.     

Systematic studies on adsorption of lead on sea nodule residues

Lead adsorption from aqueous solutions was studied with the aim of detoxifying industrial effluents before their safe disposal onto land or into river waters. Sea nodule residue (SNR), a waste material containing oxides and oxyhydroxides of manganese, iron, silicon, etc., was used as an effective adsorbent for lead in this study. The effect of various parameters such as contact time, initial lead concentration, pulp density, particle size of the adsorbent, pH, and temperature was studied to optimize the conditions for maximum adsorption. Adsorption followed first-order kinetics and 99% of lead adsorption was achieved at a solid:liquid ratio of 1:330, in the pH range 5.5-6.0 at a particle size of -150 microm in 8 h for solution containing 200 ppm lead. The adsorption capacity was found to be 99.0 mg of lead per gram of SNR and the adsorption isotherms followed the Langmuir and Freundlich adsorption models. The mechanism of adsorption of lead onto the sea nodule residue was also investigated. It was possible to reduce the lead level from 25-200 ppm to acceptable levels (0.1 ppm) by adsorption over this solid waste.