Manifestation of nonuniformity in the electron charge distribution in H<Stack><Subscript>2</Subscript><Superscript>+</Superscript></Stack> molecular ions colliding with each other

The anomalous character of threshold properties in the ion-molecule reactions H ₂ ⁺ + H ₂ ⁺ → H ₃ ⁺ + p and H ₂ ⁺ + H ₂ ⁺ → H + p + H + p has been theoretically analyzed. It has been shown that these reactions proceed through the formation of the intermediate H ₄ ⁺⁺ complex. Molecules H ₂ ⁺ in the collision process are described by a chemical model, where the positive charge is concentrated in one of the nuclei. The calculations of the reaction cross sections are in good agreement with the experimental data. It has been shown that the chemical model of the H ₂ ⁺ molecule can be consistently explained only in terms of dynamic interactions, i.e., polarization forces and van der Waals forces.     

Optical characteristics of radiation from N<Stack><Subscript>2</Subscript><Superscript>*</Superscript></Stack> dimers in a barrier discharge

The spectral and power characteristics of radiation of the second positive system of nitrogen (C ³Π _u → B ³Π _g ) in Ar-N₂ and Ar-N₂-Cl₂ mixtures excited by barrier discharge have been studied experimentally. Addition of argon to N₂ increased the radiation power by sixfold. In the triple mixture Ar-N₂-Cl₂ = 210/0.5/0.005, minor chlorine additions increased the intensity of the C ³Π _u → _B ³Π _g transition by 26% compared to Ar-N₂ mixtures. Radiation power density of 2.7 mW/cm² has been achieved. In both binary and triple mixtures, the second positive system of nitrogen was the major contributor to radiation, while the contributions of the fourth positive system of N ₂ ^* (D ³Σ _u ⁺ → B ³Π _g ), the Vegard-Kaplan transition of N ₂ ^* (A ³Σ _u ⁺ → X ¹Σ _g ⁺ ), and the D′ → A′ band of Cl ₂ ^* were negligibly small.     

High-frequency EPR of Cr<Superscript>2+</Superscript> ions in CdGa<Subscript>2</Subscript>S<Subscript>4</Subscript>

High-frequency broad-band (65–240 GHz) EPR is used to study impurity centers of bivalent chromium in a CdGa₂S₄ crystal. It is found that the EPR spectra correspond to tetragonal symmetry. The spin Hamiltonian H = βB · g · S + B ₂ ⁰ O ₂ ⁰ + B ₄ ⁰ O ₄ ⁰ + B ₄ ⁴ O ₄ ⁴ with the parameters B ₂ ⁰ =23659±2 MHz, B ₄ ⁰ =1.9±1 MHz, |B ₄ ⁴ |=54.2±2 MHz, g_‖=1.93±0.02, and g_⊥=1.99±0.02 is used to describe the observed spectra. It is concluded that chromium ions occupy one of the tetrahedrally coordinated cation positions.     

Secondary ion emission from a GaAs single crystal upon bombardment with Bi<Stack> <Subscript> <Emphasis Type="Italic">m</Emphasis> </Subscript> <Superscript>+</Superscript> </Stack> cluster ions

Secondary-ion mass spectra and energy distributions upon bombarding a gallium arsenide single crystal using Bi_m⁺(m = 1–5) cluster ions with energies of 2–12 keV are investigated. The gallium cluster ion yield grew nonadditively with the number of atoms in the cluster projectiles. A quasi-thermal component found in the energy spectra of secondary Ga⁺ and Ga₂⁺ ions is indicative of the occurrence of the thermal spike mode upon cluster ion bombardment. The quasi-thermal component in the yield of atomic Ga⁺ ions upon bombardment with Bi₂⁺–Bi₅⁺–ions is 35–75%.     

Investigating the Double Beta Decay of <Superscript>58</Superscript>Ni

Double beta decay (β + EC, EC/EC) of ⁵⁸Ni is investigated at France’s Modane Underground Laboratory (4800 m water equivalent) using the OBELIX ultralow-background HPGe detector with a sensitive volume of 600 cm³ and a natural nickel sample of ~68% 58Ni with a mass of ~21.7 kg. After preliminary analysis of the experimental data accumulated over ~144 days, new experimental limits are obtained for the 2νβ⁺EC decay of ⁵⁸Ni to the 0⁺ ground state and the 2 ₁ ⁺ , 811 keV excited state of ⁵⁸Fe, and for the 2νEC/EC decay of ⁵⁸Ni to the 2 ₁ ⁺ , 811 keV and 2 ₂ ⁺ , 1675 keV excited states of ⁵⁸Fe. The limits are T_1/2(β⁺EC,0→0⁺) > 1.7 × 10²² yr, T_1/2(β⁺EC,0→2 ₁ ⁺ ) > 2.3 × 10²² yr, T_1/2(EC/EC,0→2 ₁ ⁺ ) > 3.3 × 10²² yr, and T_1/2(EC/EC,0→2 ₂ ⁺ ) > 3.4 × 10²² yr. Experimental limit T_1/2(0νEC/EC–res, 1918 keV > 4.1 × 10²² yr is obtained for resonant neutrinoless radiative EC/EC decay with an energy of 1918.3 keV. All limits are at 90% CL.     

Critical behavior of La<Subscript>0.825</Subscript>Sr<Subscript>0.175</Subscript>MnO<Subscript>2.912</Subscript> anion-deficient manganite in the magnetic phase transition region

For La _0.825 ³⁺ Sr _0.175 ² +Mn³⁺O _2.912 ^2? anion-deficient manganite, the specific magnetization, the dynamic magnetic susceptibility, and the heat capacity are investigated. This material is found to be an inhomogeneous ferromagnet below the Curie point T _C ≈ 122 K, which is much lower than the Curie point determined for the stoichiometric composition (T _C ≈ 268 K). An increase in magnetic field by two orders of magnitude leads to an increase in the Curie temperature by ΔT ≈ 12 K. The presence of oxygen vacancies leads to the frustration of a part, namely, V _fr ≈ 22%, of the indirect Mn³⁺-O-Mn³⁺ exchange interactions, but the spin glass state is not realized. The ferromagnetic matrix of the material under study is characterized by a scatter in the exchange interaction intensities. The heat capacity is found to exhibit an anomalous behavior. Based on the Banerjee magnetic criterion, it is established that the ferromagnet-paramagnet transition observed for La _0.825 ³⁺ Sr _0.175 ²⁺ Mn³⁺O _2.912 ^2? anion-deficient manganite is a second-order thermodynamic phase transition. The mechanism and origin of the critical behavior of the system under investigation are discussed.     

Synthesis and electrical properties of new perovskite-like <Emphasis Type="Italic">A</Emphasis>Mn<Subscript>3</Subscript>V<Subscript>4</Subscript>O<Subscript>12</Subscript> (A = Ca,Ce, and Sm) compounds

AMn₃V₄O₁₂ (A = Ca, Ce, and Sm) compounds with a perovskite structure are synthesized at high pressures and temperatures. The crystalline structure of these compounds (space group \(Im\bar 3\)Z = 2) is determined via X-ray analysis. If ions in the A sublattice are changed in the order Ca²⁺–Sm³⁺–Ce³⁺, the valence is redistributed from Ca²⁺Mn₃²⁺V₄⁴⁺O₁₂ to Sm³⁺Mn₃²⁺V₄^3.75+O₁₂, and to Ce³⁺Mn₃²⁺V₄^3.75+O₁₂. The temperature dependences of the electrical resistivity are studied.     

Effect of the Lattice and Spin-Phonon Contributions on the Temperature Behavior of the Ground State Splitting of Gd<Superscript>3+</Superscript> in SrMoO<Subscript>4</Subscript>

The temperature behavior of the EPR spectra of the Gd³⁺ impurity center in single crystals of SrMoO₄ in the temperature range T = 99–375 K is studied. The analysis of the temperature dependences of the spin Hamiltonian b ₂ ⁰ (T) = b₂(F) + b₂(L) and P ₂ ⁰ (T) = P₂(F) + P₂(L) (for Gd¹⁵⁷) describing the EPR spectrum and contributing to the Gd³⁺ ground state splitting ΔE is carried out. In terms of the Newman model, the values of b₂(L) and P₂(L) depending on the thermal expansion of the static lattice are estimated; the b₂(F) and P₂(F) spin-phonon contributions determined by the lattice ion oscillations are separated. The analysis of b ₂ ⁰ (T) and P ₂ ⁰ (T) is evidence of the positive contribution of the spin-phonon interaction; the model of the local oscillations of the impurity cluster with close frequencies ω describes well the temperature behavior of b₂(F) and P₂(F).     

Measurement of the spin structure function <Emphasis Type="Italic">g</Emphasis><Subscript>1</Subscript> at HERMES

New, preliminary results are presented for the deuteron structure function g ₁ ^d , where the kinematic range has been extended to 0.0021<x<0.85 and Q ²>0.1 GeV², to include 7 new data points at low x with respect to previously released results. Within the present statistics, the structure function ratio g ₁ ^d /F ₁ ^d is found to be independent of Q ².     

Optical and x-ray diffraction studies of the symmetry of distorted phases of the (NH<Subscript>4</Subscript>)<Subscript>3</Subscript>ZrF<Subscript>7</Subscript> crystal

(NH₄)₃ZrF₇ single crystals were grown, and polarization-optical and x-ray diffraction studies were performed on powders and crystalline plates of various cuts over a wide temperature range. Phase transitions are revealed at temperatures T _1↑ = 280 K, T _2↑ = 279.6 K, T _3↑ = 260–265 K, and T _4↑ = 238 K on heating and at T _1↓ = 280 K, T _2↓ = 269–270 K, T _3↓ = 246 K, and T _4↓ = 235 K on cooling. The sequence of changes in symmetry is established to be as follows: O _h ⁵ (Z = 4) ? D _2h ²⁵ (Z = 2) ? C _2h ³ (Z = 2) ? C _i ¹ (Z = 108) ? monoclinic²(Z = 216).     

Specific features of the three-electron <Emphasis Type="Italic">KL</Emphasis><Stack><Subscript>2, 3</Subscript><Superscript>2</Superscript></Stack> ionization of silicon atoms upon near-threshold photoabsorption

The shape and relative intensity of the group of the Kα_5–8 satellites (radiative transitions KL _{2, 3} ² )-L _{2, 3} ³ of Si atoms are experimentally studied upon photoabsorption near and far from the KL _{2, 3} ² ionization threshold. The satellites were excited near the ionization threshold by lines of the characteristic L spectrum and bremsstrahlung radiation from Nb and Mo anodes and far from the threshold by the L spectrum and bremsstrahlung radiation from an Ag anode and by monochromatized Kα_{1, 2} radiation from a Ti anode. It is established that the probability P(L _{2, 3} ² ) of formation of two additional 2p vacancies during KL _{2, 3} ² photoabsorption of Si atoms near the energy threshold is by a factor of 1.5 lower than that during photoionization in a more distant energy region beyond the threshold. At the same time, the P(L _{2, 3} ² )/P(L _{2, 3}) ratio remains invariable for the absorbed photons throughout the energy range studied. It is demonstrated that, as the KL _{2, 3} ² ionization threshold is approached, an intensity redistribution occurs among the components of the group of the Kα_5–8 lines, which reflects a decrease in the excitation cross section ratio σ(⁴ P)/σ(² P) of the ⁴ P and ² P terms of the KL _{2, 3} ² configuration. A conclusion is drawn that the effects of suppression of the generation of P terms of higher multiplicity during the KL _{2, 3} and KL _{2, 3} ² near-threshold photoionizations are of a common nature.     

Nonergodic state of relaxation ferroelectric Cd<Subscript>2</Subscript>Nb<Subscript>2</Subscript>O<Subscript>7</Subscript> in a constant electric field

The temperature dependence of the residual polarization of the nonergodic relaxation state (NERS) obtained from the measurements of pyroelectric current during zero-field heating (ZFH) in the temperature interval from 10 to 295 K is investigated for the Cd₂Nb₂O₇ relaxation system in two cases: (1) after sample cooling in a constant electric field E (FC) from T = 295 K to a preset temperature, which is much lower than the “freezing” temperature of the relaxation state (T _f ≈ 182 K), field removal, and subsequent cooling in zero field (ZFC) to T = 10 K and (2) after ZFC from T = 295 K to the same temperature below T _f , application of the same field, and FC to T = 10 K. The behavior of the P _r ^FC (T) and P _r ^ZFC (T) dependences is analyzed. In the field E < 2 kV/cm, the P _r ^ZFC curves as functions of 1/T have a broad low-intensity peak in the region T ≈ T _f , which vanishes in stronger fields, when the P _r ^FC (1/T) curves intersect at T ≈ T _f and have no anomalies. The difference in the behavior of P _r ^ZFC (T) and P _r ^FC (T) indicates the difference in the nature of NERS formed during ZFC and FC of the system upon a transition through T _f . In the ZFC mode, NERS exhibits glasslike behavior; in the FC regime, features of the ferroelectric behavior even in the weak field. Analogous variations of P _r ^ZFC (T) and P _r ^FC (T) in a classical ferroelectric KDP are also given for comparison.     

Recombination population of excited states of the hydrogen atom in an He-H<Subscript>2</Subscript> plasma

The population of excited states of the hydrogen atom in an afterglow plasma produced by a pulsed discharge in helium (40 Torr) with a small admixture of hydrogen ([H₂] ≈ 10¹² cm^?3) has been studied spectroscopically. The contribution from electron-ion recombination Γ ₃ ^rec to the production rate of atoms H(n = 3) has been separated. On the basis of an experiment in which the response of the spectral line intensities to the perturbation of the electron temperature in the afterglow phase was observed, the dependence Γ ₃ ^rec (T _e ～ T _e ^?(0.9–1.0) has been obtained in the region kT _e = 0.026–0.064 eV.     

Study of the formation and unimolecular fragmentation of Si<Subscript><Emphasis Type="Italic">n</Emphasis></Subscript>O<Stack><Subscript><Emphasis Type="Italic">m</Emphasis></Subscript><Superscript>+</Superscript></Stack> clusters under ion bombardment

The dependences of the emission and fragmentation of clusters sputtered by Xe⁺ ions from the surface of Si _n O _m ⁺ on the oxygen pressure near the bombarded surface are studied using secondary ion mass spectrometry. It is shown that the process of Si _n O _m ⁺ cluster formation under ion bombardment can be described within the framework of the mechanism of combinatorial synthesis by taking into account the mutual reversibility of the reactions of formation and unimolecular decay.     

37 Years with the light scalar mesons. The learned lessons

Attention is paid to the production mechanisms of light scalars that reveal their nature. We reveal the chiral shielding of the σ(600) meson. We show that the kaon loop mechanism of the ? radiative decays, ratified by experiment, is four-quark transition and points to the four-quark nature of light scalars. We show also that the light scalars are produced in the two photon collisions via four-quark transitions in contrast to the classic P wave tensor qq? mesons that are produced via two-quark transitions γγ → qq?. We study the mechanism of production of the light scalar mesons in the D _s ⁺ → π⁺π^? e ⁺ν decays: D _s ⁺ → ss?e ⁺ν → [σ(600) + f ₀(980)]e ⁺ν → π⁺π^? e ⁺ν, and compare it with the mechanism of production of the light pseudoscalar mesons in the D _s ⁺ → (η/η′)e ⁺ν decays: D _s ⁺ → ss?e ⁺ν → (η/η′)e ⁺ν. As a result we find support to four-quark nature of light scalars. In the end, we outline the future research program.     

Metastable helium cluster He<Stack><Subscript>4</Subscript><Superscript>*</Superscript></Stack>

The existence of a metastable cluster He ₄ ^* with total spin S = 2 is predicted. The cluster consists of two covalently bound excited spin-polarized triplet He ₂ ^* molecules and is rectangular in shape. The electron wavefunctions, the dependence of the energy He ₄ ^* system on the distance between the He ₂ ^* triplet molecules, the atomic spacing, the frequency spectrum of natural oscillations of the cluster, and other characteristics are calculated from first principles. It is shown that the metastable state is formed if one of the excited He ₂ ^* molecules is in the ³Σ _u ⁺ state, while the other is in the ³Π_g state. The radiation lifetime τ of the metastable cluster He ₄ ^* is calculated; it is found to range from 100 to 200 s, which is much longer than the lifetime τ ≈ 20 s of the triplet molecule He ₂ ^* (³Σ _u ⁺ ). The height U ≈ 0.5 eV of the potential barrier preventing the departure from the local energy minimum is determined. The energy E_acc ≈ 9 eV/atom accumulated in the He ₄ ^* cluster is calculated; this energy considerably exceeds the energy of known chemical energy carriers. It is shown that the accumulated energy is released virtually completely during decomposition of the He ₄ ^* cluster into individual helium atoms. This means that helium clusters are a promising material with a high accumulated energy density (HEDM).     

Low-energy Ce spin excitations in CeFeAsO and CeFeAsO<Subscript>0.84</Subscript>F<Subscript>0.16</Subscript>

We use inelastic neutron scattering to study the low-energy spin excitations of polycrystalline samples of nonsuperconducting CeFeAsO and superconducting CeFeAsO_0.84F_0.16. Two sharp dispersionless modes are found at 0.85 and 1.16 meV in CeFeAsO below the Ce antiferromagnetic (AF) ordering temperature of T _N ^Ce ? 4 K. On warming to above T _N ^Ce ? 4 K, these two modes become one broad dispersionless mode that disappears just above the Fe ordering temperature T _N ^Fe ? 140 K. For superconducting CeFeAsO_0.84F_0.16, where Fe static AF order is suppressed, we find a weakly dispersive mode center at 0.4 meV that may arise from short-range Ce-Ce exchange interactions. Using a Heisenberg model, we simulate powder-averaged Ce spin wave excitations. Our results show that we need both Ce spin wave and crystal electric field excitations to account for the whole spectra of low-energy spin excitations.     

Dipole moment functions for electronic transitions from ion-pair states of the second tier of molecular iodine

The emission spectra caused by the transitions from the ion-pair states and f0 _g ⁺ and G1_g of the I₂ molecule are obtained by excitation of individual rovibronic levels of the molecule by the method of optical-optical double resonance. The emission spectra from the state F0 _u ⁺ populated due to collisions I₂(f) + I₂(X) are also measured. By modeling the experimental emission spectra, the dipole moment functions for the electronic transitions f _g ⁺ -B0 _u ⁺ , A0 _u ⁺ , and B″0 _u ⁺ ; G1_g-A0 _u ⁺ and B″0 _u ⁺ ; and F0 _u ⁺ -X0 _g ⁺ and a′0 _g ⁺ of the iodine molecule are reconstructed.     

Band interaction in <Superscript>162</Superscript>Dy, <Superscript>168</Superscript>Er,and <Superscript>172</Superscript>Yb

Coriolis interaction between levels of two rotational bands in ¹⁷²Yb with K ^π = 2⁺ and 3⁺ and in ¹⁶⁸Er between levels with K ^π = 0^?, 1^?, and 2^? is studied. The values of the interaction parameters are obtained. The mutual influence of two bands in ¹⁶²Dy with ΔK = 2, K _i ^π = 0 ₂ ⁺ and 2 ₁ ⁺ due to Coriolis interaction is demonstrated.     

The origin of frustration of magnetic coupling in the NiFeCrO<Subscript>4</Subscript> ferrite

The magnetostriction of the NiFeCrO₄ ferrite is investigated for the first time. It is found that the frustration of magnetic coupling occurs only in the B sublattice of the NiFeCrO₄ ferrite, whereas the A sublattice has a usual magnetic structure. The inference is made that the frustration of magnetic coupling in the B sublattice is caused not only by the negative direct BB exchange interaction Cr _B ³⁺ -Cr _B ³⁺ but also by the positive indirect AB exchange interaction Fe _A ³⁺ -O^2?-Cr _B ³⁺ . Reasoning from the experimental data and an analysis of the exchange interactions in the NiFeCrO₄ ferrite sample, it is demonstrated for the first time that the magnetic moments of Fe _A ³⁺ ions in this ferrite deviate from collinearity. It is established that, at low temperatures, the B sublattice of the NiFeCrO₄ ferrite is responsible for the total magnetic moment n_0exp.