Features of the Formation of the Spin Polarization of an Alkali Metal at the Resolution of Hyperfine Sublevels in the <Superscript>2</Superscript><Emphasis Type="Italic">S</Emphasis><Subscript>1/2</Subscript> State

The optical orientation of the angular momenta of alkali atoms in the presence of a buffer gas (molecular nitrogen) has been studied experimentally. It has been shown that, even at a low concentration of molecular nitrogen in the cell, the excitation of ¹³³Cs atoms from the lower hyperfine level with F = 3, which belongs to the ground ²S_1/2 state, results in a larger amplitude of the magnetic resonance than the excitation from the hyperfine level with F = 4. This result has been theoretically explained under the assumption that the spin state of the alkali atomic nucleus does not change at collision with a nitrogen molecule, which is accompanied by a nonradiative transition of the alkali atom from the excited ²P_1/2 state to the ground ²S_1/2 state.     

Hyperfine magnetic anomaly in isotopic pairs <Superscript>151, 152</Superscript>Eu and <Superscript>152, 153</Superscript>Eu

High-resolution laser spectroscopy measurements of optical hyperfine splitting on the ^{151, 152, 153}Eu isotopes were performed on the atomic transition 4f ⁷6s ^{2 8} S _7/2 → 4f ⁷6s6p ⁶ P _5/2 at λ ≈ 564.58 nm. Values of the nuclear magnetic dipole and electric quadrupole moments are obtained from the measured hyperfine splitting and the magnetic hyperfine anomalies in the isotope pairs ^{151, 152}Eu and ^{152, 153}Eu are deduced. The absolute values of the hyperfine anomaly in both cases are unusually large: 5 (1)%. The possible sources causing these anomalies are discussed.     

Interrelationship between the polarization moments of different parity upon optical pumping of atoms under the magnetic resonance conditions: I. Experiment on the <Superscript>133</Superscript>Cs and <Superscript>4</Superscript>He atoms

In an experiment with an optical pumping of ¹³³Cs atoms in the 6² S _1/2 ground state, the line shape of the D ^2f magnetic resonance signal for the transverse alignment component oscillating at a double frequency f of a radio-frequency (RF) magnetic field is found to strongly depend on the polarization of pumping radiation. On passage from a linearly polarized pumping light to a circularly polarized (CP) light with a sufficiently strong RF field the ordinary three-peak line with the highest central peak transforms into a two-peak line with a minimum at the center, so that the D ^2f signal line resembles the M ^f signal line of a transverse orientation oscillating at the RF field frequency f. This suggests that the orientation (the first-rank polarization moment (PM)) arising upon CP pumping affects the alignment (the second-rank PM); i.e., the PMs of Cs atoms with different parities of their ranks become coupled. No influence of the polarization of a pumping radiation on the line shape of the D ^2f signal is observed in a similar experiment with the ⁴He atoms in the 2³ S ₁ metastable state.     

Structure of neutron-rich Isotopes <Superscript>8</Superscript>Li and <Superscript>9</Superscript>Li and allowance for it in elastic scattering

The differential cross sections for elastic proton scattering on the unstable neutron-rich nuclei ⁸Li and ⁹Li at E = 700 and 60 MeV per nucleon were considered. The ⁸Li nucleus was treated on the basis of the three-body α-t-n model, while the ⁹Li nucleus was considered within the α-t-n and ⁷Li-n-n models. The cross sections in question were calculated within Glauber diffraction theory. A comparison of the results with available experimental data made it possible to draw conclusions on the quality of the wave functions and potential used in the calculations.     

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.     

Structure of the spatial periphery of the <Superscript>11</Superscript>Li and <Superscript>11</Superscript>Be isobars

On the basis of the shell model with an extended basis, the structure of ⁹Li-⁹Be to ¹¹Li-¹¹Be nuclei is examined with allowance for the competition of ^jj coupling and Majorana exchange forces via considering the sequential addition of neutrons, and the respective wave functions are determined. A formalism for calculating the spectroscopic factor for a dineutron and for individual neutrons in nuclei whose wave functions incorporate the mixing of shell configurations is developed. The reactions ⁹Li(t, p)¹¹Li and ⁹Be(t, p)¹¹Be treated with allowance for the mechanisms of dineutron stripping and a sequential transfer of two neutrons are considered as an indicator of the proposed structure of lithium and berylliumisotopes. The parameters of the optical potentials, the wave functions for the bound states of transferred particles, and the interaction potentials corresponding to them are determined from a comparison of the theoretical angular distribution of protons from the reaction ⁹Be(t, p)¹¹Be with its experimental counterpart. It is shown that a dineutron periphery of size about 6.4 fm is present in the ¹¹Li nucleus and that a single-neutron periphery of size about 8 fm is present in the ¹¹Be nucleus.     

Frequency shift of the magnetic resonance in 5<Emphasis Type="Italic">s</Emphasis><Superscript>2</Superscript><Emphasis Type="Italic">S</Emphasis><Subscript>1/2</Subscript> rubidium atoms due to spin-exchange collisions with optically oriented cesium atoms

For the case of cesium atoms optically oriented in a mixture of cesium and rubidium vapors, the temperature dependence of the frequency shift of a magnetic resonance excited in a set of Zeeman sublevels for two hyperfine states of ⁸⁷Rb 5s ² S _1/2 atoms. It is shown that, in a weak magnetic field of about 2 × 10^?6 T, this shift is determined by the spin-exchange interaction of rubidium atoms with optically oriented ¹³³Cs atoms.     

Structure of the local environment and hyperfine interactions of <Superscript>57</Superscript>Fe probe atoms in DyNiO<Subscript>3</Subscript> nickelate

The local structure of DyNiO₃ nickelate at both sides of the insulator (T < T _im) ? metal (T > T _im) phase transition was studied by probe ⁵⁷Fe Mössbauer spectroscopy. The character of change in the hyperfine parameters of probe iron atoms specifically near the phase-transition temperature (T ≈ T _im) was analyzed.     

Magnetic order in the structurally disordered helicoidal magnet Cr<Subscript>1/3</Subscript>NbS<Subscript>2</Subscript>: NMR at <Superscript>53</Superscript>Cr nuclei

The Cr_1/3NbS₂ magnet is studied by nuclear magnetic resonance (NMR) at ⁵³Cr nuclei in a zero applied magnetic field. The following two frequency ranges are distinguished in the ⁵³Cr NMR spectrum at T = 4.2 K: ν ₁ = 64–68 MHz and ν ₂ = 49–51 MHz. They can be related to two valence states of chromium ions, namely, Cr⁴⁺ and Cr³⁺. The components of the electric field gradient, the hyperfine fields, and the magnetic moment at chromium atoms are determined. The NMR data demonstrate that the magnetic moments of chromium lie in plane ab and form a magnetic structure consisting of regions with a helicoidal magnetic order and regions where this order is broken.     

Role of the nuclear and electromagnetic interactions in the coherent dissociation of the relativistic <Superscript>7</Superscript>Li nucleus into the <Superscript>3</Superscript>H + <Superscript>4</Superscript>He channel

The differential cross section in the transverse momentum Q and a total cross section of (31 ± 4) mb for the coherent dissociation of a 3-A-Ge V/c ⁷Li nucleus through the ³H + ⁴He channel have been measured on emulsion nuclei. The observed Q dependence of the cross section is explained by the predominant supposition of the nuclear diffraction patterns on light (C, N, O) and heavy (Br, Ag) emulsion nuclei. The contributions to the cross section from nuclear diffraction (Q ≤ 400 Me V/c) and Coulomb (Q ≤ 50 Me V/c) dissociations are calculated to be 40.7 and 4 mb, respectively.     

Electron paramagnetic resonance of Cr<Superscript>3+</Superscript>-Li<Superscript>+</Superscript> centers in (Cr,Li): Mg<Subscript>2</Subscript>SiO<Subscript>4</Subscript> synthetic forsterite

Synthetic single crystals of chromium-and lithium-doped forsterite, namely, (Cr,Li): Mg₂SiO₄, are studied using electron paramagnetic resonance spectroscopy. It is revealed that, apart from the known centers Cr³⁺(M1) and Cr³⁺(M2) (with local symmetries C_i and C_s, respectively), these crystals involve two new types of centers with C₁ symmetry, namely, Cr³⁺(M1)′ and Cr³⁺(M2)′ centers. The standard parameters D and E in a zero magnetic field [zero-field splitting (ZFS) parameters expressed in GHz] and principal components of the g tensor are determined as follows: D=31.35, E=8.28, and g=(1.9797, 1.9801, 1.9759) for Cr³⁺(M1)′ centers and D=15.171, E=2.283, and g=(1.9747, 1.9769, 1.9710) for Cr³⁺(M2)′ centers. It is found that the lowsymmetric effect of misalignment of the principal axes of the ZFS and g tensors most clearly manifests itself (i.e., its magnitude reaches 19°) in the case of Cr³⁺(M2)′ centers. The structural models Cr³⁺(M1)-Li⁺(M2) and Cr³⁺(M2)-Li⁺(M1) are proposed for the Cr³⁺(M1)′ and Cr³⁺(M2)′ centers, respectively. The concentrations of both centers are determined. It is demonstrated that, upon the formation of Cr³⁺-Li⁺ ion pairs, the M1 position for chromium appears to be two times more preferable than the M2 position. Reasoning from the results obtained, the R₁ line (the ²E → ⁴A₂ transition) observed in the luminescence spectra of (Cr,Li): Mg₂SiO₄ crystals in the vicinity of 699.6 nm is assigned to the Cr³⁺(M1)′ center.     

Puzzle of the <Superscript>6</Superscript>Li quadrupole moment: Steps toward solving it

The problem of the origin of the quadrupole deformation in the ⁶Li ground state is investigated with allowance for the three-deuteron component of the ⁶Li wave function. Two long-standing puzzles related to the tensor interaction in the ⁶Li nucleus are known: that of an anomalous smallness of the ⁶Li quadrupole moment (being negative, it is smaller in magnitude than the ⁷Li quadrupole moment by a factor of 5) and that of an anomalous behavior of the tensor analyzing power T_2q in the scattering of polarized ⁶Li nuclei on various targets. It is shown that a large (in magnitude) negative exchange contribution to the ⁶Li quadrupole moment from the three-deuteron configuration cancels almost completely the “direct” positive contribution due to the αd folding potential. As a result, the total quadrupole moment proves to be close to zero and highly sensitive to fine details of the tensor nucleon-nucleon interaction in the ⁴He nucleus and of its wave function.     

Hyperfine structure of the <Emphasis Type="Italic">S</Emphasis> levels of the muonic helium ion

Corrections of the α⁵ and α⁶ orders to the energy spectrum of the hyperfine splitting of the 1S and 2S levels of the muonic helium ion are calculated with the inclusion of the electron vacuum polarization effects, nuclear-structure corrections, and recoil effects. The values ΔE ^hfs(1S) = ?1334.56 meV and ΔE ^hfs(2S) = ?166.62 meV obtained for hyperfine splitting values can be considered as reliable estimates for comparison with experimental data. The hyperfine structure interval Δ₁₂ = 8ΔE ^hfs(2S) ? ΔE ^hfs(1S) = 1.64 meV can be used to verify QED predictions.     

<Superscript>53</Superscript>Cr NMR study of CuCrO<Subscript>2</Subscript> multiferroic

The magnetically ordered phase of the CuCrO₂ single crystal has been studied by the nuclear magnetic resonance (NMR) method on ⁵³Cr nuclei in the absence of an external magnetic field. The ⁵³Cr NMR spectrum is observed in the frequency range ν_res = 61–66 MHz. The shape of the spectrum depends on the delay tdel between pulses in the pulse sequence τ_π/2–t _del–τ_π–t _del–echo. The spin–spin and spin–lattice relaxation times have been measured. Components of the electric field gradient, hyperfine fields, and the magnetic moment on chromium atoms have been estimated.     

Submillimeter ESR spectra of Fe<Superscript>2+</Superscript> ions in synthetic and natural beryl crystals

Electron spin resonance spectra of non-Kramers bivalent iron (Fe²⁺) ions have been detected in synthetic and natural beryl crystals with an iron impurity. The observed ESR spectra have been attributed to resonance transitions of Fe²⁺ ions from the ground (singlet) state to excited (doublet) levels with the splitting Δ = 12.7 cm^–1 between the levels. The experimental angular and frequency dependences of the resonance field of the ESR signal have been described by the spin Hamiltonian with the effective spin S = 1. The analysis of the ESR data and optical absorption spectra indicates that the Fe²⁺ ions are situated in tetrahedral positions and substitute Be²⁺ cations in the beryl structure.     

Study of absorption spectra of Er<Superscript>3+</Superscript> in KTaO<Subscript>3</Subscript> crystals

The first results of the study of optical absorption spectra of KTaO₃: Er³⁺ crystals are presented. In the 350–660-nm region, lines are observed deriving from intraconfigurational electronic transitions from the ⁴ I _15/2 ground state to levels of the ⁴ F _9/2, ⁴ S _3/2, ² H _11/2, ⁴ F _7/2, ⁴ F _5/2(⁴ F _3/2), ² G _9/2, and ⁴ G _11/2 excited states of the Er³⁺ ions. A comprehensive study of transitions to the ⁴ F _9/2, ⁴ S _3/2, ² H _11/2, and ⁴ F _7/2 levels at 77 K is carried out. The number of lines observed for the above transitions fits the theoretically possible number for ?-? electronic transitions in Er³⁺ ions in the cubic crystal field. In the case of a differently charged substituted ion, this situation occurs only under nonlocal impurity charge compensation. The energies of the excited state levels for the transitions under study are determined.     

Measurement of the excitation function for the <Superscript>7</Superscript>Li(t,p)<Superscript>9</Superscript>Li reaction using detection of delayed neutrons

The excitation function for the ⁷Li(t, p)⁹Li reaction is measured at the EGP-10 electrostatic tandem accelerator (VNIIEF) at the incident triton energies E _t = 5–11.3 MeV. The measurements were based on detection of delayed neutrons resulting from the decay of ⁹Li nuclei. The neutrons were detected by the 4π detector consisting of ³He counters enclosed in a polyethylene moderator. A pulsed mode was used for irradiating the LiF target (210 μg cm^?2) of natural isotopic composition on the tantalum backing. Absolutization of the excitation function was performed against the ⁷Li(p, n) reaction cross sections.     

Momentum distributions of <Superscript>4</Superscript>He nuclei from the <Superscript>6</Superscript>He and <Superscript>6</Superscript>Li breakup

Experimental data on the momentum distributions of ⁴He nuclei originating from ⁶He and ⁶Li breakup on various targets are presented over a wide beam energy range. The experiment with ⁶He was performed at the DRIBs accelerator complex for radioactive beams at the Joint Institute for Nuclear Research (JINR, Dubna). The intensity of the ⁶He beam used was 5 × 10⁶ particles per second and its energy was 10 MeV per nucleon. The momentum distributions of breakup products were measured by means of the MSP-144 magnetic spectrometer. The distribution width was shown to be virtually independent of the target mass. A small value of this width, σ ～ 28 MeV/c, confirms the presence of a halo in ⁶He. The measurements performed with ⁶Li beams of energy 18 and 46 MeV per nucleon at the U-400M accelerator yielded a width value of σ ～ 50 MeV/c for the momentum distributions of ⁴He nuclei, which is intermediate between that for ⁶He and those for stable nuclei. A compilation of the widths of the momentum distributions of fragments originating from the breakup of various nuclei is presented versus the binding energy of one or two neutrons in these nuclei, the target mass and the projectile energy.     

Data on photoneutron reactions from various experiments for <Superscript>133</Superscript>Cs, <Superscript>138</Superscript>Ba and <Superscript>209</Superscript>Bi nuclei

Basic methods for determining cross sections for photoneutron partial reactions are examined. They are obtained directly in experiments with quasimonoeneregetic annihilation photons or from the cross section for the (γ, xn) = (γ, 1n) + 2(γ, 2n) + 3(γ, 3n) +... neutron-yield reaction in experiments with bremsstrahlung photons by introducing corrections based on statistical nuclear-reaction theory. The difference in the conditions of these experiments, which leads to discrepancies between their results because of sizable systematic errors, is analyzed. Physical criteria are used to study the reliability of data on the photodisintegration of ¹³³Cs, ¹³⁸Ba, and ²⁰⁹Bi nuclei. The cross sections for partial and total reactions satisfying the reliability criteria are evaluated within the experimental–theoretical method (σ^eval(γ, in) = F_i^theor × σ^expt(γ, xn)) on the basis of the experimental cross sections σ^expt(γ, xn) and the results of the calculations within the combined model of photonuclear reactions.     

Nuclear spin resonance of <Superscript>53</Superscript>Cr in ferromagnetic CuCr<Subscript>2</Subscript>S<Subscript>4</Subscript>: Sb

The influence of variable valence on NSR spectra of ⁵³Cr nuclei in ferromagnetic CuCr_2?xSb_xS₄ (x = 0, 0.02, 0.07) at T = 77 K is considered. For quadrupole nuclei in locally anisotropic positions, the effects of variable valence result in averaging of not only the resonance frequency but also of the quadrupole and magnetic anisotropy constants. The significant difference between the experimental and calculated values of these constants indicates the important role of the intrinsic electronic contribution to the anisotropy of hyperfine fields of compounds containing Cr⁴⁺ ions. Additional lines caused by intrinsic and induced defects in the structure are observed in the spectra of doped and undoped compounds.