The geometric structure of pure SF6 and mixed Ar/SF6 clusters investigated by core level photoelectron spectroscopy

The S 2p core level photoelectron spectra of Sulphurhexafluoride clusters have been investigated together with heterogeneous Ar/SF₆ clusters, created by doping Ar host clusters (with a mean size of 3600 atoms) with the molecule. Surface and bulk features are resolved both in the argon 2p and the sulphur 2p core level photoelectron spectra. For the latter level such features were only observed in the pure cluster case; a single feature characterizes the S 2p core level spectra of SF₆ doped argon clusters. From the chemical shifts, investigated with respect to SF₆ doping pressure. It can be concluded that the host clusters get smaller with increasing doping pressures and that the SF₆ molecules predominantly stay below the cluster surface, whereas the Argon core stays intact. We have neither observed features corresponding to SF₆ on the cluster surface, nor features corresponding to molecules deep inside the bulk in any of the spectra from the pick-up experiments.     

Detection of SF<Subscript>6</Subscript> molecules sublimating from the surface of (CO<Subscript>2</Subscript>)<Subscript><Emphasis Type="Italic">N</Emphasis></Subscript> nanoparticles in a cluster beam by the infrared multiphoton excitation method

It has been found that SF₆ molecules captured by large van der Waals clusters (CO₂) _N (where N ≥ 10² is the number of monomers in a cluster) in intersecting molecular and cluster beams sublimate from the surface of clusters after a certain time and carry information on the velocity and temperature (internal energy) of clusters. Experiments have been carried out for detecting these molecules by means of a pyroelectric detector and the infrared multiphoton excitation method. The multiphoton absorption spectra of molecules sublimating from the surface of clusters have been obtained. The temperature of the (CO₂) _N nanoparticles in the cluster beam has been estimated using these spectra and comparing them with the infrared multiphoton absorption spectra of SF₆ in the initial molecular beam.     

Local geometry and electronic structure of free NaCl clusters

Experimental x-ray spectra of the Cl L _{2, 3} absorption edge for free NaCl clusters of various sizes are presented. X-ray absorption spectra of the Na₄Cl₄ cluster have been theoretically calculated by the total multiple scattering method. A distorted cube is determined as the most probable geometric structure of the Na₄Cl₄ cluster. Curves of the partial densities of states are obtained for the cluster.     

Measurement of nanoparticle temperature in a (CO2) N cluster beam using SF6 molecules as tiny probe thermometers

A temperature measurement technique using SF₆ molecules as tiny probe thermometers is described, and results are presented, for large (CO₂) _N van der Waals clusters (with N ≥ 10²) in a cluster beam. The SF₆ molecules captured by (CO₂) _N clusters in crossed cluster and molecular beams sublimate (evaporate) after a certain time, carrying information about the cluster velocity and internal temperature. Experiments are performed using detection of these molecules with an uncooled pyroelectric detector and infrared multiphoton excitation. The multiphoton absorption spectra of molecules sublimating from clusters are compared with the IR multiphoton absorption spectra of SF₆ in the incoming beam. As a result, the nanoparticle temperature in the (CO₂) _N cluster beam is estimated as T _cl < 150 K. Time-of-flight measurements using a pyroelectric detector and a pulsed CO₂ laser are performed to determine the velocity (kinetic energy) of SF₆ molecules sublimating from clusters, and the cluster temperature is found to be T _cl = 105 ± 15 K. The effects of various factors on the results of nanoparticle temperature measurements are analyzed. The potential use of the proposed technique for vibrational cooling of molecules to low temperatures is discussed.     

Photoelectron spectroscopy of size-selected cluster ions using synchrotron radiation

A new experimental setup for photoelectron spectroscopy of size-selected cluster ions using synchrotron VUV radiation as generated by the Swiss Light Source is presented. An intense positively charged cluster ion beam is produced in a high-intensity magnetron sputter source. The clusters are subsequently mass selected in a sector magnet. To maximize the residence time of the cluster ions in the ionization region of the velocity map imaging spectrometer, the cluster ion beam is decelerated where it crosses the light beam. First experiments on (MoO₃) _n ⁺ (n = 69 and 59) cluster cations show that the approach is capable of delivering photoelectron spectra of size-selected transition metal cluster ions.     

Photoemission investigations on nanostructured TiO2 grown by cluster assembling

Nanostructured titanium dioxide (ns-TiO₂) films were grown by supersonic cluster beam deposition method. Transmission electron microscopy demonstrated that films are mainly composed by TiO₂ nanocrystals embedded in an amorphous TiO₂ phase while their electronic structure was studied by photoemission spectroscopy. The cluster assembled ns-TiO₂ films are expected to exhibit several structural and chemical defects owing to the large surface to volume ratio of the deposited clusters. Ultraviolet photoemission spectra (hv = 50 eV) from the valence band unveil the presence of a restrained amount of surface Ti 3d defect states in the band gap, whereas Ti 2p core level X-ray photoelectron (hv = 630 eV) spectra do not manifestly disclose these defects.     

Small copper clusters: Study of the local atomic and electronic structure by the XANES and DFT methods

The Cu L ₃ edge X-ray absorption spectra of free Cu _n clusters containing 5–15 atoms were obtained for the first time. It is shown that the geometry of small clusters, including the bond length and bond angle, can be studied by analyzing the X-ray absorption spectra. The experimental X-ray absorption spectra of Cu₁₃ clusters were theoretically interpreted using the self-consistent method of full multiple scattering, the finite difference method in the muffin-tin potential approximation, and the full potential method for the icosahedral cluster structure. Good agreement between the theoretical and experimental spectra is achieved for the calculation in the full potential approximation.     

Cluster generation by laser ablation

We produced niobium clusters over a wide size range (Nb_n: 1≤n≤1000) by a laser ablation method in a low-pressure condensation source. Mass spectra of the neutral clusters, positive and negative ion clusters were measured by a time-of-flight mass spectrometer with a high mass resolution and a wide mass range. Similarity and difference in their mass spectra were examined to understand the cluster generation process in different charged states.     

The electronic structure of NbO: Theory and experiment

Experimental photoelectric- and X-ray emission spectra obtained from NbO are compared with the molecular orbital calculations of NbO₆^10? cluster using X_α discrete variational method. The agreement between theory and experiment establishes the validity of the cluster approach to determine the electronic structure of the 4d transition metal chalcogenides.     

Fluid synthesis and structure of a new polymorphic modification of boron nitride

A new previously unknown phase of boron nitride with a hardness of 0.41–0.63 GPa has been pre-pared by the supercritical fluid synthesis. The presence of a new phase is confirmed by the X-ray spectra and IR absorption spectra, where new reflections and bands are distinguished. The fundamental reflection of the X-ray diffraction pattern is d = 0.286–0.291 nm, and the characteristic band in the infrared absorption spectrum is observed at 704 cm^?1. The X-ray diffraction pattern and the experimental and theoretical infrared absorption spectra show that a new synthesized boron nitride phase can be a cluster crystal (space group 211) with a simple cubic lattice. Cage clusters of a fullerene-like morphology B₂₄N₂₄ with point symmetry O are arranged in lattice sites.     

Charge-transfer transitions and optical spectra of chromites

Specific features of charge-transfer states and charge-transfer transitions of the O_2p → Cr_3d type in octahedral complexes (CrO₆)^9? have been considered in the cluster approximation. Reduced matrix elements of the electric-dipole moment operator on many-electron wave functions, which are the initial and final states for charge-transfer transitions, are calculated. The results are parameterized, and the relative intensities of different allowed charge-transfer transitions in the absence of mixing of different charge-transfer configurations with identical symmetry are calculated. This mixing is taken into account within the Tanabe-Sugano theory, and the true energies and intensities of many-electron charge-transfer transitions are obtained. The Coulomb interaction between 2p electrons of O^2? ligands and 3d electrons of the central Cr³⁺ ion in (CrO₆)^9? cluster is considered. The influence of this interaction on the optical spectra is found to be insignificant. Simulation of the optical spectra of chromium oxides has shown the presence of a band of complex charge-transfer transitions composed of 33 lines with a total width of about 8 eV. The model spectrum is in adequate agreement with the experimental data, which indicates limited applicability of the widespread view that charge-transfer transition spectra have a simple structure.     

A molecular cluster model of the Vo. center in paratellurite

A RHF-SCF electronic structure calculation has been performed by means of the MELD program package for a cluster model of the V_o^.-center, a basic radiation-induced defect in paratellurite identified recently by ESR as an oxygen vacancy defect with a net positive charge. A “perfect” Te₂O₇H₆ cluster has been constructed with an effective core potential and an STO-3G valence basis set complemented by a single Gaussian d-function on Te. The charge distribution inside this cluster has been optimized by adjusting positions and nuclear charges of the peripheral “hydrogens”. A Mulliken analysis of a Te₂O₆H₆ defect cluster derived from the perfect one by removing an O⁻ ion and relaxing both Te's showed that the unpaired spin is primarily concentrated on the Te nearest to the oxygen vacancy with appreciable spin densities on nearby atoms and a substantially modified distribution of the surplus charge. The open-shell wave function has also been employed to derive hyperfine constants, taking into account relativistic corrections in independently calculated values of |;ψ_5s(0)|² and 〈r⁻³〉_5p for the Te atom, but neglecting core polarization. The calculated spin populations and hyperfine parameters account for all major features of the ESR spectra.     

Rotation-vibration energy cluster formation in XH2D and XHD2 molecules (X = Bi, P, and Sb)

We investigate theoretically the energy cluster formation in highly excited rotational states of several pyramidal XH₂D and XHD₂ molecules (X = Bi, P, and Sb) by calculating, in a variational approach, the rotational energy levels in the vibrational ground states of these species for J?70. We show that at high J the calculated energy levels of the di-deuterated species XHD₂ exhibit distinct fourfold cluster patterns highly similar to those observed for H₂X molecules. We conclude from eigenfunction analysis that in the energy cluster states, the XHD₂ molecule rotates about a so-called localization axis which is approximately parallel to one of the X-D bonds. For the mono-deuterated XH₂D isotopologues, the rotational spectra are found to have a simple rigid-rotor structure with twofold clusters.     

Site-specific valence-band X-ray photoelectron spectra of a SrTiO3 single-crystal by X-ray standing wave technique

Site-specific valence-band X-ray photoelectron spectra of SrTiO₃ (111) were successfully obtained by using X-ray standing wave technique. Contributions of the Ti and SrO₃ derived states to the valence-band spectra were clearly separated. The spectra provided not only site-specific but also bulk-sensitive information on the SrTiO₃ crystal because of the use of a high-energy synchrotron X-ray source (hν=4750 eV) for photoelectron excitation with the large escape depth. The electronic structures calculated by the DV-Xα method using a (Sr₈Ti₂₇O₁₀₈)⁹²⁻ cluster model well reproduced the observed structures in the valence-band spectra. The partial density of states of both Ti and Sr ions in SrTiO₃ were mainly distributed over the bottom of the valence-band to produce the covalent bonding with O ions.     

Determination of time dependence of domain growth by direct observation of small metal clusters

The in situ growth of small gold clusters on amorphous carbon substrates has been measured by direct observation, utilizing high (single-atom) resolution scanning transmission electron microscopy. The time dependence of the width of the distribution of characteristic microscopic spatial frequencies in the samples was seen to obey a power-law behavior, HWHM = (t − t₀)^−a. The exponent measured, a = 0.15 ± 0.06, was in good agreement with scaling theories for domain growth. An excess of single atoms and small clusters was observed at all times, over that predicted by a uniform growth model of clusters. It is suggested that these support single-atom and few-atom cluster migration and activated accretion as important components of cluster growth at all times.     

Endohedral nitride cluster fullerenes: Formation and spectroscopic analysis of L3−xMxN@C2n (0≤x≤3; N=39,40)

The class of endohedral fullerenes is demonstrated in its large variety by the new type of nitride cluster fullerenes. These endohedral fullerenes were obtained as the most abundant fullerenes in the soot extract. This is reached by the concept of reactive gas atmosphere in arc burning fullerene production. The chemical reactions in the gas atmosphere during the production are described. Different nitride cluster fullerenes were produced with selectivity up to 90% by using reactive gas addition to the cooling gas of the arc burning process. Fullerenes prepared by this method are Sc₃N@C₈₀, Sc_3−xEr_xN@C₈₀ (x=1,2,3), Sc₃N@C₇₈, Y₃N@C₈₀, Ho₃N@C₈₀ and Tb₃N@C₈₀.By studying Vis/NIR spectra, it is demonstrated that nitride cluster fullerenes are generally large band-gap endohedrals. Therefore, the M₃N@C₈₀ structures are very stable and suitable for applications. This stability is caused by a charge transfer from the cluster to the carbon cage and the formation of a M₃N–carbon cage bond as well as covalent metal–nitrogen bonds. Infrared spectroscopy of M₃N@C₈₀ was used to study the metal and cage size influence on the structure of the nitride cluster.     

Molecular rearrangement and cluster formation in the secondary ion mass spectra (SIMS) of fluoride salts

Secondary ion clusters with mass greater than 700 amu, e.g., K(KF)₁₂⁺ and up to 27 atoms, e.g., Na(NaF)₁₃⁺, have been observed in the static SIMS spectra of MF (M = Li, Na, K), NaBF₄, and KPF₆. The long series of detected cluster ions of the type M(MF)_n⁺ indicates that there is a high degree of stability associated with these clusters. The observation of such clusters in the NaBF₄ and KPF₆ spectra suggest that there is significant molecular rearrangement occurring in the secondary ion emission process from such salts. The secondary ion Intensities provide a crude fit to the Saha-Eggert equation, yielding an electron temperature of ~12,000 K. The data are consistent with the plasma model of surface ionization in which rearrangement and cluster formation occur in the plasma.     

UPS spectra of H2O,CH3OH and C5H9OH adsorbed onto Cu(111)/Na and Na(cp)

The present communication presents ultraviolet photoemission spectra (UPS) of three different “alcohols”; water (H₂O), methanol (CH₃OH), and cyclopentanol (C₅H₉OH), chemisorbed onto a Cu(111) surface partially covered by sodium atoms as well as onto closely packed sodium films, a free electron adsorbent. Whereas all three alcohols ROH bind reversibly and associatively to Cu(111) they react with adsorbed sodium atoms to metal bound alcoxides RO. The chemisorption bond, characterized by the interaction between O 2pπ orbitals and metal atoms as an electron donor, the alcoxide being the acceptor, is similar for all groups R. The O 2pπ orbitals shift to higher UPS binding energies with increasing electron density, i.e. decreasing r_s/a_o of the sodium overlayer. Only for HONa, the sterically smallest group R, does the alcoxide growth continue in three dimensions. Although, possibly failing to reproduce the electron density profile of a free electron surface, Hartree-Fock-Slater cluster calculations of small models ROH and RONa₃ enable correlations to be made between UPS intensity peaks and one electron orbitals.     

Probing electronic and structural properties of the reduced [2Fe−2S] cluster by orientation-selective1H ENDOR spectroscopy: Adrenodoxin versus rieske iron-sulfur protein

The¹H electron-nuclear double resonance (ENDOR) spectra in frozen buffer solutions of the reduced [2Fe?2S] clusters in adrenodoxin (Adx) and in the “Rieske” iron-sulfur protein (ISP) from the bovine mitochondrial bc1 complex were measured at low temperatures (5–20 K) and analyzed by spectra reconstruction. A single paramagnetic species with iron valence states (II) and (III) connected uniquely to the cluster irons was found in both proteins. For Adx, the experimental spectra from 23 field positions across the nearly axial (g _max=2.0241,g _int=1.9347, andg _min=1.9331) electron paramagnetic resonance (EPR) spectrum were analyzed. Four larger hyperfine couplings were assigned to the cysteine β-protons near the Fe(III) ion. Transfer into the crystal structure showed that the Fe(III) ion was coordinated to the residues Cys55 and Cys92. The spin density was estimated as +1.60 for the Fe(III) and ?0.6 for the Fe(II) ion, respectively. Theg-tensor direction with respect to the cluster showed strong similarities with the earlier assignment inArthospira platensis ferredoxin (Canne C., Ebelshauser M., Gay E., Shergill J.K., Cammack R., Kappl R., Hüttermann J.: J. Biol. Inorg. Chem. 5, 514, 2000). An Adx mutant (T54A) exhibiting a change (70 mV) in redox potential showed no significant influence at the [2Fe?2S] cluster. The Rieske ISP was subjected to the same analysis. The ENDOR spectra from 35 field positions across the rhombic (g _max=2.028,g _int=1.891, andg _min=1.757) EPR spectrum were simulated. Three major proton contributions were identified from the orientation behavior. Two were assigned to cysteine β-protons and one to a β-proton of His141. In contrast to Adx, the direction of theg _max-component was found to lie roughly in the FeS-core plane and the largest proton coupling occurred alongg _int. The spin population was estimated as about +1.6 for the oxidized and ?0.55 for the reduced iron.     

Raman scattering and electron-phonon coupling in mized-valent and superconducting compounds

Polarized Raman spectra of mixed-valent Sm_0.75Y_0.25S are presented and compared with those of semiconducting SmS and superconducting YS and TiN. The spectra are explained in terms of cluster deformabilities which also govern the phonon anomalies. The existence of a breathing Γ₁⁺ deformability of Sm in Sm_1?xY_xS, of S in YS, of N in TiN and a quadrupolar Γ₁₂⁺ deformability of N in TiN is demonstrated. The relationship between these results and the microscopic theory is discussed.