Bound-exciton cathodoluminescence in ZnSe crystals and two-phonon resonance

A study has been made of the cathodoluminescence of ZnSe crystals annealed in vacuum [ZnSe(Vac)], in vacuum and, subsequently, in antimony melt [(ZnSe(Vac)(Sb)], or in a zinc melt with subsequent annealing in antimony [ZnSe(Zn)(Sb)]. The emission of all samples contained the I ₁ ^s,d -nLO series. The LO-phonon replicas of the emission line I ₁ ^s observed in ZnSe(Vac) samples are accompanied by single-plasmon satellites. The plasmon energy determining the replica separation is ?ω_p?10 meV. The emission lines of ZnSe(Zn)(Sb) samples have the smallest half-width. We report the first observation of anomalous broadening of the zero-phonon line I ₁ ^s in ZnSe(Vac) samples caused by a high zinc vacancy content. A theory on the shape of the emission spectrum under two-phonon resonance is developed including bound-exciton interaction with mixed plasmon-phonon vibrational modes. It is shown that the splitting of the I ₁ ^d line at T?2 K may originate from resonance exciton-phonon interaction between exciton-impurity complexes.     

The Structure of Radiative Tunnel Recombination Sites in Emulsion Microcrystals of AgBr(I)

To identify the structure of emissive tunnel recombination sites in the emulsion microcrystals of silver bromide AgBr(I) with iodine contaminations and to determine the role of an emulsion medium in their formation, the temperature dependence of the luminescence spectra in the range from 77 to 120 K, the kinetics of the growth of the maximum luminescence intensity value at λ ≈ 560 nm, and the luminescence flash spectrum stimulated by the infrared light are investigated. Two types of the AgBr_{1 – x}(I _x ) (x = 0.03) microcrystals—namely, obtained in an aqueous solution and on a gelatin substrate—are used in the studies. It is established that the emissive tunnel recombination sites with a luminescence maximum at λ ≈ 560 nm in AgBr_{1 – x}(I _x ) (x = 0.03) are the {(I _a ^- I _a ^- )Ag _i ⁺ } donor–acceptor complexes with the I _a ^- iodine ions located in neighbor anionic sites of the AgBr(I) crystal lattice, next to which the Ag _i ⁺ interstitial silver ion is positioned. With an increase in the temperature, the {(I _a ^- I _a ^- )Ag _i ⁺ } sites undergo structural transformation into the {(I _a ^- I _a ^- )Ag_in⁺} sites, where n = 2, 3, …. Moreover, the {(I _a ^- I _a ^- )Ag _in ⁺ } sites (n = 2) after the capture of an electron and hole also provide the tunnel recombination with a luminescence maximum at λ ≈ 720 nm. The influence of an emulsion medium consists in that gelatin interacts with the surface electron-localization sites, i.e., the interstitial silver ions Ag _in ⁺ , n = 1, 2, and forms the complexes {Ag _in ⁰ G⁺} (n = 1, 2) with them. The latter are deeper electron traps with a small capture cross section as compared to the Ag _in ⁺ sites (n = 1, 2) and that manifest themselves in that the kinetics of the luminescence growth in AgBr(I) to a stationary level at λ ≈ 560 nm is characterized by the presence of “flash firing.” At the same time, the luminescence flash stimulated by IR light, for which the Ag _in ⁺ (n = 1, 2) electron-localization sites are responsible, is absent. It is supposed that the electrons localized on the {Ag _in ⁺ G⁺} complexes (n = 2) retain the capability for emissive tunnel recombination with holes localized on paired iodine sites with a luminescence maximum at λ ≈ 750 nm.     

Multiplasmon replicas of the emission lines of bound excitons in ZnSe crystals

This paper reports on the results of investigations into the cathodoluminescence spectra of ZnSe crystals grown from a vapor phase and annealed in a bismuth melt [ZnSe(Bi) crystals], as well as in a bismuth melt containing aluminum [ZnSe(Bi,Al) crystals], at a temperature of 1200 K for 120 h. The longitudinal optical (LO) phonon replicas of the emission lines of free excitons are dominant in the cathodoluminescence spectra of the samples subjected to annealing in the bismuth melt with subsequent quenching. The emission spectra of all the samples studied, including the initial crystals, contain series of I ₁ ^s,d -nLO lines of bound excitons. It is revealed that plasmon replicas are characteristic not only of the I ₁ ^s -nLO lines. A weak exciton-plasmon interaction manifests itself in the form of single-plasmon Stokes satellites of the I ₁ ^d -nLO spectral lines. A theory describing the multiphoton optical transitions of bound excitons with the participation of mixed plasmonphonon vibrational modes is proposed. This theory makes it possible to calculate the shape function of an emission spectrum without recourse to model concepts. The results of the theoretical calculations are in agreement with the experimental data.     

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.     

Nuclear magnetic relaxation of spin<Emphasis Type="Italic">I</Emphasis>=1/2 scalar coupled to a quadrupolar nucleus in the presence of cross-correlation effects

The rigorous treatment of relaxation for the dipolar-multipolarAX spin system (I=1/2,S>1/2) in the presence of the dipolarI-S coupling, anisotropy chemical shift and quadrupolar interaction ofS spin is proposed. The calculations of the spin evolution under the relaxation Hamiltonian are based on the second-order time-dependent perturbation theory and are carried out in the operator representation. For this task the double commutator identities of the type [[I _±S _z ⁿ ,A _q ^μp ]A _?q ^μp ] and [[I _zS _z ⁿ ,A _q ^μp ]A _?q ^μp. ] are derived. The fist-order differential equations for the evolution of longitudinal two-spin orderI _zS _z ⁿ , z=magnetization ofS spinS _z ⁿ and coherences <I _±S _z ⁿ > in the spin systemIS with scalar coupling between spin 1/2 and quadrupolar spinS>1/2 were obtained. These equations are used to get equations for the evolutions of each component of the multiplet structure of spinI. The imaginary part of the cross-correlation spectral density function and indirect spin-spin coupling Hamiltonian are taken into account. Equations for the longitudinal components of theI spin spectrum in the presence of cross-correlation effects were obtained also. Longitudinal and transverse relaxation times and cross-relaxation times in the presence of cross-correlation D-CSA, Q-CSA, Q-D were analyzed.     

Vibronic-spin-orbit interactions in heterocyclic analogues of fluorene with the N and O heteroatoms

The transition dipole moments P _0n ^s for the transitions from the electronic triplet state ³ B ²(ππ*) to vibrational sublevels of the vibrational out-of-plane modes n of the carbazole and dibenzofuran molecules are calculated. The values of the radiative deactivation rate constant k _rad ^s of the triplet sublevels T ^s are determined along with the components k _SO ^s and k _VSO ^s of this constant, which depend on the intramolecular spin-orbit (SO) and vibronic-spin-orbit (VSO) interaction. It is ascertained that k _rad ^z > k _rad ^y . For different structural units of the molecules (the heteroatom and the carbon atoms of the dibenzene fragment), the effect of the SO coupling on the constant k _VSO～Σ_{s, n} (P _0n ^s )² is studied. A competition between the effects on k _VSO from the SO coupling in the carbon atoms and in the light N and O heteroatoms is revealed. This competition accounts for the weak influence of the heteroatom on this component of the rate constant k _rad in these molecules. It is ascertained that the intensity distribution among the vibronic lines in the phosphorescence spectra of carbazole and dibenzofuran I _0n ～Σs (P _0n ^s )² is different due to the substantially different influence of the N and O heteroatoms on the deactivation of the triplet sublevel T ^y .     

Stability of Coulomb systems

The stability of m ₁ ⁺ m ₂ ⁺ m ₃ ^? m ₄ ^? Coulomb systems formed by particles of unit charge against dissociation is considered as a function of the particle mass. It is shown that, from the stability of the m ₁ ⁺ m ₂ ⁺ m ₃ ^? three-particle system, it follows that the m ₁ ⁺ m ₂ ⁺ m ₃ ^? m ₄ ^? four-particle system containing an additional particle of mass satisfying the condition m ₄ ^? ? m ₃ ^? is stable. The results of calculations of the stability domain for m ₁ ⁺ m ₂ ⁺ m ₃ ^? systems asymmetric in particle masses are reported. The stability of 39 asymmetric exotic four-particle molecules and mesic molecules against dissociation is established.     

The nature of the lower excited state of the special pair of bacterial photosynthetic reaction center of <Emphasis Type="Italic">Rhodobacter Sphaeroides</Emphasis> and the dynamics of primary charge separation

Quantum-chemical calculations of the structure in the ground and lower singlet excited states and the vibrations (in the ground state) of special pair P of photosynthetic reaction center of purple bacteria (RCPb) Rhodobacter Sphaeroides, consisting of two bacteriochlorophyll molecules P_A and P_B, have been carried out. It is shown that excitation of the special pair is followed by fast relaxation dynamics, accompanied by the transformation of the initial P* state into the P_A^δ+P_B^δ- state (δ ~ 0.5) with charge separation. This behavior is due to the presence of several nonplanar vibrations with participation of the acetyl group of macrocycle P_В in the nuclear wave packet on the potential surface of the P* state; these vibrations facilitate destabilization of the lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) of the macrocycle P_A and formation of the P_A^δ+P_B^δ- state. The structural transformations in the P* state are due to its linking character in the contact region of the acetyl group-containing pyrrole rings of P_A and P_B. The transition from the P* state to specifically the P_A^δ+P_B^δ- state is related to the fact that the acetyl group P_A is involved in the intermolecular hydrogen bond with amino acid residue His^L168; for this reason, this group and the pyrrole ring linked with it can hardly participate in structural transformations. The electronic matrix element Н₁₂ of the electron transfer from the special pair in the P_A^δ+P_B^δ- state to a molecule of accessory bacteriochlorophyll В_А greatly exceeds that for the transfer to В_B. This circumstance and the fact that the P_A^δ+P_B^δ- state is energetically more favorable than the P* state facilitate the preferred directionality of the electron transfer in RCPb Rhodobacter Sphaeroides with participation of the cofactors located in its subunit L.     

Spectroscopy of the <Stack><Subscript>Λ</Subscript><Superscript>7</Superscript></Stack>He nucleus in a three-cluster model

The _Λ ⁷ He hypernucleus is considered within the _Λ ⁵ He + n + n cluster model. The hyperon—nucleon interaction is described by a one-boson-exchange potential that is constructed on the basis of the NSC97f model. Phenomenological potentials are used to describe the αΛ and αN interactions. For the _Λ ⁵ Hen interaction, use is made of the folding-model potential. The calculations of the hyperon binding energy in the ground state of the _Λ ⁷ He hypernucleus on the basis of Faddeev equations in configuration space yield a result (5.35 MeV) that agrees well with preliminary experimental data (5.4 MeV). The problem of calculating the hyperon binding energy within the three-body approach is discussed. In calculating the energy spectrum of _Λ ⁷ He, use is made of a version of the method of analytic continuation in the coupling constant. Low-lying excited states of this nucleus can be classified as an analog of the corresponding states of the ⁶He nucleus with allowance for the clustering of the _Λ ⁵ He+n+n system in the ⁶He(J ^π)+Λ(s) form.     

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.     

Hysteresis of the magnetoresistance of granular HTSC YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7 − δ</Subscript> in weak fields

Hysteresis of the magnetoresistance of ceramic YBa₂Cu₃O_～6.95 HTSC samples is studied at T = 77.3 K in an external magnetic field H _ext changing in 0 → H _max → 0 cycles, where H _max is the maximum magnitude of H _ext. Information is obtained about the dependences of the critical fields of Josephson weak links H _c2J , the lower critical fields of superconducting grains H _c1A , and the critical fields H _BG-VG of the Bragg glass-vortex glass phase transition in the vortex matter on transport current I, magnetic field, and the mutual orientation of I and H _ext. It is found that the magnetoresistance δρ⁺/ρ_{273 K} measured with increasing H _ext is significantly higher than Δρ^?/ρ_{273 K} and that H _c2J ⁺ < H _c2J ^? , H _c1A ⁺ < H _c1A ^? , and H _BG-VG ⁺ < H _BG-VG ^? .     

Probabilities of forbidden magnetic-dipole transitions in the hydrogen atom and hydrogen-like ions

Explicit formulasfor strongly forbidden magnetic-dipole transitiions between states njl and n′jl in the hydrogen atom and light hydrogen-like ions are derived. The expressions for transition probabilities are presented in the form W _{n′jl; njl} ^(M1) = D _n′n ^lj αm _e (αZ)¹⁰ (in relativistic units), where m _e is the electron mass, α is the fine-structure constant, and Z is the nuclear charge; the constants D _n′n ^lj are presented in an analytical form. Before now, only the D ₂₁ ^01/2 coefficient corresponding to the 1s _1/2–2s _1/2 transition was known in explicit form. The results obtained can be used in designing an experiment on parity violation in the hydrogen atom.     

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.     

Influence of the dimension of a polycrystalline film and the optical anisotropy of crystallites on the effective dielectric constant of the film

The dimension D of a polycrystalline film and the optical anisotropy m = ε_z/ε_x of uniaxial crystallites with the principal components ε_x = ε_y and ε_z of the tensor of the dielectric constant have been shown to produce a strong influence on the effective dielectric constant ε_D^* and the effective refractive index n_D^* = (ε_D^*)^1/2 of the film in the optical transparency region, as well as on the boundaries of the intervals B_Dl ≤ ε_D^* ≤ B_Du. The intervals Δ₂(m) = B_2l–B_2u and Δ₃(m) = B_3l–B_3u are separated by a gap for m in the range 1 < m < 2, whereas the theoretical dependence ε₂^*(m) is separated by a gap from the interval Δ₃(m) for m in the range 1 < m < 4. This is confirmed by a comparison of the experimental (n_oP) and theoretical (n_D^*) ordinary refractive indices for uniaxial polycrystalline films of the conjugated polymer poly(p-phenylene vinylene) (PPV) with uniaxial crystallites and appropriate values of m. In the visible transparency region of the PPV films with a change in m(λ) in the range 2 < m(λ) < 3 due to the dependence of the components ε_x,z(λ) on the light wavelength λ, the refractive indices n_oP²(λ) = ε_oP(λ) are consistent with the theoretical values of ε₂^*(λ) and lie outside the interval Δ₃(m). For m(λ) > 3 near the electronic absorption band of the crystallites, the values of ε_oP(λ) lie in the region of the overlap of the intervals Δ₂(m) and Δ₃(m). The boundaries mc of the range 1 < m < m_c are determined, for which the interval Δ₂(m) is separated by a gap from the dependences ε₃^*(m) corresponding to the effective medium theory with spherical crystallites and hierarchical models of a polycrystal, as well as from the proposed new dependence ε₃^*(m).     

Hyperfine Interactions in Pb<Superscript>3+</Superscript>F<Stack><Subscript>8</Subscript><Superscript>−</Superscript></Stack>F<Stack><Subscript>a</Subscript><Superscript>−</Superscript></Stack> clusters in fluorite crystals

The parameters of hyperfine interactions in Pb³⁺F ₈ ^? F _a ^? tetragonal clusters of MeF₂ crystals (Me=Ca, Sr, Ba) are interpreted. The contributions of the spin polarization to the parameters of the proper hyperfine interaction and additional (ligand) hyperfine interactions are calculated in the approximation of weak binding between a charge-compensating ion F _a ^? and a cubic fragment in the tetragonal cluster. It is demonstrated that correct inclusion of the contributions from the spin polarization to the ligand isotropic hyperfine interaction for the F _a ^? ion leads to anomalously large parameters of this interaction for MeF₂ crystals. These results are in agreement with experimental data.     

Dominant decays of scalar leptoquarks and scalar gluons in the minimal four-color symmetry model

Fermionic and weak decays of the scalar leptoquarks S = S ₁ ⁽⁺⁾ , S ₁ ^(?) , and S _m and the scalar gluons F = F ₁ and F ₂ predicted by the minimal model involving four-color symmetry and the Higgs mechanism of quark-and lepton-mass splitting are considered. The widths and the branching ratios are calculated for these decays, and the results are analyzed versus the couplings and masses of decaying particles. It is shown that, at relatively small mass splittings Δm within scalar doublets (Δm < m _W), the fermionic decays S ₁ ⁽⁺⁾ → tl _j ⁺ , S ₁ ^(?) → v _i \(\tilde b\), S _m → t \(\tilde \nu \) _j, F ₁ → t \(\tilde b\), and F ₂ → t \(\tilde t\), which are characterized by few-GeV widths for m _S, m _F < 1 TeV and decay branching ratios close to unity, are dominant, but that, for Δm > m _W, the weak decays S → S′W and F → F’W compete with the above fermionic decays. In the case of m _S < m _t, the processes S ₁ ⁽⁺⁾ → cl _j ⁺ , S ₁ ^(?) → v _i \(\tilde b\), S _m → bl _j ⁺ , and S _m → c \(\tilde \nu \) _j, whose total branching ratios are Br(S ₁ ⁽⁺⁾ → cl ⁺) ≈ Br(S ₁ ^(?) → v \(\tilde b\)) ≈ 1, Br(S _m → bl ⁺) ≈ 0.9, and Br(S _m → c \(\tilde \nu \)) ≈ 0.1, appear to be dominant decays of scalar leptoquarks. Searches for these decays at LHC and the Tevatron are of interest.     

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.     

Novel features of the fragment mass variance in fission of hot nuclei

On the basis of data obtained by the incomplete fusion reactions ⁷Li(43A MeV)+²³²Th and ¹⁴N(34A MeV)+¹⁹⁷Au, the energy dependence of the variance (σ _M ² ) of the fragment mass in fission of highly heated nuclei has been investigated for total excitation energies E _tot ^* ranging from 50 up to 350 MeV. The dependence σ _M ² E _tot ^* shows some unexpected features when E _tot ^* exceeds a value of about 70 MeV. After this value, the steady increase of σ _M ² expected from its temperature dependence changes to some kind of plateau between 100 and 200 MeV. Further on, at E _tot ^* in excess of about 250 MeV, the variance is found to increase again sharply. In order to analyze this behavior quantitatively, a dynamical stochastic model has been developed. The model employs the one-body dissipation mechanism and describes the decay of highly excited and rotating nuclei by fission and light-particle evaporation. It satisfactorily explains the measured prior-to-scission neutron multiplicities and the experimental mass variances up to E _tot ^* ?250 MeV, but the stochastic treatment does not reveal any increase in σ _M ² at higher excitation energies in contradiction with the data.     

Magnetoresistance of granular YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7 − δ</Subscript> HTSC in weak magnetic field: Angular dependence

The magnetoresistance of ceramic YBa₂Cu₃O_～6.5 HTSC samples is studied as a function of the mutual orientation of the current I and external magnetic field H _ext at T = 77.3 K in magnetic fields of up to ～500 Oe. It is found that, if the demagnetization factor D is taken into account, the effective critical field of complete penetration of Josephson vortices into weak links H _c2J ^eff does not depend on the mutual orientation of I and H _ext. The lower critical field H _c1A ^eff associated with the beginning of penetration of Abrikosov vortices into superconducting grains increases substantially with the angle between I and H _ext. The strongest variation with the mutual orientation of I and H _ext is exhibited by the critical field of the Bragg glass-vortex glass first-order phase transition H _BG-VG ^eff and by the magnetoresistance jump at this phase transition.