Temperature determination of (CF<Subscript>3</Subscript>I)<Subscript><Emphasis Type="Italic">N</Emphasis></Subscript> clusters in a beam by time-of-flight measurements of IR-laser excited SF<Subscript>6</Subscript> molecules sublimating from the surface of clusters

The method is described and the experimental results are presented on the temperature determination of the (CF₃I) _N clusters in a beam (N ⩽ 10² is a number of monomers in a cluster) using SF₆ molecules from intersecting molecular beam as probe thermometers. The SF₆ molecules are captured by clusters in the crossed cluster and molecular beams and, after a certain time, sublimate from the surface of clusters carrying information on the velocity and temperature (internal energy) of clusters. Using time-of-flight (TOF) method the kinetic energy (velocity) of sublimated SF₆ molecules was measured and the temperature of clusters was determined to be T _cl = (88 ± 15) K.     

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.     

Direct observation of intracluster reactions induced in (CF<Subscript>3</Subscript>I)<Subscript><Emphasis Type="Italic">n</Emphasis></Subscript> clusters by femtosecond ultraviolet radiation

Intracluster reactions that are induced in (CF₃I) _n clusters by femtosecond ultraviolet radiation, including the reaction of the formation of the I₂⁺ molecular ion, have been directly observed. It has been shown that there are two channels of the formation of I₂⁺ ions with the characteristic times τ₁ ≈ 1 ps and τ₂ ≈ 7 ps. A model of these reactions has been proposed that is in good agreement with the experimental data.     

Average decay rate constants and excitation energies for metal clusters sputtered by SF<Stack> <Subscript>5</Subscript> <Superscript>+</Superscript> </Stack> and inert gas ions

Results from a comparative SIMS study of the fragmentation of metal clusters sputtered with atomic Xe⁺ and molecular SF₅⁺ ions are presented. It is shown that the average decay rate constants and, hence, the excitation energies of clusters of the same stoichiometry do not depend on the type of bombarding ions.     

Polarization modes in the Ba<Subscript>2</Subscript>Mg<Subscript>2</Subscript>Fe1<Subscript>2</Subscript>O<Subscript>22</Subscript> multiferroic

The spectra of complex permittivity of a Ba₂Mg₂Fe₁₂O₂₂ single crystal belonging to the family of Y-type hexaferrites have been measured over a wide temperature range (10–300 K) with the aim of determining the dynamic parameters of the phonon and magnetic subsystems in the terahertz and infrared frequency ranges (3–4500 cm⁻¹). A factor-group analysis of the vibrational modes has been performed, and the results obtained have been compared with the experimentally observed resonances. The oscillator parameters of all nineteen phonon modes of E _u symmetry, which are allowed by the symmetry of the Ba₂Mg₂Fe₁₂O₂₂ crystal lattice, have been calculated. It has been found that, at temperatures below 195 and 50 K, the spectral response exhibits new absorption lines due to magnetic excitations.     

Effect of collisions on the resonance vibrational polarizability of gaseous SF<Subscript>6</Subscript> and CF<Subscript>4</Subscript> molecules

The density dependences of the absorption cross sections and refractivity are experimentally studied for the SF₆ and CF₄ molecules in pure gases in the region of their ν₃ infrared vibrational-rotational antisymmetric modes. The dispersions of the refractive index are determined for both compounds by the Kramers-Kronig transformation of the spectral data obtained, and, for the SF₆ isotopomers, they are also measured by the method of two-color interferometry. Strong nonlinear dependences of optical parameters and their dispersions on the gas density are observed. The values of second optical virial coefficient B _R (ν) obtained for pure SF₆ are more than an order of magnitude greater than the values found earlier for mixtures of SF₆ with buffer rare gases. The results of calculations of the second virial coefficients of the absorption cross section and refractivity in terms of the DID model of interacting dipoles are in agreement with the experimental data in the band wings. Correlations between the behavior of the spectral dependence of functions B _R (ν) and the parameters of model intermolecular potentials used in the calculations are found.     

Properties of ferromagnetic resonance in Fe<Subscript>73.5</Subscript>CuNb<Subscript>3</Subscript>Si<Subscript>13.5</Subscript>B<Subscript>9</Subscript> nanocrystalline alloys

The structural properties and parameters of ferromagnetic resonance have been studied for Fe_73.5CuNb₃Si_13.5B₉ nanocrystalline alloys produced from the initial amorphous state via annealing under different conditions. The dependence of the linewidth of the ferromagnetic resonance on the grain size ΔH ∼ D ⁶ has been found. The result is discussed within the framework of the random magnetic anisotropy model.     

Silicon isotope separation utilizing infrared multiphoton dissociation of Si<Subscript>2</Subscript>F<Subscript>6</Subscript> irradiated with two-frequency CO<Subscript>2</Subscript> laser lights

Silicon isotope separation has been performed utilizing the infrared multiphoton dissociation (IRMPD) of Si₂F₆ irradiated with two-frequency CO₂ laser lights. The two-frequency excitation method improved the separation efficiency by keeping the high enrichment factors. For example, Si₂F₆ with the ²⁸Si fraction of 99.4% was obtained at 40.0% dissociation of Si₂F₆ after the simultaneous irradiation of 100 pulses with 966.23 cm^-1 photons (0.089 J/cm²) and 954.55 cm^-1 photons (0.92 J/cm²), while 1000 pulses were needed to obtain 99.0% of ²⁸Si at 27.2% dissociation in the case of single frequency irradiation at 954.55 cm^-1 (0.92 J/cm²). The single-step enrichment factors of ²⁹Si and ³⁰Si increased with increasing Si₂F₆ pressure. The reason for this enhancement has been discussed in terms of the rotational and vibrational relaxations by collisions with ambient gases. PACS 42.62.Cf; 82.30.Lp; 82.50.Bc     

Isotope effects in the vibrational spectrum of the SF<Subscript>6</Subscript> molecule

The IR absorption spectra of solutions of SF₆ in liquid argon are studied at a temperature of 93 K in the concentration interval 10^?5–10^?7 mole fractions. A sample with a natural abundance of isotopes and a monoisotope ³⁴SF₆ sample are studied. The frequencies, half-widths, and relative intensities of bands in the vibrational spectrum of all isotopomers of the molecule are determined. For the ³⁴SF₆ molecule, the ratio of integral absorption coefficients of fundamental bands A(ν₄)/A(ν₃)=0.07(6) is larger than ³²SF₆:A(ν₄)/A(ν₃)=0.66(4) for the ³²SF₆ molecule, which corresponds to the same signs of P′₃ and P′₄. The change in the intensity of the ν₂+ν₆ and ν₅+ν₆ bands upon isotopic substitution is explained by the change in the resonance contributions of due to the isotope shift of the ν₃ band.     

Inhomogeneous magnetic state in the electron-doped Sr<Subscript>0.98</Subscript>La<Subscript>0.02</Subscript>MnO<Subscript>3</Subscript> Manganite According to <Superscript>55</Superscript>Mn NMR data

⁵⁵Mn NMR spectra in the magnetically ordered state in Sr_0.98La_0.02MnO₃ manganite have been obtained and the magnetic susceptibility has been measured. It has been shown that the microscopic phase separation into the antiferromagnetic matrix and ferromagnetic clusters, which can be presented as magnetic polarons, is observed in the long-range magnetic order region.     

Frustrated exchange interactions formation at low temperatures and high hydrostatic pressures in La<Subscript>0.70</Subscript>Sr<Subscript>0.30</Subscript>Mn<Subscript>O2.85</Subscript>

The magnetic and thermal properties of the anion-deficient La_0.70Sr_0.30MnO_2.85 manganite are investigated in wide temperature (4–350 K) range, including under hydrostatic pressure (0–1.1 GPa). Throughout the pressure range investigated, the sample is spin glass with diffused phase transition into paramagnetic state. It is established, that spin glass state is a consequence of exchange interaction frustration of the ferromagnetic clusters embeded into antiferromagnetic clusters. The magnetic moment freezing temperature T _f of ferromagnetic clusters increases under pressure, freezing temperature dependence on pressure is characterized by derivative value ∼4.5 K/GPa, while the magnetic ordering T _MO temperature dependence is characterized by derivative value ∼13 K/GPa. The volume fraction of sample having ferromagnetic state is V _fer ∼ 13% and it increases under a pressure of 1.1 GPa by ΔV _fer ≈ 6%. Intensification of ferromagnetic properties of the anion-deficient La_0.70Sr_0.30MnO_2.85 manganite under hydrostatic pressure is a consequence of oxygen vacancies redistribution and unit cell parameters decrease. The most likely mechanism of frustrated exchange interactions formation is discussed.     

Specific features of the electronic structure of the CeCu<Subscript>6</Subscript>, CePd<Subscript>3</Subscript>, CeSi<Subscript>2</Subscript>, and CeF<Subscript>3</Subscript> systems

The electronic structure of cerium systems, the hybridization of 4 f and outer-shell electrons, and the influence of the position of the localized 4 f level with respect to the Fermi level E _F in the conduction band have been investigated. The CeCu₆, CePd₃, CeSi₂, and CeF₃ systems have been studied using X-ray photoelectron spectroscopy. The densities of states have been calculated by the tight-binding linearized muffin-tin orbital method within the atomic sphere approximation, which takes into account the covalent character of bonds and the nonspherical distribution of the electron density. The results obtained from the calculations of the total density of states are in good agreement with the valence band X-ray photoelectron data for the systems under investigation. It has been shown that the differences in the properties of the cerium systems are determined by the specific features of their electronic structure. A strong interatomic interaction is characteristic of heavy-fermion systems.     

High resolution study of anion formation in low-energy electron attachment to SF<Subscript>6</Subscript> molecules in a seeded supersonic beam

Using two variants of the Laser Photoelectron Attachment (LPA) method involving a differentially-pumped, seeded supersonic beam (0.05% and 12.5% of SF₆ molecules in helium carrier gas, nozzle temperatures T₀= 300–600 K, stagnation pressures p₀= 1–5 bar) and mass spectrometric ion detection, we have investigated the energy dependence of anion formation in low-energy electron collisions with SF₆ molecules at high energy resolution. Using the standard LPA method, the yield for SF₆^- as well as SF₅^- and F^- anions was studied with an energy width around 1 meV over the electron energy range 0–200 meV. In addition, a variant of the LPA method with extended energy range (denoted as EXLPA) was developed and applied to measure the yield for SF₆^- and SF₅^- formation over the energy range 0–1.5 eV with an energy width of about 20 meV. The cross-section for formation of SF₆^- decreases by five orders of magnitude over the range 1–500 meV and is only weakly dependent on nozzle temperature. The yield for SF₅^- formation shows — apart from a weak zero energy peak which grows strongly with rising temperature — a broad maximum (located around 0.6 eV for T₀= 300 K and shifting to lower energies with rising T₀) and a monotonical decrease towards higher energies. SF₅^- attachment spectra taken at elevated temperatures exhibit changes with rising stagnation pressure which directly reflect rovibrational cooling of the SF₆ molecules with rising pressure. The SF₅^-/SF₆^- intensity ratio at near-zero energy and the low-energy shape of the broad peak in the SF₅^- spectra are used as thermometers for the internal temperature of the SF₆ molecules in the seeded supersonic beam which (at p₀= 1 bar) are found to be 50–100 K lower than the nozzle temperature. The energy dependence of the yield for F^- formation is similar to that for SF₆^-, but the F^- signals are three to four orders of magnitude lower than those for SF₆^-; in view of the rather high endothermicity of F^- formation the origin of the F^- signals is discussed in some detail.     

Luminescence and electronic excitations in Li<Subscript>6</Subscript>Gd(BO<Subscript>3</Subscript>)<Subscript>3</Subscript>: Ce<Superscript>3+</Superscript> crystals

This paper reports on a study of the luminescence emitted by Li₆Gd(BO₃)₃: Ce³⁺ crystals under selective photoexcitation to lower excited states of the host ion Gd³⁺ and impurity ion Ce³⁺ within the 100–500-K temperature interval, where the mechanisms of migration and relaxation of electronic excitation energy have been shown to undergo noticeable changes. The monotonic 10–15-fold increase in intensity of the luminescence band at 3.97 eV has been explained within a model describing two competing processes, namely, migration of electronic excitation energy over chains of Gd³⁺ ions and vibrational energy relaxation between the ⁶ I _j and ⁶ P _j levels. It has been shown that radiative transitions in Ce³⁺ ions from the lower excited state 5d ¹ to ² F _5/2 and ² F _7/2 levels of the ground state produce two photoluminescence bands, at 2.08 and 2.38 eV (Ce1 center) and 2.88 and 3.13 eV (Ce2 center). Possible models of the Ce1 and Ce2 luminescence centers have been discussed.     

Exchange Bias in Layered GdBaCo<Subscript>2</Subscript>O<Subscript>5.5</Subscript> Cobaltite

It is established that excess oxygen content δ influences the exchange bias (EB) in layered GdBa-Co₂O_{5 + δ} cobaltite. The EB effect arises in p-type (δ > 0.5) cobaltite and disappears in n-type (δ < 0.5) cobaltite. The main parameters of EB in GdBaCo₂O_5.52(2) polycrystals are determined, including the field and temperature dependences of EB field H _EB , blocking temperature T _B , exchange coupling energy J _i of antiferromagnet–ferromagnet (AFM–FM) interface, and dimensions of FM clusters. The training effect inherent in systems with EB has been studied. The results are explained in terms of exchange interaction between the FM and AFM phases. It is assumed that the EB originates from the coexistence of Co³⁺ and Co⁴⁺ ions that leads to the formation of monodomain FM clusters in the AFM matrix of cobaltite.     

Multiphoton ionization of CF<Subscript>3</Subscript>I clusters by ultraviolet laser radiation

The results of the investigation of the multiphoton ionization of (CF₃I) _n clusters by ultraviolet laser radiation are reported. The yields of the I₂⁺ and I⁺ ions, which are the products of the multiphoton ionization, have been measured as functions of the intensity of the ultraviolet radiation at the wavelengths of 308 and 232.5 nm. The degree of multiphoton ionization has been determined and appears to depend on the wavelength of radiation. The velocity distributions of the products have been measured in detail for various wavelengths and various polarizations of radiation. The anisotropy parameters of the velocity distributions of the produced ions and their kinetic energy have been determined. After analysis of the data, a mechanism of the multiphoton ultraviolet ionization of the clusters under investigation has been proposed. This mechanism depends on the used wavelengths.     

Spontaneous generation of electric voltage in a single crystal of Sm<Subscript>0.55</Subscript>Sr<Subscript>0.45</Subscript>MnO<Subscript>3</Subscript>

Spontaneous generation of electric voltage has been found in a Sm_0.55Sr_0.45MnO₃ single crystal grown by floating zone melting with cooling in oxygen. The maximum voltage of 60 μV has been observed in the region of temperatures corresponding, simultaneously, to destruction of the CE-type antiferromagnetic ordering and charge ordering in some clusters. The maximum voltage remains unchanged for 24 h and decreases by 45% in a magnetic field of 14.2 kOe. It has been shown that the spontaneous voltage is caused by the presence of regions with different electric charges in the sample.     

Nature of the low-energy excitations of a charge-ordered phase of La<Subscript>0.25</Subscript>Ca<Subscript>0.75</Subscript>MnO<Subscript>3</Subscript> manganites

The nature of the low-energy excitations of polycrystalline and nanostructured La_0.25Ca_0.75MnO₃ samples has been analyzed in order to investigate the mechanisms of charge ordering in manganites. It has been found that the electrodynamic response spectra of La_0.25Ca_0.75MnO₃ in the energy range of 0.5 to 90 meV and the temperature range of 5 to 300 K have no resonance features that could be attributed to the collective excitations of the charge-ordered phase. It has been shown that the absorption lines observed at frequencies of 20–40 and 80–100 cm^–1 are attributed to usual acoustic phonons becoming optically active owing to the structure phase transition and the appearance of a fourfold superstructure with a quadruple period along the crystallographic a axis. The suppression of the superstructure has been revealed in samples with nanocrystallites (≤40 nm). This suppression indicates a relatively weak coupling of the charge and magnetic order parameters with the phonon subsystem.     

Research of the Spin-Hamiltonian Parameters and Defect Structure for the Trigonal Mn<Superscript>4+</Superscript> Centers in Y<Subscript>2</Subscript>Ti<Subscript>2</Subscript>O<Subscript>7</Subscript>:Mn<Superscript>4+</Superscript> Crystal

The high-order perturbation formulas founded on the two-mechanism model are applied in this paper to compute the spin-Hamiltonian parameters (g factors g _//, g _⊥ and zero-field splitting D) of the trigonal Mn⁴⁺ centers in Y₂Ti₂O₇:Mn⁴⁺ crystal. In this model, besides the contributions from the traditional crystal-field (CF) mechanism (in the CF theory) related to CF excited states, those from the charge-transfer (CT) mechanism connected with CT excited states are contained. The calculated results are reasonably coincident with the observed values. The calculations show that the contributions of CT mechanism to spin-Hamiltonian parameters (in particular, the g factors) for (MnO₆)^8? clusters are large and cannot be neglected. The defect structure of trigonal (MnO₆)^8? clusters in Y₂Ti₂O₇:Mn⁴⁺ crystals is also evaluated. The results are discussed.     

EPR and optical absorption studies of Cu<Superscript>2+</Superscript> ions in [ZnPd(CN)<Subscript>4</Subscript>(C<Subscript>4</Subscript>H<Subscript>12</Subscript>N<Subscript>2</Subscript>O<Subscript>2</Subscript>)] single crystals

A Cu²⁺-doped single crystal of catena-trans-bis(N-(2-hydroxyethyl)-ethylenediamine) zinc(II)-tetra-m-cyanopaladate(II) [ZnPd(CN)₄(C₄H₁₂N₂O₂)] complex has been investigated by electron paramagnetic resonance (EPR) technique at room temperature. EPR spectra indicate that Cu²⁺ ions substitute for magnetically equivalent Zn²⁺ ions and form octahedral complexes in [ZnPd(CN)₄(C₄H₁₂N₂O₂)] hosts. The crystal field affecting the Cu²⁺ ion is nearly axial. The optical absorption studies show two bands at 322 nm (30864 cm⁻¹) and 634 nm (15337 cm⁻¹) which confirm the axial symmetry. The spin Hamiltonian parameters and the relevant wave function are determined.