The spatial distribution of argon clusters in gas jet

The spatial distribution of argon clusters in gas jet is tested using the Rayleigh scattering method. A pulsed laser is used to acquire the whole evolution of the cluster inside one event. The measured results at a fixed axial position show that the argon clusters grow in less than one millisecond after the nozzle is opened and the cluster size keeps constant during the whole open period of 20 ms. Further results show that the scattering signal along the radial direction is almost Gaussian-distributed and the full width half maximum (FWHM) increases almost linearly when the distance from nozzle increases. The scattering signal along the axial direction is Landau-distributed and the area near the nozzle is most effective for laser cluster interaction.     

Vibrational predissociation of UF6 clusters in supersonic Laval nozzle flow

The time-dependent change in the concentration of UF₆ monomers populated in the ground state was monitored in a supersonic Laval nozzle flow with an infrared diode-laser spectrometer in which the frequency of the laser beam was fixed at the = 1 0 transition (627.7 cm^–1) of the ₃ vibrational mode of the²³⁸UF₆ monomer. The concentration of UF₆ monomers in the ground state increased immediately after a single shot from a Raman laser tuned to a vibrational mode of UF₆ clusters (614.8 cm^–1) was applied to the gas in the nozzle. Subsequently, this concentration leveled off and slowly returned to the previous level. These results indicate that the population of UF₆ monomers in the ground state increased as a result of the predissociation of UF₆ clusters vibrationally excited with Raman laser radiation. It is demonstrated that one can utilize this procedure for vibrational predissociation of UF₆ clusters as a technique to increase the concentration of UF₆ monomers in an irradiation zone for molecular laser isotope separation of uranium.     

Formation and photodecomposition of cationic titanium oxide clusters

In this paper we report on the titanium oxide cluster cations Ti _x O _y ⁺, generated by laser ablation of a titanium target in the region of the nozzle expansion of oxygen. The mass distribution of the clusters produced is recorded with a time-of-flight mass spectrometer. Three different series, namely TiO(TiO₂) _n ⁺ , TiO(TiO₂) _n O₂⁺, and (TiO₂) _n ⁺, appear in the spectra. Two different ablation wavelengths (infrared at 1064 nm and ultraviolet at 308 nm) are used to generate the titanium oxide clusters. At the shorter wavelength the maximum size of the clusters formed decreases. The interaction of the UV photons with the Ti _x O _y ⁺ clusters is further investigated in a separate two-laser arrangement with an IR laser for ablation and after some mm downstream with an UV system for the cluster beam irradiation. These studies indicate that the intensity of the T _x O _y ⁺ clusters with x≥4, y≥7 is strongly influenced by the absorption of UV photons. This is attributed mainly to dissociation into smaller ones.     

Growth of van der Waals clusters in steady supersonic Laval nozzle flow: change in number density of SF6 monomers undergoing clustering

A mixture of SF₆ (4% molar fraction) and inert Ar gas was continuously fed into a plane-symmetric guided Laval nozzle. The clustering of SF₆ was observed using an FT-IR spectrometer in the nozzle flow (optical pathlength: 10 mm). The density and vibrational temperature of the SF₆ monomers were experimentally determined from the absorption peaks of the monomers. We theoretically calculated the density of the SF₆ monomers as a function of distance from the throat of the nozzle using a cluster formation model based on the RRK (Rice–Ramsperger–Kassel) theory to represent the dynamics of dissociation of the vibrationally excited clusters. We found that the model adequately simulated the decrease in SF₆ monomers and 10 collisions of the vibrationally excited SF₆ clusters with Ar atoms were necessary for the stabilization of the clusters under our experimental conditions. Received: 20 June 2000 / Revised version: 26 October 2000 / Published online: 21 February 2001     

Selecting molecules embedded in nanodroplets (clusters) of superfluid helium

A method of selecting molecules embedded in nanodroplets (clusters) of superfluid helium is proposed, which is based on the selective vibrational excitation of embedded molecules by intense IR laser radiation. This action leads to a significant decrease in size of the excited clusters, after which these clusters are separated with respect to size via scattering of the cluster beam on a crossing atomic beam. The method is described in detail and the possibility of selecting SF₆ molecules in liquid helium nanodroplets using the excitation by CO₂ laser radiation and the angular separation via scattering on a xenon atomic beam is demonstrated. The results show that, by using this technique, it is possible to separate molecules with respect to isotope (element) composition. Advantages and drawbacks of the method are analyzed.     

Condensation of monosilane-argon and monosilane-helium mixtures in free jets

Condensation of gaseous monosilane-argon and monosilane-helium mixtures was investigated in free jets by a Rayleigh scattering laser diagnostic technique. The condensation of a SiH₄-Ar mixture begins to develop at a lower stagnation pressure and at a shorter distance from the nozzle, and proceeds at a higher rate, as compared to condensation in monosilane-helium and pure argon jets. The results of Rayleigh scattering measurements in condensing monosilane-argon jets scale with the parameter P ₀ d ^0.8. An analysis of the results obtained in this study and found in literature suggests that simultaneous monosilane-argon condensation lead-ing to the formation of mixed clusters takes place in the monosilane-argon mixture.     

Role of Ga-doping in ironben gallium alloy clusters

The structural and magnetic properties of Fe_n-mGa_m (n=3～6, m=0～2; n=13, m=0～3) alloy clusters have been studied using density functional theory. The substitutional doping is favourable for small clusters with up to six atoms at low Ga concentration and substitutional Ga atoms in 13-atom clusters prefer surface sites. The Ga-doping generally could reduce the energetic stability but enhance the electronic stability of Fe clusters, along with a decrease of the local magnetic moments of Fe atoms around Ga dopants. These findings provide a microscopic insight into Fe-Ga alloys which are well-known magnetostriction materials.     

Influence of radiation-induced clusters on transport properties of silicon

Summary A calculation method for the scattering cross-section σ of charged carriers on radiation-induced cluster defects has been developed using a spherical cluster model with rectangular potential barrier shape, of radius and height of 15 nm and 0.6 eV, respectively. Values of the cluster cross-section around 2·10⁻¹¹ cm² have been obtained for charged carrier energies from 10⁻⁴ eV to over 600 eV. Applying the relaxation-time approximation of the Boltzmann equation, the influence of clusters on silicon transport properties has been observed to be close to the acoustic-phonon one. The dependence of the Hall factor on radiation-induced clusters has been determined numerically for temperatures ranging from 5 K to 400 K. The results indicate that the presence of clusters of such dimensions would not change significantly the Hall coefficientR _H.     

Spectroscopy of alkali atoms and molecules attached to liquid He clusters

Large helium clusters, ranging in size from a few hundred to several thousand atoms, are produced in a nozzle expansion. Combining this source with a pick-up scattering cell in which the clusters can be seeded with chromophores allows us to probe the influence of the helium environment on the atoms and molecules attached to the clusters. Using an alkali as chromophore we recorded laser induced fluorescence spectra of Na atoms and molecules attached to helium clusters. Apart from the spectrum of the Na monomer, we have found spectroscopic bands which can unambiguous be assigned to two bound Na atoms. The first of this bands is due to 1^{1 +} (A) 1¹ _g ⁺ (X) excitations of the covalently bound singlet Na₂ molecule while the second is due to 1³ _g ⁺ 1^{3 +} excitations for the van der Waals bound triplet Na₂ dimer. Both bands have been vibrationally resolved. Furthermore we found very large fluorescence intensities in the region 605–635 nm which are likely due to the excitation of a species containing three Na atoms attached to a helium cluster.     

Water-silica interaction in clusters

The interaction of water with SiO ₂ is an important problem in geophysics, materials physics, and environmental science. In this paper, we present recent results on studies of H ₂ O-silica clusters from first-principles Born-Oppenheimer molecular dynamics calculations. Bond strength and chemical stability are investigated as a function of cluster size and chemical composition. Both physisorption and chemisorption of water molecules on the clusters are discussed via analysis of energetics. Calculations of clusters are compared with the results from extended surfaces. The validity of clusters as models of surfaces is discussed.     

Spin-polarised DFT study of cobalt-doped gallium clusters

The structural, electronic and magnetic properties of small gallium clusters doped with Cobalt have been studied using spin-polarised density functional theory. The binding energy per atom, second-order differences of total energies and fragmentation energies of equilibrium geometries of the host Ga_n+1 and doped Ga_nCo (n = 1–12) clusters are computed. Doped clusters are found to be more stable than pure Ga clusters; Ga₃Co, Ga₅Co and Ga₈Co clusters are exceptionally stable. Doping with Co changes the highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO–LUMO) gap, and also affects the magnetic moments of clusters.     

Theoretical study of small Mo clusters and molecular nitrogen adsorption on Mo clusters

This paper studies the small molybdenum clusters of Mo_n (n=2--8) and their adsorption of N₂ molecule by using the density functional theory (DFT) with the generalized gradient approximation. The optimized structures of Mo_n clusters show the onset of a structural transition from a close-packed structure towards a body-centred cubic structure occurred at n=7. An analysis of adsorption energies suggests that the Mo₂ is of high inertness and Mo₆ cluster is of high activity against the adsorption of N₂. Calculated results indicate that the N₂ molecule prefers end-on mode by forming a linear or quasi-linear structure Mo--N--N, and the adsorption of nitrogen on molybdenum clusters is molecular adsorption with slightly elongated N--N bond. The electron density of highest occupied molecular orbital and lowest unoccupied molecular orbital, and the partial density of states of representative cluster are also used to characterize the adsorption properties of N₂ on the sized Mo_n clusters.     

Palladium clusters in nanoporous carbon samples: Structural properties

The structure of nanoporous carbon samples (prepared from silicon and boron carbides) with incorporated palladium clusters was studied. X-ray and electron diffraction studies show that most Pd clusters have an fcc lattice. Using small-angle X-ray scattering measurements, the metal cluster sizes are determined under certain assumptions. The sizes are not very close to the cluster sizes (4–14 nm) that are found from photomicrographs obtained using a transmission electron microscope (TEM). The difference is likely due to the local character of the TEM measurements. Apart from relatively coarse clusters of the sizes indicated above, the samples contain very small clusters, which are smaller in size than a micropore. Such small clusters are particularly large in number in C(SiC)B: Pd, where they are 0.5–0.7 nm in size. The content of small clusters in C(B₄C)B: Pd is substantially lower, and they are somewhat larger in size (1.2–1.6 nm). The possible reasons of the ferromagnetism observed in these samples are discussed. It is assumed that the magnetism may be due to the small clusters, which do not have cubic symmetry. Original Russian Text ? A.M. Danishevskiĭ, R.N. Kyutt, A.A. Sitnikova, B.D. Shanina, D.A. Kurdyukov, S.K. Gordeev, 2009, published in Fizika Tverdogo Tela, 2009, Vol. 51, No. 3, pp. 604–608.     

CARS spectroscopy of molecules and clusters in supersonic jets

Supersonic molecular beams of D₂, CH₄, NH₃, and C₂H₄ are investigated in the expansion region employing collinear coherent anti-Stokes Raman spectroscopy (CARS). The analysis of rotationally resolved CARS spectra allows the determination of temperatures in the beam. The rotational relaxation as a function of stagnation pressure and separation from the nozzle is studied by recording theQ branch for D₂ and the ₃ R andS branches for CH₄. Rotational temperatures for NH₃ are determined by investigating the complete ₃ band. At strong stagnation conditions broad structures arise which can be attributed to the formation of NH₃ clusters. For C₂H₄ the ₅ band with resolved rotational structure is reported. Again, at larger distances from the nozzle, broad structures are observed. They are assigned to the ₁ and ₅ vibrations in the C₂H₄ cluster.     

甲烷团簇与氘团簇的团簇源特性比较

利用瑞利散射法测量了特定喷嘴产生的甲烷团簇及氘团簇的大小随背压的变化曲线。将团簇大小的实验值与Hagena理论进行了比较,当背压超过3MPa时,两种团簇的实验值都明显高于理论值。实验发现,特定喷嘴在相同的背压下,常温(298K)甲烷团簇尺寸小于低温(80K)氘团簇尺寸,而甲烷团簇中氢原子数至少是氘团簇中氘原子数的1.98倍。     

密度泛函理论对ZrnPd团簇结构和性质的研究

用B3LYP/LANL2DZ方法对Zr_nPd(n =1—13)团簇的平衡几何结构、能量、频率、电子性质和磁性进行了计算.研究表明,Pd原子位于表面的异构体更为稳定,其中Zr₇Pd,Zr₁₂Pd团簇稳定性高,是幻数团簇,此外,相对于Zr_nCo与Zr_nFe团簇,Zr_nPd团簇参与化学反应的能力较弱,化学稳定性更     

DFT study of size-dependent geometries,stabilities and electronic properties of Si2Agn clusters: comparison with pure silver clusters

The equilibrium geometric structures, stabilities and electronic properties of Si₂Ag_n (n = 1–8) and pure silver Ag_n (n ≤ 10) clusters have been systematically investigated by using meta-generalised gradient approximation. Due to sp³ hybridisation, the lowest energy structures of Si₂Ag_n clusters for n > 2 favour the three-dimensional structure. The silicon atoms prefer to be located at the surface of the host silver clusters. By analysing the relative stabilities, it is found that the di-bridged structure Si₂Ag₂ isomer is the most stable structure for Si₂Ag_n (n = 1–8) clusters. The highest occupied–lowest unoccupied molecular orbital gaps, exhibiting a pronounced even–odd alternation, indicate that the doped clusters with even number of atoms have enhanced chemical stability than those with odd number of atoms. The results of Wiberg bond indices and electronic localisation function show that the stronger Si–Si and Si–Ag interaction may be the main driving force for the higher stability of Si₂Ag_n clusters.     

Radiation defect clusters in electron-irradiated silicon

Radiation defect formation in Czochralski-grown (pulled) and vacuum float-zone n-Si (ρ=15 to 150 ωcm) irradiated by electrons with energy E_e=2.5 to 1200 MeV has been studied. The results have been obtained from an analysis of the dose, energy and temperature dependences of the concentration n, Hall mobility μ _n , and charge-carrier lifetime τ. The experimentally observed peculiarities in the variation of n, μ _n , and τ irradiation by electrons with E10 MeV have been explained taking into account the formation of radiation defect clusters in the bulk of the crystal induced by such electrons. The correlation coefficients K_i have bsen obtained which take into consideration the “evolution” of primary displacement cascades in the location of which clusters are formed. The parameters of radiation defect clusters produced by electrons of different energies have been calculated using these coefficients.     

Defect studies in small CdTe clusters

We study Cd vacancy formation in prototype stoichiometric and non-stoichiometric CdTe clusters with and without passivation. For certain clusters like Cd₁₃Te₁₆, vacancy leads to severe distortion of the geometry due to propagation of defect. Annealing of the vacancy out of the cluster is observed in all unpassivated clusters. Passivated clusters retain their initial geometry and vacancy induced structural distortions are not seen in these clusters since the defect gets localized. Vacancy also induces intragap states. However, it was observed that the passivation of the dangling bonds created by the vacancy removes the intragap states. In an attempt to have CdTe clusters with extrinsic carriers, we substituted a Cd atom by its adjacent atoms Pd/Ag/In/Sn in these CdTe clusters. Substitutional doping of Cd by metal atoms increases the stability of unpassivated clusters. For certain clusters, metal atom doping leads to a half-metallic character. Pd/Ag-doped clusters are p-type semiconductors whereas In-doped clusters are n-type semiconductors. Sn doping in these clusters does not result in n-type semiconductors.     

密度泛函理论研究Hg与Auqn(n=1—6, q=0,+1,-1) 团簇的相互作用

采用密度泛函理论中的广义梯度近似对Hg与小团簇Au ^q_n (n=1—6, q=0, +1, -1)的相互作用进行了系统研究. 结果表明,除Au₅^+,-团簇外,前线分子轨道理论可以成功预测大部分Au _n Hg ^q 复合物的最低能量结构. Au_n团簇对Hg的吸附受团簇尺寸大小和团簇所携带电荷的影 关键词： 密度泛函理论 汞 金团簇 吸附能